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.

Hacking PayPal's Express Checkout

Do you know what is happening in the background when you buy something in an online shop using PayPal?

In this post we will tackle the following problems:

• How can PayPal's API be tested?
• How does PayPal's Express Checkout work? You can find the detailed report here.
• How can we debit more money than authorized?

How to Break Microsoft Rights Management Services

In this post, we provide a security analysis of Microsoft Rights Management Services (RMS) and present two working attacks: 

1. We completely remove the RMS protection of a Word document on which we only have a view-only permission, without having the right to edit it. This shows that in contrast to claims made by Microsoft, Microsoft RMS can only be used to enforce all-or-nothing access. 
2. We extend this attack to be stealthy in the following sense: We show how to modify the content of an RMS write-protected Word document issued by our victim. The resulting document still claims to be write protected, and that the modified content was generated by the victim

This work is going to be presented at WOOT'16.

Curious Padding oracle in OpenSSL (CVE-2016-2107)

Today, a new OpenSSL security advisory came out and it patched my recent finding, Padding oracle in AES-NI CBC MAC check (CVE-2016-2107).

In this post, I will give some background on this attack and how I found it. Before reading the whole post, note that this vulnerability is very hard to exploit (even if it is given the high severity score). Also note that it is not a new general padding oracle attack with a new logo. It is just an oracle coming from an invalid check of decrypted message content, specifically introduced in OpenSSL (ok, if you really want to have a name for it, call it UnluckyHMAC ...because our HMAC is sad not to be able to validate bytes :) ).

XML Parser Evaluation

XML Parser Evaluation

For some time now, we've been researching in excruciating detail the prevalence of DTD attacks on different XML parsers.

For a quick recap which attacks are possible, see our DTD Cheat Sheet post.


In this post, we present you the results in a nutshell.
The information presented here is based on this masterthesis which covers the respective results in greater detail.

Test Methodology

We identified 16 test vectors, each testing a specific attack vector (e.g. XXE, various kinds of DoS, XXE parameter entity,...). We ran these tests against the default parser configuration and call these therefore core tests.

Additional tests are based on the same test vectors, however, we executed them against custom (modified) parser configurations, indicating the effect of specific features of a parser.

The complete test set is available on github.

Results

We analyzed the following parsers and summarized the test results in Table 1. In addition, we show which attacks cannot be mitigated indicated by an asterisk.

DTD Cheat Sheet

When evaluating the security of XML based services, one should always consider DTD based attack vectors, such as XML External Entities (XXE) as,for example, our previous post XXE in SAML Interfaces demonstrates.

In this post we provide a comprehensive list of different DTD attacks.

The attacks are categorized as follows:

• Denial-of-Service Attacks
• Classic XXE
• Advanced XXE
• Server-Side Requst Forgery (SSRF)
• XInclude
• XSLT

RuhrSec - our non-profit IT security conference

We are proud to announce our first security conference - RuhrSec. The conference takes place in Bochum at our university (28.-29.4.2016). It is a non-profit conference, i.e. all profit resulting from the sold tickets will go to Gänseblümchen NRW e.V. (thanks to our sponsors and to the great university conditions, we hope it will be much :) ). As this is our first conference, we carefully invited some top-class speakers (mainly our friends) to present their recent work. Given the program that we have now, I think we do not have to shame and we can keep up with the best conferences.