tl;dr
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A carefully crafted attack request can cause the contents of the HTTP parser’s buffer to be appended to the attacking request’s header, making it appear to come from the attacker. Since it is generally safe to echo back contents of a request, this can allow an attacker to get an otherwise correctly designed server to divulge information about other requests. It is theoretically possible that it could enable header-spoofing attacks, though such an attack has not been demonstrated.
- Versions affected: All versions of the 0.5/0.6 branch prior to 0.6.17, and all versions of the 0.7 branch prior to 0.7.8. Versions in the 0.4 branch are not affected.
- Fix: Upgrade to v0.6.17, or apply the fix in c9a231d to your system.
Details
A few weeks ago, Matthew Daley found a security vulnerability in Node's HTTP implementation, and thankfully did the responsible thing and reported it to us via email. He explained it quite well, so I’ll quote him here:
There is a vulnerability in node's http_parser binding which allows information disclosure to a remote attacker:
In node::StringPtr::Update, an attempt is made at an optimization on certain inputs (node_http_parser.cc, line 151). The intent is that if the current string pointer plus the current string size is equal to the incoming string pointer, the current string size is just increased to match, as the incoming string lies just beyond the current string pointer. However, the check to see whether or not this can be done is incorrect; "size" is used whereas "size_" should be used. Therefore, an attacker can call Update with a string of certain length and cause the current string to have other data appended to it. In the case of HTTP being parsed out of incoming socket data, this can be incoming data from other sockets.
Normally node::StringPtr::Save, which is called after each execution of http_parser, would stop this from being exploitable as it converts strings to non-optimizable heap-based strings. However, this is not done to 0-length strings. An attacker can therefore exploit the mistake by making Update set a 0-length string, and then Update past its boundary, so long as it is done in one http_parser execution. This can be done with an HTTP header with empty value, followed by a continuation with a value of certain length.
The attached files demonstrate the issue:
$ ./node ~/stringptr-update-poc-server.js & [1] 11801 $ ~/stringptr-update-poc-client.py HTTP/1.1 200 OK Content-Type: text/plain Date: Wed, 18 Apr 2012 00:05:11 GMT Connection: close Transfer-Encoding: chunked 64 X header: This is private data, perhaps an HTTP request with a Cookie in it. 0
The fix landed on 7b3fb22 and c9a231d, for master and v0.6, respectively. The innocuous commit message does not give away the security implications, precisely because we wanted to get a fix out before making a big deal about it.
The first releases with the fix are v0.7.8 and 0.6.17. So now is a good time to make a big deal about it.
If you are using node version 0.6 in production, please upgrade to at least v0.6.17, or at least apply the fix in c9a231d to your system. (Version 0.6.17 also fixes some other important bugs, and is without doubt the most stable release of Node 0.6 to date, so it's a good idea to upgrade anyway.)
I'm extremely grateful that Matthew took the time to report the problem to us with such an elegant explanation, and in such a way that we had a reasonable amount of time to fix the issue before making it public.
Hey, what about node 0.4.x, is it not affected? That is what I understand from the post, but since it is not mentioned explicitly I thought I’d ask.
No, 0.4 is not affected. I add a tl;dr section just how stating this. Thanks 🙂
Note that 0.4 has many problems of its own, but as far as we know, there are no security-related issues.
I totally don’t intend to troll or sound ungrateful here — I’m just genuinely curious:
Why publicize the details of the issue before production apps have a chance to upgrade? Why not first just announce the critical security update and recommend everyone upgrade, and wait maybe a week or so to share the details after?
Regardless of this, great work to Matt and the team for finding and fixing this!
That’s a tricky question. The reasoning behind explaining the details of the issue is to give production apps the ability to determine whether they’re affected, how badly they’re affected, and whether it’s best to upgrade their node installation or refactor their app to reduce their exposure to the problem.
That can’t be done without explaining exactly what the issue is. The alternative would be to contact all the affected users directly, but since we don’t keep track of who uses node, that’s not possible. And, since so many people do use it, it would be effectively the same as a public announcement.
Thanks for explaining, Isaac.
Even if you don’t disclose the details, the commits to fix the problem are public anyway, so it’s trivial to reverse engineer (which people do).
In this case, I’d argue that the commits don’t really give away too much about how the error happens, unless you knew what to go look for, and were willing to spend a lot of time reading the code through carefully.
That’s why it’s so great that Matthew found it 🙂
What’s the checksum for the release package? It’s not mentioned in the changelog for this release.
Shasums:
Are there known regressions in this release? I am seeing decreased stability in production over 0.6.16.
Hm. Can you post an issue at https://github.com/joyent/node/issues?
If it helps, it turns out that the fix went in for 0.6.16, so you can roll back to that one safely.