OWASP API Security Project Media Coverage

A list of must read articles on OWASP API Security Project: 

• 10/3/19, APISecurity.IO (UMV: 1,510): Issue 51: Gartner Releases Full Report on API Security
• 10/2/19, ADT Magazine (UMV: 117,500): API Security Project Identifies Top 10 Vulnerabilities
• 9/26/19, Dark Reading (UMV: 57,800): Why You Need to Think About API Security
• 9/24/19, The Daily Swig (UMV: 30,500): OWASP Reveals Top 10 Security Threats Facing API Ecosystem
• 9/20/19, Security Boulevard (UMV: 29,100): New OWASP List Highlights API Security Holes
• 9/13/19, Security Boulevard (UMV: 29,100): Why You Need to Be Thinking About API Security
• 9/13/19, CyberWire (UMV: 49,380): Daily Briefing: OWASP API Security Project
• 9/12/19, Dark Reading (UMV: 57,800): APIs Get Their Own Top 10 Security list
• Also included in Dark Reading's weekly newsletter on 9/19/19

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PDFex: Major Security Flaws In PDF Encryption

After investigating the security of PDF signatures, we had a deeper look at PDF encryption. In co­ope­ra­ti­on with our friends from Müns­ter Uni­ver­si­ty of Ap­p­lied Sci­en­ces, we discovered severe weaknesses in the PDF encryption standard which lead to full plaintext exfiltration in an active-attacker scenario.

To guarantee confidentiality, PDF files can be encrypted. This enables the secure transfer and storing of sensitive documents without any further protection mechanisms.

The key management between the sender and recipient may be password based (the recipient must know the password used by the sender, or it must be transferred to them through a secure channel) or public key based (i.e., the sender knows the X.509 certificate of the recipient).

In this research, we analyze the security of encrypted PDF files and show how an attacker can exfiltrate the content without having the corresponding keys.

So what is the problem?

The security problems known as PDFex discovered by our research can be summarized as follows:

1. Even without knowing the corresponding password, the attacker possessing an encrypted PDF file can manipulate parts of it.
   More precisely, the PDF specification allows the mixing of ciphertexts with plaintexts. In combination with further PDF features which allow the loading of external resources via HTTP, the attacker can run direct exfiltration attacks once a victim opens the file.
2. PDF encryption uses the Cipher Block Chaining (CBC) encryption mode with no integrity checks, which implies ciphertext malleability.
   This allows us to create self-exfiltrating ciphertext parts using CBC malleability gadgets. We use this technique not only to modify existing plaintext but to construct entirely new encrypted objects.

Who uses PDF Encryption?

PDF encryption is widely used. Prominent companies like Canon and Samsung apply PDF encryption in document scanners to protect sensitive information.

Further providers like IBM offer PDF encryption services for PDF documents and other data (e.g., confidential images) by wrapping them into PDF. PDF encryption is also supported in different medical products to transfer health records, for example Innoport, Ricoh, Rimage.

Due to the shortcomings regarding the deployment and usability of S/MIME and OpenPGP email encryption, some organizations use special gateways to automatically encrypt email messages as encrypted PDF attachments, for example CipherMail, Encryptomatic, NoSpamProxy. The password to decrypt these PDFs can be transmitted over a second channel, such as a text message (i.e., SMS).

Technical details of the attacks

We developed two different attack classes on PDF Encryption: Direct Exfiltration and CBC Gadgets.

Attack 1: Direct Exfiltration (Attack A)

The idea of this attack is to abuse the partial encryption feature by modifying an encrypted PDF file. As soon as the file is opened and decrypted by the victim sensitive content is sent to the attacker. Encrpyted PDF files does not have integrity protection. Thus, an attacker can modify the structure of encrypted PDF documents, add unencrypted objects, or wrap encrypted parts into a context controlled the attacker.

In the given example, the attacker abuses the flexibility of the PDF encryption standard to define certain objects as unencrypted. The attacker modifies the Encrypt dictionary (6 0 obj) in a way that the document is partially encrypted – all streams are left AES256 encrypted while strings are defined as unencrypted by setting the Identity filter. Thus, the attacker can freely modify strings in the document and add additional objects containing unencrypted strings.

The content to be exfiltrated is left encrypted, see Contents (4 0 obj) and EmbeddedFile (5 0 obj). The most relevant object for the attack is the definition of an Action, which can submit a form, invoke a URL, or execute JavaScript. The Action references the encrypted parts as content to be included in requests and can thereby be used to exfiltrate their plaintext to an arbitrary URL. The execution of the Action can be triggered automatically once the PDF file is opened (after the decryption) or via user interaction, for example, by clicking within the document.

This attack has three requirements to be successful. While all requirements are PDF standard compliant, they have not necessarily been implemented by every PDF application:

• Partial encryption: Partially encrypted documents based on Crypt Filters like the Identity filter or based on other less supported methods like the None encryption algorithm.
• Cross-object references: It must be possible to reference and access encrypted string or stream objects from unencrypted attacker-controlled parts of the PDF document.
• Exfiltration channel: One of the interactive features allowing the PDF reader to communicate via Internet must exist, with or without user interaction. Such Features are PDF Forms, Hyperlinks, or JavaScript.

Please note that the attack does not abuse any cryptographic issues, so that there are no requirements to the underlying encryption algorithm (e.g., AES) or the encryption mode (e.g., CBC).

In the following, we show three techniques how an attack can exfiltrate the content.

Exfiltration via PDF Forms (A1)

The PDF standard allows a document's encrypted streams or strings to be defined as values of a PDF form to be submitted to an external server. This can be done by referencing their object numbers as the values of the form fields within the Catalog object, as shown in the example on the left side. The value of the PDF form points to the encrypted data stored in 2 0 obj.

To make the form auto-submit itself once the document is opened and decrypted, an OpenAction can be applied. Note that the object which contains the URL (http://p.df) for form submission is not encrypted and completely controlled by the attacker. As a result, as soon as the victim opens the PDF file and decrypts it, the OpenAction will be executed by sending the decrypted content of 2 0 obj to (http://p.df).

Exfiltration via Hyperlinks (A2)

If forms are not supported by the PDF viewer, there is a second method to achieve direct exfiltration of a plaintext. The PDF standard allows setting a "base" URI in the Catalog object used to resolve all relative URIs in the document.

This enables an attacker to define the encrypted part as a relative URI to be leaked to the attacker's web server. Therefore the base URI will be prepended to each URI called within the PDF file. In the given example, we set the base URI to (http://p.df).

The plaintext can be leaked by clicking on a visible element such as a link, or without user interaction by defining a URI Action to be automatically performed once the document is opened.

In the given example, we define the base URI within an Object Stream, which allows objects of arbitrary type to be embedded within a stream. This construct is a standard compliant method to put unencrypted and encrypted strings within the same document. Note that for this attack variant, only strings can be exfiltrated due to the specification, but not streams; (relative) URIs must be of type string. However, fortunately (from an attacker's point of view), all encrypted streams in a PDF document can be re-written and defined as hex-encoded strings using the hexadecimal string notation.

Nevertheless, the attack has some notable drawbacks compared to  Exfiltration via PDF Forms:

• The attack is not silent. While forms are usually submitted in the background (by the PDF viewer itself), to open hyperlinks, most applications launch an external web browser.
• Compared to HTTP POST, the length of HTTP GET requests, as invoked by hyperlinks, is limited to a certain size.
• PDF viewers do not necessarily URL-encode binary strings, making it difficult to leak compressed data.

Exfiltration via JavaScript (A3)

The PDF JavaScript reference allows JavaScript code within a PDF document to directly access arbitrary string/stream objects within the document and leak them with functions such as *getDataObjectContents* or *getAnnots*.

In the given example, the stream object 7 is given a Name (x), which is used to reference and leak it with a JavaScript action that is automatically triggered once the document is opened. The attack has some advantages compared to Exfiltration via PDF Forms and Exfiltration via Hyperlinks, such as the flexibility of an actual programming language.

It must, however, be noted that – while JavaScript actions are part of the PDF specification – various PDF applications have limited JavaScript support or disable it by default (e.g., Perfect PDF Reader).

Attack 2: CBC Gadgets (Attack B)

Not all PDF viewers support partially encrypted documents, which makes them immune to direct exfiltration attacks. However, because PDF encryption generally defines no authenticated encryption, attackers may use CBC gadgets to exfiltrate plaintext. The basic idea is to modify the plaintext data directly within an encrypted object, for example, by prefixing it with an URL. The CBC gadget attack, thus does not necessarily require cross-object references.

Note that all gadget-based attacks modify existing encrypted content or create new content from CBC gadgets. This is possible due to the malleability property of the CBC encryption mode.

This attack has two necessary preconditions:

• Known plaintext: To manipulate an encrypted object using CBC gadgets, a known plaintext segment is necessary. For AESV3 – the most recent encryption algorithm – this plain- text is always given by the Perms entry. For older versions, known plaintext from the object to be exfiltrated is necessary.
• Exfiltration channel: One of the interactive features: PDF Forms or Hyperlinks.

These requirements differ from those of the direct exfiltration attacks, because the attacks are applied "through" the encryption layer and not outside of it.

Exfiltration via PDF Forms (B1)

As described above, PDF allows the submission of string and stream objects to a web server. This can be used in conjunction with CBC gadgets to leak the plaintext to an attacker-controlled server, even if partial encryption is not allowed.

A CBC gadget constructed from the known plaintext can be used as the submission URL, as shown in the example on the left side. The construction of this particular URL gadget is challenging. As PDF encryption uses PKCS#5 padding, constructing the URL using a single gadget from the known Perms plaintext is difficult, as the last 4 bytes that would need to contain the padding are unknown.

However, we identified two techniques to solve this. On the one hand, we can take the last block of an unknown ciphertext and append it to our constructed URL, essentially reusing the correct PKCS#5 padding of the unknown plaintext. Unfortunately, this would introduce 20 bytes of random data from the gadgeting process and up to 15 bytes of the unknown plaintext to the end of our URL.

On the other hand, the PDF standard allows the execution of multiple OpenActions in a document, allowing us to essentially guess the last padding byte of the Perms value. This is possible by iterating over all 256 possible values of the last plaintext byte to get 0x01, resulting in a URL with as little random as possible (3 bytes). As a limitation, if one of the 3 random bytes contains special characters, the form submission URL might break.

Exfiltration via Hyperlinks (B2)

Using CBC gadgets, encrypted plaintext can be prefixed with one or more chosen plaintext blocks. An attacker can construct URLs in the encrypted PDF document that contain the plaintext to exfiltrate. This attack is similar to the exfiltration hyperlink attack (A2). However, it does not require the setting of a "base" URI in plaintext to achieve exfiltration.

The same limitations described for direct exfiltration based on links (A2) apply. Additionally, the constructed URL contains random bytes from the gadgeting process, which may prevent the exfiltration in some cases.

Exfiltration via Half-Open Object Streams (B3)

While CBC gadgets are generally restricted to the block size of the underlying block cipher – and more specifically the length of the known plaintext, in this case, 12 bytes – longer chosen plaintexts can be constructed using compression. Deflate compression, which is available as a filter for PDF streams, allows writing both uncompressed and compressed segments into the same stream. The compressed segments can reference back to the uncompressed segments and achieve the repetition of byte strings from these segments. These backreferences allow us to construct longer continuous plaintext blocks than CBC gadgets would typically allow for. Naturally, the first uncompressed occurrence of a byte string still appears in the decompressed result. Additionally, if the compressed stream is constructed using gadgets, each gadget generates 20 random bytes that appear in the decompressed stream. A non-trivial obstacle is to keep the PDF viewer from interpreting these fragments in the decompressed stream. While hiding the fragments in comments is possible, PDF comments are single-line and are thus susceptible to newline characters in the random bytes. Therefore, in reality, the length of constructed compressed plaintexts is limited.

To deal with this caveat, an attacker can use ObjectStreams which allow the storage of arbitrary objects inside a stream. The attacker uses an object stream to define new objects using CBC gadgets. An object stream always starts with a header of space-separated integers which define the object number and the byte offset of the object inside the stream. The dictionary of an object stream contains the key First which defines the byte offset of the first object inside the stream. An attacker can use this value to create a comment of arbitrary size by setting it to the first byte after their comment.

Using compression has the additional advantage that compressed, encrypted plaintexts from the original document can be embedded into the modified object. As PDF applications often create compressed streams, these can be incorporated into the attacker-created compressed object and will therefore be decompressed by the PDF applications. This is a significant advantage over leaking the compressed plaintexts without decompression as the compressed bytes are often not URL-encoded correctly (or at all) by the PDF applications, leading to incomplete or incomprehensible plaintexts. However, due to the inner workings of the deflate algorithms, a complete compressed plaintext can only be prefixed with new segments, but not postfixed. Therefore, a string created using this technique cannot be terminated using a closing bracket, leading to a half-open string. This is not a standard compliant construction, and PDF viewers should not accept it. However, a majority of PDF viewers accept it anyway.

Evaluation

During our security analysis, we identified two standard compliant attack classes which break the confidentiality of encrypted PDF files. Our evaluation shows that among 27 widely-used PDF viewers, all of them are vulnerable to at least one of those attacks, including popular software such as Adobe Acrobat, Foxit Reader, Evince, Okular, Chrome, and Firefox.

You can find the detailed results of our evaluation here.

What is the root cause of the problem?

First, many data formats allow to encrypt only parts of the content (e.g., XML, S/MIME, PDF). This encryption flexibility is difficult to handle and allows an attacker to include their own content, which can lead to exfiltration channels.

Second, when it comes to encryption, AES-CBC – or encryption without integrity protection in general – is still widely supported. Even the latest PDF 2.0 specification released in 2017 still relies on it. This must be fixed in future PDF specifications and any other format encryption standard, without enabling backward compatibility that would re-enable CBC gadgets.

A positive example is JSON Web Encryption standard, which learned from the CBC attacks on XML and does not support any encryption algorithm without integrity protection.

Authors of this Post

Jens Müller
Fabian Ising
Vladislav Mladenov
Christian Mainka
Sebastian Schinzel
Jörg Schwenk

Acknowledgements

Many thanks to the CERT-Bund team for the great support during the responsible disclosure process.Related word

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Read More :- "PDFex: Major Security Flaws In PDF Encryption"

DOWNLOAD SENTRY MBA V1.4.1 – AUTOMATED ACCOUNT CRACKING TOOL

Sentry MBA is an automated account cracking tool that makes it one of the most popular cracking tools. It is used by cybercriminals to take over user accounts on major websites. With Sentry MBA, criminals can rapidly test millions of usernames and passwords to see which ones are valid on a targeted website. The tool has become incredibly popular — the Shape Security research team sees Sentry MBA attack attempts on nearly every website we protect.  Download Sentry MBA v1.4.1 latest version.

FEATURES

Sentry MBA has a point-and-click graphical user interface, online help forums, and vibrant underground marketplaces to enable large numbers of individuals to become cybercriminals. These individuals no longer need advanced technical skills, specialized equipment, or insider knowledge to successfully attack major websites.

Sentry MBA attack has three phases,

• Targeting and attack refinement
• Automated account check
• Monetization

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Printer Security

Printers belong arguably to the most common devices we use. They are available in every household, office, company, governmental, medical, or education institution.

From a security point of view, these machines are quite interesting since they are located in internal networks and have direct access to sensitive information like confidential reports, contracts or patient recipes.

TL;DR: In this blog post we give an overview of attack scenarios based on network printers, and show the possibilities of an attacker who has access to a vulnerable printer. We present our evaluation of 20 different printer models and show that each of these is vulnerable to multiple attacks. We release an open-source tool that supported our analysis: PRinter Exploitation Toolkit (PRET) https://github.com/RUB-NDS/PRET

Full results are available in the master thesis of Jens Müller and our paper.

Furthermore, we have set up a wiki (http://hacking-printers.net/) to share knowledge on printer (in)security.
The highlights of the entire survey will be presented by Jens Müller for the first time at RuhrSec in Bochum.

Background

There are many cool protocols and languages you can use to control your printer or your print jobs. We assume you have never heard of at least half of them. An overview is depicted in the following figure and described below.

 

Device control

This set of languages is used to control the printer device. With a device control language it is possible to retrieve the printer name or status. One of the most common languages is the Simple Network Management Protocol (SNMP). SNMP is a UDP based protocol designed to manage various network components beyond printers as well, e.g. routers and servers.

Printing channel

The most common network printing protocols supported by printer devices are the Internet Printing Protocol (IPP), Line Printer Daemon (LPD), Server Message Block (SMB), and raw port 9100 printing. Each protocol has specific features like print job queue management or accounting. In our work, we used these protocols to transport malicious documents to the printers.

 

Job control language

This is where it gets very interesting (for our attacks). A job control language manages printer settings like output trays or paper size. A de-facto standard for print job control is PJL. From a security perspective it is very useful that PJL is not limited to the current print job as some settings can be made permanent. It can further be used to change the printer's display or read/write files on the device.

 

Page description language

A page description language specifies the appearance of the actual document. One of the most common 'standard' page description languages is PostScript. While PostScript has lost popularity in desktop publishing and as a document exchange format (we use PDF now), it is still the preferred page description language for laser printers. PostScript is a stack-based, Turing-complete programming language consisting of about 400 instructions/operators. As a security aware researcher you probable know that some of them could be useful. Technically spoken, access to a PostScript interpreter can already be classified as code execution.

 

Attacks

Even though printers are an important attack target, security threats and scenarios for printers are discussed in very few research papers or technical reports. Our first step was therefore to perform a comprehensive analysis of all reported and published attacks in CVEs and security blogs. We then used this summary to systematize the known issues, to develop new attacks and to find a generic approach to apply them to different printers. We estimated that the best targets are the PostScript and PJL interpreters processing the actual print jobs since they can be exploited by a remote attacker with only the ability to 'print' documents, independent of the printing channel supported by the device.

We put the printer attacks into four categories.

 

Denial-of-service (DoS)

Executing a DoS attack is as simple as sending these two lines of PostScript code to the printer which lead to the execution of an infinite loop:

Denial-of-service

%!
{} loop

Other attacks include:

• Offline mode. The PJL standard defines the OPMSG command which 'prompts the printer to display a specified message and go offline'.
• Physical damage. By continuously setting the long-term values for PJL variables, it is possible to physically destroy the printer's NVRAM which only survives a limited number of write cycles.
• Showpage redefinition. The PostScript 'showpage' operator is used in every document to print the page. An attacker can simply redefine this operator to do nothing.

Protection Bypass

Resetting a printer device to factory defaults is the best method to bypass protection mechanisms. This task is trivial for an attacker with local access to the printer, since all tested devices have documented procedures to perform a cold reset by pressing certain key combinations.
However, a factory reset can be performed also by a remote attacker, for example using SNMP if the device complies with RFC1759 (Printer MIB):

Protection Bypass

# snmpset -v1 -c public [printer] 1.3.6.1.2.1.43.5.1.1.3.1 i 6

Other languages like HP's PML, Kyocera's PRESCRIBE or even PostScript offer similar functionalities.

Furthermore, our work shows techniques to bypass print job accounting on popular print servers like CUPS or LPRng.

Print Job Manipulation

Some page description languages allow permanent modifications of themselves which leads to interesting attacks, like manipulating other users' print jobs. For example, it is possible to overlay arbitrary graphics on all further documents to be printed or even to replace text in them by redefining the 'showpage' and 'show' PostScript operators.

Information Disclosure

Printing over port 9100 provides a bidirectional channel, which can be used to leak sensitive information. For example, Brother based printers have a documented feature to read from or write to a certain NVRAM address using PJL:

Information Disclosure

@PJL RNVRAM ADDRESS = X

Our prototype implementation simply increments this value to dump the whole NVRAM, which contains passwords for the printer itself but also for user-defined POP3/SMTP as well as for FTP and Active Directory profiles. This way an attacker can escalate her way into a network, using the printer device as a starting point.
Other attacks include:

• File system access. Both, the standards for PostScript and PJL specify functionality to access the printers file system. As it seems, some manufacturers have not limited this feature to a certain directory, which leads to the disclosure of sensitive information like passwords.
  
• Print job capture. If PostScript is used as a printer driver, printed documents can be captured. This is made possible by two interesting features of the PostScript language: First, permanently redefining operators allows an attacker to 'hook' into other users' print jobs and secondly, PostScript's capability to read its own code as data allows to easily store documents instead of executing them.
  

• Credential disclosure. PJL passwords, if set, can easily retrieved through brute-force attacks due to their limited key space (1..65535). PostScript passwords, on the other hand, can be cracked extremely fast (up to 100,000 password verifications per second) thanks to the performant PostScript interpreters.
  

PRET

To automate the introduced attacks, we wrote a prototype software entitled PRET. The main idea of PRET is to facilitate the communication between the end-user and the printer. Thus, by entering a UNIX-like command PRET translates it to PostScript or PJL, sends it to the printer, and evaluates the result. For example, PRET converts a UNIX command ls to the following PJL request:

Information Disclosure

@PJL FSDIRLIST NAME="0:\" ENTRY=1 COUNT=65535

It then collects the printer output and translates it to a user friendly output.

PRET implements the following list of commands for file system access on a printer device:

Evaluation

As a highly motivated security researcher with a deep understanding of systematic analysis, you would probably obtain a list of about 20 - 30 well-used printers from the most important manufacturers, and perform an extensive security analysis using these printers.
However, this was not our case. To overcome the financial obstacles, we collected printers from various university chairs and facilities. While our actual goal was to assemble a pool of printers containing at least one model for each of the top ten manufacturers, we practically took what we could get. The result is depicted in the following figure:

The assembled devices were not brand-new anymore and some of them were not even completely functional. Three printers had physically broken printing functionality so it was not possible to evaluate all the presented attacks. Nevertheless, these devices represent a good mix of printers used in a typical university or office environment.

Before performing the attacks, we of course installed the newest firmware on each of the devices. The results of our evaluation show that we could find multiple attacks against each printer. For example, simple DoS attacks with malicious PostScript files containing infinite loops are applicable to each printer. Only the HP LaserJet M2727nf had a watchdog mechanism and restarted itself after about ten minutes. Physical damage could be caused to about half of the tested device within 24 hours of NVRAM stressing. For a majority of devices, print jobs could be manipulated or captured.

PostScript, PJL and PML based attacks can even be exploited by a web attacker using advanced cross-site printing techniques. In the scope of our research, we discovered a novel approach – 'CORS spoofing' – to leak information like captured print jobs from a printer device given only a victim's browser as carrier.
A proof-of-concept implementation demonstrating that advanced cross-site printing attacks are practical and a real-world threat to companies and institutions is available at http://hacking-printers.net/xsp/.

Our next post will be on adapting PostScript based attacks to websites.

Authors of this Post

Jens Müller
Juraj Somorovsky

Vladislav Mladenov

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PKCE: What Can(Not) Be Protected

This post is about PKCE [RFC7636], a protection mechanism for OAuth and OpenIDConnect designed for public clients to detect the authorization code interception attack.

At the beginning of our research, we wrongly believed that PKCE protects mobile and native apps from the so called „App Impersonation" attacks. Considering our ideas and after a short discussion with the authors of the PKCE specification, we found out that PKCE does not address this issue.

In other words, the protection of PKCE can be bypassed on public clients (mobile and native apps) by using a maliciously acting app.

OAuth Code Flow

In Figure 1, we briefly introduce how the OAuth flow works on mobile apps and show show the reason why we do need PKCE.
In our example the user has two apps installed on the mobile phone: an Honest App and an Evil App. We assume that the Evil App is able to register the same handler as the Honest App and thus intercept messages sent to the Honest App. If you are more interested in this issue, you can find more information here [1].

Figure 1: An example of the "authorization code interception" attack on mobile devices. 

Step 1: A user starts the Honest App and initiates the authentication via OpenID Connect or the authorization via OAuth. Consequentially, the Honest App generates an Auth Request containing the OpenID Connect/OAuth parameters: client_id, state, redirect_uri, scope, authorization_grant, nonce, …. 

Step 2: The Browser is called and the Auth Request is sent to the Authorization Server (usually Facebook, Google, …).

• The Honest App could use a Web View browser. However, the current specification clearly advice to use the operating system's default browser and avoid the usage of Web Views [2]. In addition, Google does not allow the usage of Web View browser since August 2016 [3].

Step 3: We asume that the user is authenticated and he authorizes the access to the requested resources. As a result, the Auth Response containing the code is sent back to the browser.

Step 4: Now, the browser calls the Honest App registered handler. However, the Evil App is registered on this handler too and receives the code.

Step 5: The Evil App sends the stolen code to the Authorization Server and receives the corresponding access_token in step 6. Now, the Evil App can access the authorized ressources.

• Optionally, in step 5 the App can authenticate on the Authorization Server via client_id, client_secret. Since, Apps are public clients they do not have any protection mechanisms regarding the storage of this information. Thus, an attacker can easy get this information and add it to the Evil App.

Proof Key for Code Exchange - PKCE (RFC 7636)

Now, let's see how PKCE does prevent the attack. The basic idea of PKCE is to bind the Auth Request in Step 1 to the code redemption in Step 5. In other words, only the app generated the Auth Request is able to redeem the generated code.

Figure 2: PKCE - RFC 7636 

Step 1: The Auth Request is generated as previosly described. Additionally, two parameters are added:

• The Honest App generates a random string called code_verifier
• The Honest App computes the code_challenge=SHA-256(code_verifier)
• The Honest App specifies the challenge_method=SHA256

Step 2: The Authorization Server receives the Auth Request and binds the code to the received code_challenge and challenge_method.

• Later in Step 5, the Authorzation Server expects to receive the code_verifier. By comparing the SHA-256(code_verifier) value with the recieved code_challenge, the Authorization Server verifies that the sender of the Auth Request ist the same as the sender of the code.

Step 3-4: The code leaks again to the Evil App.

Step 5: Now, Evil App must send the code_verifier together with the code. Unfortunatelly, the App does not have it and is not able to compute it. Thus, it cannot redeem the code.

 PKCE Bypass via App Impersonation

Again, PKCE binds the Auth Request to the coderedemption.

The question rises, if an Evil App can build its own Auth Request with its own code_verifier, code_challenge and challenge_method.The short answer is – yes, it can.

Figure 3: Bypassing PKCE via the App Impersonation attack

Step 1: The Evil App generates an Auth Request. The Auth Request contains the client_id and redirect_uri of the Honest App. Thus, the User and the Authorization Server cannot recognize that the Evil App initiates this request. 

Step 2-4: These steps do not deviate from the previous description in Figure 2.

Step 5: In Step 5 the Evil App sends the code_verifier used for the computation of the code_challenge. Thus, the stolen code can be successfully redeemed and the Evil App receives the access_token and id_token.

OAuth 2.0 for Native Apps

The attack cannot be prevented by PKCE. However, the IETF working group is currently working on a Draft describing recommendations for using OAuth 2.0 for native apps.

References

[1] http://mews.sv.cmu.edu/papers/ccs-14.pdf

[2] https://tools.ietf.org/html/draft-ietf-oauth-native-apps-06#section-8.1

[3] https://developers.googleblog.com/2016/08/modernizing-oauth-interactions-in-native-apps.html

Authors

Vladislav Mladenov
Christian Mainka (@CheariX)

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Top 15 Best Operating System Professional Hackers Use

Top 15 Best Operating System Professional Hackers Use

Top 15 Best Operating System Professional Hackers Use

A hacker is someone who seeks and exploits the weaknesses of a computer system or network computing. Hackers may be motivated by a multitude of reasons, such as profit, protest, challenge, enjoyment or to assess these weaknesses to help in removing them.

The listed operating systems are based on the Linux kernel so it is all free operating systems.

1. Kali Linux

Kali Linux maintained and funded by Offensive Security Ltd. and it is first on our list. Kali Linux is a Debian-derived Linux distribution designed for digital forensics and penetration testing. It was developed by Mati Aharoni and Devon Kearns of Offensive Security through rewriting BackTrack, its previous forensics Linux distribution based on Ubuntu. Kali Linux has a specific project for the withdrawal of compatibility and portability of Android-specific devices, called Kali Linux NetHunter. It is the first open test platform penetration Source for Nexus Android devices, created as a joint effort between the member of the Kali "BinkyBear" Security and offensive community. It supports Wireless 802.11 frame injection, one-click configurations MANA Evil access point, keyboard HID (Teensy as attacks), as well as attacks MITM USB Mala.

2. Back Box

Back Box is an evaluation penetration testing Linux distribution and Ubuntu-based security aimed at providing an analysis of computer network systems and toolkit. Desktop environment back box includes a complete set of tools needed for ethical hacking and security testing.

3. Parrot Security OS

Parrot Security OS is a GNU / Linux distribution based on Debian. Fue built in order to perform penetration tests (safety information), vulnerability assessment and mitigation, Computer Forensics and Anonymous Surfing. Ha been developed by the team of Frozen box.

Parrot is based on the stable branch (Jessie) of Debian, a Linux 4.1 kernel hardened customized with a branch grsecurity patched available. The desktop environment is MATE fork of Gnome 2, and the default display manager is LightDM. The project is certified to run on machines with 265MB of RAM minimum follow and it is suitable for both 32-bit (i386) and 64-bit (amd64), with a special edition that works on 32-bit machines of age (486). Moreover, the project is available for Armel and armhf architectures. It even offers an edition (both 32 bit and 64 bit) developed for servers only for pen testing cloud.

4. Live Hacking OS

Live Hacking OS is a Linux distribution packed with tools and utilities for ethical hacking, penetration testing, and countermeasure verification. It includes embedded GUI GNOME user. There is a second variation available which has only the command line and requires much fewer hardware requirements.

5. DEFT Linux

DEFT stands for Digital Evidence and Forensic Toolkit and it is a distribution of Linux open source software built around the DART (Toolkit for Advanced Response Digital) and is based on the Ubuntu operating system. It has been designed from scratch to offer some of the best computer forensics open source and incident response tools that can be used by individuals, IT auditors, investigators, military, and police.

6. Samurai Web Testing Framework

The Samurai Web Testing Framework is a live Linux environment which has been pre-configured to function as a web pen-testing environment. The CD contains the best of open source and free tools that focus on testing and websites that attack. In the development of this environment, it is based on our selection of tools that we use in our practice of security. Hence, it includes the tools that were used in the four steps of a pen-test web.

7. Network Security Toolkit

The Network Security Toolkit (NST) is a Live CD based on Linux that provides a set of security tools computing and open source network to carry out routine security tasks and diagnostic networks and tracing. The distribution can be used as an analysis of network security, validation and monitoring tool for servers hosting virtual machines. NST has management capabilities similar to Fedora packages and maintains its own repository of additional packages.

8. Bugtraq

Bugtraq is a mailing list dedicated to safety issues in computers. On-topic issues new discussions about vulnerabilities, security-related notices providers, operating methods, and how to fix them. This is a mailing list of large volume, and almost all new vulnerabilities are there. Bugtraq computer freaks and experienced developers are discussed, is available in Debian, Ubuntu and openSUSE 32 and 64-bit architectures.

9. NodeZero

NodeZero is an open source system based on the operating core derived from the most popular Linux distribution in the world, Ubuntu, and designed to be used for penetration testing operations. The distribution can be downloaded as an ISO image live DVD, which will also take place on computers that support both 32-bit (x86) and 64-bit (x86_64) instruction set. Besides the fact that it allows you to start the live system, start menu contains several advanced features such as the ability to perform a diagnostic test of system memory, boot from local disk options, start the installer directly and to start in safe graphics mode, text mode or in debug mode.

Default graphical desktop environment NodeZero is powered by GNOME, which uses the classic GNOME interface. It has a design of two panels and uses the default software repositories of Ubuntu.

10. Pentoo

Pentoo is a Live CD and Live USB OS designed for penetration testing and security assessment. It is based on Gentoo Linux, Pentoo is offered both as 32-bit and 64-bit live cd which is installable. Pentoo is also available as a superposition of an existing Gentoo installation. It has conductors packet injection patched wifi, GPGPU cracking software, and plenty of tools for penetration testing and security assessment. The kernel includes Pentoo grsecurity and PAX hardening and additional patches with the binary compiled from a string of hardened with the latest nightly versions of some tools that are available.

#11 Live Hacking OS

Well, this Linux distro actually comes with some useful hacking tools which are often used in penetration testing or ethical hacking purpose. Live Hacking OS consists of the GNOME inbuilt. The operating system is really easy to operate and it can work on less RAM.

#12 Knoppix STD

This is another best Linux distro which focuses on tools for computer security. Knoppix STD brings some advanced tools for Password cracking, Firewalls, Network Utilities, Honeypots, Wireless Networking and more. This is one of the most used operating systems for Hackers.

#13 Cyborg Hawk

Cyborg Hawk is a new operating system which is based on Ubuntu Linux. Well, lots of hackers talk about Cyborg hawk and its one of the most powerful and cutting-edge penetration testing distribution that has ever been created. The operating system houses more than 700 tools for different purposes.

#14 Blackbuntu

Well, this is another operating system which is based on Linux and it was specially developed for penetration testing. Well, the operating system is very famous amongst hackers and it offers an awesome platform to learn Information security.

#15 Weakerth4n

Well, this is another best operating system which is used by professional hackers. WeakerTh4n actually comes with lots of hacking tools and it's actually a modern operating system for WiFi Hacking. Some of the wireless tools include SQL Hacking, Password Cracking, WiFi attacks, Cisco exploitation and more.

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