Automating REST Security Part 3: Practical Tests for Real-World APIs

If you have read our two previous blogposts, you should now have a good grasp on the structural components used in REST APIs and where there are automation potentials for security analysis. You've also learned about REST-Attacker, the analysis tool we implemented as a framework for automated analysis.

In our final blogpost, we will dive deeper into practical testing by looking at some of the automated analysis tests implemented in REST-Attacker. Particularly, we will focus on three test categories that are well-suited for automation. Additionally, we will look at test results we acquired, when we ran these tests on the real-world API implementation of the services GitHub, Gitlab, Microsoft, Spotify, YouTube, and Zoom.

Author

Christoph Heine

Overview

• Part 1: Challenges in Analyzing REST Security
• Part 2: Tool-based REST Analysis with REST-Attacker
• Part 3: Practical Tests for Real-World APIs

Automating REST Security Part 2: Tool-based Analysis with REST-Attacker

Our previous blog post described the challenges in analyzing REST API implementations. Despite the lack of REST standardization, we learned that similarities between implementations exist and that we can utilize them for tool-based REST security analysis.

This blog post will now look at our own implementation. REST-Attacker is a free software analysis tool specifically built to analyze REST API implementations and their access control measures. Using REST-Attacker as an example, this blog post will discuss how a REST security tool can work and where it can improve or streamline the testing process, especially in terms of automation.

Author

Christoph Heine

Overview

• Automating REST Security Part 1: Challenges
• Automating REST Security Part 2: Tool-based Analysis with REST-Attacker
• Automating REST Security Part 3: Practical Tests for Real-World APIs 

Automating REST Security Part 1: Challenges

Although REST has been a dominant choice for API design for the last decade, there is still little dedicated security research on the subject of REST APIs. The popularity of REST contrasts with a surprisingly small number of systematic approaches to REST security analysis. This contrast is also reflected in the low availability of analysis tools and best security practices that services may use to check if their API is secure.

In this blog series, we try to find reasons for this situation and what we can do about it. In particular, we will investigate why general REST security assessments seem more complicated than other API architectures. We will likewise discuss how we may still find systematic approaches for REST API analysis despite REST's challenges. Furthermore, we will present REST-Attacker, a novel analysis tool designed for automated REST API security testing. In this context, we will examine some of the practical tests provided by REST-Attacker and explore the test results for a small selection of real-world API implementations.

Author

Christoph Heine

Overview

• Automating REST Security Part 1: Challenges
• Automating REST Security Part 2: Tool-based Analysis with REST-Attacker
• Automating REST Security Part 3: Practical Tests for Real-World APIs 

Shadow Attacks … the smallest attack vector ever

In July 2020, we introduced a novel attack class called Shadow Attacks. In our recent research, we discovered a new variant of the attack which relies only on an Incremental Update containing a malicious trailer.
A proof-of-concept exploit working on Foxit (Version: 11.0.1.49938) can be downloaded here.

ALPACA: Application Layer Protocol Confusion-Analyzing and Mitigating Cracks in TLS Authentication

In cooperation with the university Paderborn and Münster University of Applied Sciences, we discovered a new flaw in the specification of TLS. The vulnerability is called ALPACA and exploits a weakness in the authentication of TLS for cross-protocol attacks. The attack allows an attacker to steal cookies or perform cross-site-scripting (XSS) if the specific conditions for the attack are met.

Security Analysis in an OpenID Connect Lab Environment

In this post, Christian Fries shows an approach to unveil security flaws in OpenID Connect Certified implementations with well-known attack methods. One goal of the master's thesis Security Analysis of Real-Life OpenID Connect Implementations was to provide a platform for developers and security researchers to test implementations in a reproducible and maintainable OIDC lab environment.

We included six OpenID Provider (OP) and eight Relying Party (RP) services in the lab environment. For the comprehensive security analysis, we tested the implementations against eleven Relying Party attacks and seven OpenID Provider attacks in different variations with our tool PrOfESSOS. In addition, we carried out manual tests as well. We have disclosed twelve implementation flaws and reported them to the developers in a responsible disclosure process.

Two developer teams fixed (✔) the vulnerabilities before the deadline of the master's thesis. One Redirect URI Manipulation vulnerability was rejected (✖). This particular case can be permissible for only one registered URI for reasons of interoperability and fault tolerance. We informed three further development teams (✦).

Name	Vulnerability	Fixed	CVE
MITREid Connect	PKCE Downgrade Attack	✦
mod auth openidc	ID Spoofing, JWKS Spoofing	✔
node oidc-provider	Redirect URI Manipulation	✖
OidcRP	Replay Attack	✦
phpOIDC	Message Flow Confusion, ID Spoofing, Key Confusion	✦
pyoidc	Replay Attack, Signature Manipulation, Token Recipient Confusion	✔	CVE-2020-26244

We explain the method of how we have archived this result in the following sections.

Attacks on PDF Certification

In recent years, we have presented How to Spoof PDF Signatures and Shadow Attacks: Hiding and Replacing Content in Signed PDFs, which describe attacks on PDF signatures under various attack scenarios. The attacks focused on so-called approval signatures. However, in addition to signing PDFs, the PDF specification also specifies the certification of documents, also known as certification signatures.

To close this research gap, we performed an extensive analysis of the security of PDF certification. In doing so, we developed the Evil Annotation Attack (EAA), as well as the Sneaky Signature Attack (SSA). The attack idea exploits the flexibility of PDF certification, which allows signing or adding annotations to certified documents under different permission levels. Our practical evaluation shows that an attacker could change the visible content in 15 of 26 viewer applications by using EAA and in 8 applications using SSA by using PDF specification compliant exploits. We improved both attacks’ stealthiness with applications’ implementation issues and found only two applications secure to all attacks.

We responsibly disclosed all affected vendors. Together with the CERT-Bund (BSI), we supported the vendors in developing suitable countermeasures. The attacks are documented in CVE-2020-35931, CVE-2021-28545 and CVE-2021-28546.

Full results are available in our paper, vulnerability report and on our website.

Security and Privacy of Social Logins (III): Privacy in Single Sign-On Protocols

 This post is the second out of three blog posts summarizing my (Louis Jannett) research on the design, security, and privacy of real-world Single Sign-On (SSO) implementations. It is based on my master's thesis that I wrote between April and October 2020 at the Chair for Network and Data Security.

We structured this blog post series into three parts according to the research questions of my master's thesis: Single Sign-On Protocols in the Wild, PostMessage Security in Single Sign-On, and Privacy in Single Sign-On Protocols.

Overview

Part I: Single Sign-On Protocols in the Wild

Although previous work uncovered various security flaws in SSO, it did not work out uniform protocol descriptions of real-world SSO implementations. We summarize our in-depth analyses of Apple, Google, and Facebook SSO. We also refer to the sections of the thesis that provide more detailed insights into the protocol flows and messages.

Part II: PostMessage Security in Single Sign-On

It turned out that the postMessage API is commonly used in real-world SSO implementations. We introduce the reasons for this and propose security best practices on how to implement postMessage in SSO. Further, we present vulnerabilities on top-visited websites that caused DOM-based XSS and account takeovers due to insecure use of postMessage in SSO.

Part III: Privacy in Single Sign-On Protocols

Identity Providers (IdPs) use "zero-click" authentication flows to automatically sign in the user on the Service Provider (SP) once it is logged in on the IdP and has consented. We show that these flows can harm user privacy and enable new targeted deanonymization attacks of the user's identity.

• Introduction to XS-Leaks in Single Sign-On
• XS-Leaks in Single Sign-On: Account Leakage Attack
• XS-Leaks in Single Sign-On: Identity Leakage Attack
• Automatic Sign-In and Session Management Practices in the Wild

Security and Privacy of Social Logins (II): PostMessage Security in Single Sign-On

This post is the second out of three blog posts summarizing my (Louis Jannett) research on the design, security, and privacy of real-world Single Sign-On (SSO) implementations. It is based on my master's thesis that I wrote between April and October 2020 at the Chair for Network and Data Security.

We structured this blog post series into three parts according to the research questions of my master's thesis: Single Sign-On Protocols in the Wild, PostMessage Security in Single Sign-On, and Privacy in Single Sign-On Protocols.

Overview

Part I: Single Sign-On Protocols in the Wild

Although previous work uncovered various security flaws in SSO, it did not work out uniform protocol descriptions of real-world SSO implementations. We summarize our in-depth analyses of Apple, Google, and Facebook SSO. We also refer to the sections of the thesis that provide more detailed insights into the protocol flows and messages.

Part II: PostMessage Security in Single Sign-On

It turned out that the postMessage API is commonly used in real-world SSO implementations. We introduce the reasons for this and propose security best practices on how to implement postMessage in SSO. Further, we present vulnerabilities on top-visited websites that caused DOM-based XSS and account takeovers due to insecure use of postMessage in SSO.

• Vuln. 1) DOM-based XSS on myaccount.nytimes.com
• Vuln. 2) Account Takeover on cbsnews.com, cnet.com, and zdnet.com
• Vuln. 3) Account Takeover in SAP Customer Data Cloud (GIGYA)

Part III: Privacy in Single Sign-On Protocols (coming soon)

Identity Providers (IdPs) use "zero-click" authentication flows to automatically sign in the user on the Service Provider (SP) once it is logged in on the IdP and has consented. We show that these flows can harm user privacy and enable new targeted deanonymization attacks of the user's identity.

Security and Privacy of Social Logins (I): Single Sign-On Protocols in the Wild

This post is the first out of three blog posts summarizing my (Louis Jannett) research on the design, security, and privacy of real-world Single Sign-On (SSO) implementations. It is based on my master's thesis that I wrote between April and October 2020 at the Chair for Network and Data Security.

We structured this blog post series into three parts according to the research questions of my master's thesis: Single Sign-On Protocols in the Wild, PostMessage Security in Single Sign-On, and Privacy in Single Sign-On Protocols.

Overview

Part I: Single Sign-On Protocols in the Wild

Although previous work uncovered various security flaws in SSO, it did not work out uniform protocol descriptions of real-world SSO implementations. We summarize our in-depth analyses of Apple, Google, and Facebook SSO. We also refer to the sections of the thesis that provide more detailed insights into the protocol flows and messages.

• Identity Provider: Apple
• Identity Provider: Google
• Identity Provider: Facebook

Part II: PostMessage Security in Single Sign-On

It turned out that the postMessage API is commonly used in real-world SSO implementations. We introduce the reasons for this and propose security best practices on how to implement postMessage in SSO. Further, we present vulnerabilities on top-visited websites that caused DOM-based XSS and account takeovers due to insecure use of postMessage in SSO.

Part III: Privacy in Single Sign-On Protocols (coming soon)

Identity Providers (IdPs) use "zero-click" authentication flows to automatically sign in the user on the Service Provider (SP) once it is logged in on the IdP and has consented. We show that these flows can harm user privacy and enable new targeted deanonymization attacks of the user's identity.

Insecure Features in PDFs

In 2019, we published attacks on PDF Signatures and PDF Encryption. During our research and studying the related work, we discovered a lot of blog posts, talks, and papers focusing on malicious PDFs causing some damage. However, there was no systematic analysis of all possible dangerous features supported by PDFs, but only isolated exploits and attack concepts.

We decided to fill this gap and systematize the possibilities to use legitimate PDF features and do bad stuff. We define four attack categories: Denial of Service, Information Disclosure, Data Manipulation, and Code Execution.

Our evaluation reveals 26 of 28 popular PDF processing applications are vulnerable to at least one attack. You can download all malicious PDFs here. You can also find more technical details in our NDSS'21 paper.

This is a joined work of Jens Müller, Dominik Noss, Christian Mainka, Vladislav Mladenov, and Jörg Schwenk.

Single Sign-On Security: Security Analysis of real-life OpenID Connect Implementations

This is a guest blogpost by Lauritz Holtmann. He wrote his master thesis:

"Single Sign-On Security: Security Analysis of real-life OpenID Connect Implementations"

Lauritz summarizes his exciting results in the following. The thesis was supervised by Vladislav Mladenov, Christian Mainka, and Jörg Schwenk. You can read find his full thesis here.

OpenID Connect 1.0 and OAuth 2.0 are the Single Sign-On Protocols that are implemented in modern web applications. In this post, we outline common issue patterns that were discovered in popular OpenID Connect implementations, give concrete examples of vulnerabilities, and give recommendations for adjustments to the OpenID Connect specification.

Shadow Attacks: Hiding and Replacing Content in Signed PDFs

Last year we presented How to Spoof PDF Signatures. We showed three different attack classes. In cooperation with the CERT-Bund (BSI), we contacted the vendors of affected PDF applications to inform them about the vulnerabilities and to support them in developing countermeasures. Most vendors reacted promptly and closed the reported vulnerabilities promptly.

One of those three attack classes was Incremental Saving Attacks (ISA). The proposed countermeasures aimed to distinguish PDF objects appended to the file via updates into dangerous and non-dangerous. In other words, black and whitelisting approaches were used. 

Naturally, this countermeasure succeeds as long as the PDF update contains evil objects. So we came up with the idea to attack PDFs with only non-dangerous updates. We achieve this by adding invisible, malicious content when creating the PDF document (before it is signed) and to reveal them after signing.

Today, we present Shadow Attacks! Our evaluation of 28 PDF applications reveals that 15 of them, including Adobe Acrobat and Foxit Reader, are vulnerable.

We responsibly disclosed all affected vendors. Together with the CERT-Bund (BSI), we supported the vendors in developing suitable countermeasures. The attacks are documented in CVE-2020-9592 and CVE-2020-9596.

Full results are available in our vulnerability report and on our website.

CVE-2020-2655 JSSE Client Authentication Bypass

During our joint research on DTLS state machines, we discovered a really interesting vulnerability (CVE-2020-2655) in the recent versions of Sun JSSE (Java 11, 13). Interestingly, the vulnerability does not only affect DTLS implementations but does also affects the TLS implementation of JSSE in a similar way. The vulnerability allows an attacker to completely bypass client authentication and to authenticate as any user for which it knows the certificate WITHOUT needing to know the private key. If you just want the PoC's, feel free to skip the intro.

CTF: FluxFingers4Future - Evil Corp Solution

For this years hack.lu CTF I felt like creating a challenge. Since I work a lot with TLS it was only natural for me to create a TLS challenge. I was informed that TLS challenges are quite uncommon but nevertheless I thought it would be nice to spice the competition up with something "unusual". The challenge mostly requires you to know a lot of details on how the TLS record layer and the key derivation works. The challenge was only solved by one team (0ops from China) during the CTF. Good job!

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.

Testing SAML Endpoints for XML Signature Wrapping Vulnerabilities

A lot can go wrong when validating SAML messages. When auditing SAML endpoints, it's important to look out for vulnerabilities in the signature validation logic. XML Signature Wrapping (XSW) against SAML is an attack where manipulated SAML message is submitted in an attempt to make the endpoint validate the signed parts of the message -- which were correctly validated -- while processing a different attacker-generated part of the message as a way to extract the authentication statements. Because the attacker can arbitrarily forge SAML assertions which are accepted as valid by the vulnerable endpoint, the impact can be severe. [1,2,3]

Testing for XSW vulnerabilities in SAML endpoints can be a tedious process, as the auditor needs to not only know the details of the various XSW techniques, but also must handle a multitude of repetitive copy-and-paste tasks and apply the appropriate encoding onto each message. The latest revision of the XSW-Attacker module in our BurpSuite extension EsPReSSo helps to make this testing process easier, and even comes with a semi-automated mode. Read on to learn more about the new release! 

Probing for XML Encryption Weaknesses in SAML with EsPReSSO

Security Assertion Markup Language (SAML) is an XML-based standard commonly used in Web Single Sign-On (SSO) [1]. In SAML, the confidentiality of transferred authentication statements against intermediaries can be provided using XML Encryption [2]. However, implementing XML Encryption in a secure way can be tricky and several attacks on XML Encryption have been identified in the past [3] [4]. Therefore, when auditing a SAML endpoint, one should always consider testing for vulnerabilities in the XML Encryption implementation.

This blog post introduces our latest addition to the SAML Attacker of our BurpSuite extension EsPReSSO: the Encryption Attack tab. The new tab allows for easy manipulation of the encrypted parts within intercepted SAML responses and can, therefore, be used to quickly assess whether the SAML endpoint is vulnerable against certain XML Encryption attacks.

Scanning TLS Server Configurations with Burp Suite

In this post, we present our new Burp Suite extension "TLS-Attacker".
Using this extension penetration testers and security researchers can assess the security of TLS server configurations directly from within Burp Suite.
The extension is based on the TLS-Attacker framework and the TLS-Scanner, both of which are developed by the Chair for Network and Data Security.

You can find the latest release of our extension at: https://github.com/RUB-NDS/TLS-Attacker-BurpExtension/releases

Why Receipt Notifications increase Security in Signal

This blog post is aimed to express and explain my surprise about Signal being more secure than I thought (due to receipt acknowledgments). I hope you find it interesting, too.

Signal, and especially its state update protocol, the Double Ratchet algorithm, are widely known for significantly increasing security for instant messaging. While most users first see the end-to-end security induced by employing Signal in messaging apps, the properties achieved due to ratcheting go far beyond protecting communication against (active) attackers on the wire. Due to updating the local device secrets via the Double Ratchet algorithm, the protocol ensures that attackers, who temporarily obtain a device's local storage (on which Signal runs), only compromise confidentiality of parts of the communications with this device. Thus, the leakage of local secrets from a device only affects security of a short frame of communication. The exact duration of compromise depends on the messaging pattern among the communicating parties (i.e., who sends and receives when), as the state update is conducted during the sending and receiving of payload messages.