There has been a lot of discussion recently around the LastPass breach, especially with regards to the number of PBKDF2 iterations applied to the master password to derive the vault encryption key. Other people have already dissected this particular breach, but I want to more generally talk about PBKDF2 iterations and security models. (I’m not going to talk about Argon2 or Bcrypt or any other algorithms).Continue reading “On PBKDF2 iterations”
Just a few quick notes/updates to correct some potentially inaccurate statements that are floating around on Reddit/Twitter etc:
- The bug only impacts Java 15 and above. The original advisory from Oracle incorrectly listed earlier versions (like 7, 8 and 11) as being impacted. They have since corrected this. Note that they now only list 17 and 18, because 15 and 16 are no longer supported.
- Bouncy Castle is not impacted by this vulnerability. They have their own ECDSA implementation, and it performs the relevant check to prevent this bug.
- Although an all-zero signature value is the simplest way to exploit this, there are several alternative values that exhibit the same bug. As previously mentioned, Project Wycheproof has an excellent selection of test vectors for this bug and many variations on it, in different signature formats, and for different elliptic curves.
- On a related note, some JWT libraries were initially assumed to be unaffected because a quirk of re-encoding raw (IEEE P1363) format signatures into ASN.1 format rejected zero values. But, as pointed out above, there are other invalid values that are not rejected by this conversion that still trigger the bug. Either upgrade your JVM, or your JWT library, and ideally both.
- Some JWT libraries also apparently accept signature values in several alternative encodings, so if you are checking for bad signatures in a pre-processing step then you have even more values to check. Again, best to update to get the patches rather than trying to fix this yourself.
The long-running BBC sci-fi show Doctor Who has a recurring plot device where the Doctor manages to get out of trouble by showing an identity card which is actually completely blank. Of course, this being Doctor Who, the card is really made out of a special “psychic paper“, which causes the person looking at it to see whatever the Doctor wants them to see: a security pass, a warrant, or whatever.
It turns out that some recent releases of Java were vulnerable to a similar kind of trick, in the implementation of widely-used ECDSA signatures. If you are running one of the vulnerable versions then an attacker can easily forge some types of SSL certificates and handshakes (allowing interception and modification of communications), signed JWTs, SAML assertions or OIDC id tokens, and even WebAuthn authentication messages. All using the digital equivalent of a blank piece of paper.
It’s hard to overstate the severity of this bug. If you are using ECDSA signatures for any of these security mechanisms, then an attacker can trivially and completely bypass them if your server is running any Java 15, 16, 17, or 18 version before the April 2022 Critical Patch Update (CPU). For context, almost all WebAuthn/FIDO devices in the real world (including Yubikeys*) use ECDSA signatures and many OIDC providers use ECDSA-signed JWTs.
If you have deployed Java 15, Java 16, Java 17, or Java 18 in production then you should stop what you are doing and immediately update to install the fixes in the April 2022 Critical Patch Update.
the official announcement from Oracle also lists older versions of Java, including 7, 8 and 11. Although I’m not aware of the bug impacting those older implementations they did fix a similar bug in the (non-EC) DSA implementation at the same time, so it’s possible older versions are also impacted. There are also other security vulnerabilities reported in the same CPU, so (as always) it is worth upgrading even if you are running an older Java version. The OpenJDK advisory on the other hand lists only versions 15, 17, and 18 as affected by this specific issue (CVE-2022-21449).
Update 2: Oracle have informed me they are in the process of correcting the advisory to state that only versions 15-18 are impacted. The CVE has already been updated. Note that 15 and 16 are no longer supported, so it will only list 17 and 18 as impacted.Continue reading “CVE-2022-21449: Psychic Signatures in Java”
Datalog is a logic programming language, based on Prolog, which is seeing something of a resurgence in interest in recent years. In particular, several recent approaches to authorization (working out who can do what) have used Datalog as the logical basis for access control decisions. On the face of it, this seems like a perfect fit, and there’s a lot to recommend it. I myself have been a fan of Datalog since first coming across it at the start of my PhD studies back in 2003, and have even written papers advocating for it. However, although I think it has a lot of benefits, I think there is some confusion about some of its complexity results that means it is not always as good a fit as you may be led to believe.Continue reading “Is Datalog a good language for authorization?”
I was catching up on the always excellent Security. Cryptography. Whatever. podcast, and enjoyed the episode with Colm MacCárthaigh about a bunch of topics around TLS. It’s a great episode that touches a lot of subjects I’m interested in, so go ahead and listen to it if you haven’t already, and definitely subscribe. I want to pick up on one of the topics in the podcast in this article, and discuss part of the OAuth specs that I think deserves to be better known.Continue reading “Why the OAuth mTLS spec is more interesting than you might think”
In “Towards a standard for bearer token URLs”, I described a URL scheme that can be safely used to incorporate a bearer token (such as an OAuth access token) into a URL. That blog post concentrated on the technical details of how that would work and the security properties of the scheme. But as Tim Dierks commented on Twitter, it’s not necessarily obvious to people how you’d actually use this in practice. Who creates these URLs? How are they used and shared? In this follow-up post I’ll attempt to answer that question with a few examples of how bearer URLs could be used in practice.Continue reading “How do you use a bearer URL?”
In my previous post, I described the KEM/DEM paradigm for hybrid encryption. The key encapsulation mechanism is given the recipient’s public key and outputs a fresh AES key and an encapsulation of that key that the recipient can decapsulate to recover the AES key. In this post I want to talk about several ways that the KEM interface falls short and what to do about it:
- As I’ve discussed before, the standard definition of public key encryption lacks any authentication of the sender, and the KEM-DEM paradigm is no exception. You often want to have some idea of where a message came from before you process it, so how can we add sender authentication?
- If you want to send the same message to multiple recipients, a natural approach would be to encrypt the message once with a fresh AES key and then encrypt the AES key for each recipient. With the KEM approach though we’ll end up with a separate AES key for each recipient. How can we send the same message to multiple recipients without encrypting the whole thing separately for each one?
- Finally, the definition of public key encryption used in the KEM/DEM paradigm doesn’t provide forward secrecy. If an attacker ever compromises the recipient’s long-term private key, they can decrypt every message ever sent to that recipient. Can we prevent this?
In this article I’ll tackle the first two issues and show how the KEM/DEM abstractions can be adjusted to cope with each. In a follow-up post I’ll then show how to tackle forward secrecy, along with replay attacks and other issues. Warning: this post is longer and has more technical details than the previous post. It’s really meant for people who already have some experience with cryptographic algorithms.Continue reading “When a KEM is not enough”
There’s a persistent belief among web security people that cross-site scripting (XSS) is a “game over” event for defence: there is no effective way to recover if an attacker can inject code into your site. Brian Campbell refers to this as “XSS Nihilism”, which is a great description. But is this bleak assessment actually true? For the most part yes, but in this post I want to talk about a faint glimmer on the horizon that might just be a ray of sunshine after all.Continue reading “XSS doesn’t have to be game over”
I wasn’t expecting it so quickly, so it caught me a little off guard, but API Security in Action is now finally published. PDF copies are available now, with printed copies shipping by the end of the month. Kindle/ePub take a little bit longer but should be out in a few weeks time.
My own print copies will take a few weeks to ship to the UK, and I can’t wait to finally hold it in my hands. That’s a brighter ending to 2020.
At some point I’ll try and collect some thoughts about the process of writing it and my feelings with the finished product. But tonight I’ll settle for a glass (or two) of a nice red. Cheers!