This post is about padding oracle vulnerabilities and the tool for attacking them - “Poracle” I’m officially releasing right now. You can grab the Poracle tool on Github!
At my previous job — Tenable Network Security — one of the first tasks I ever had was to write a vulnerability check for MS10-070 — a padding oracle vulnerability in ASP.net. It’s an interesting use of a padding oracle vulnerability, since it leads to code execution, but this blog is going to be a more general overview of padding oracles. When I needed to test this vuln, I couldn’t find a good writeup on how they work. The descriptions I did find were very technical and academic, which I’m no good at. In fact, when it comes to reading academic papers, I’m clueless and easily frightened. But, I struggled through them, and now I’m gonna give you a writeup that even I’d be able to understand!
By the way, the Wikipedia page for this attack isn’t very good. If somebody wants to summarize my blog and make it into a Wikipedia page, there’s now a source you can reference. :)
On a related note, I’m gonna be speaking at Shmoocon in February: “Crypto: You’re doing it wrong”. Among other things, I plan to talk about padding oracles and hash extension attacks — I’m really getting into this crypto thing!
Everything you need to know about hash length extension attacks
You can grab the hash_extender tool on Github!
(Administrative note: I'm no longer at Tenable! I left on good terms, and now I'm a consultant at Leviathan Security Group. Feel free to contact me if you need more information!)
Awhile back, my friend @mogigoma and I were doing a capture-the-flag contest at https://stripe-ctf.com. One of the levels of the contest required us to perform a hash length extension attack. I had never even heard of the attack at the time, and after some reading I realized that not only is it a super cool (and conceptually easy!) attack to perform, there is also a total lack of good tools for performing said attack! After hours of adding the wrong number of null bytes or incorrectly adding length values, I vowed to write a tool to make this easy for myself and anybody else who's trying to do it. So, after a couple weeks of work, here it is!