Beck, variant 2

Against

Apache on Any Unix/Linux

IDs

Category

Poor resource management

Effect

Denial of Service

Source

Packetstorm

Description

Here is another (less interesting) example of Apache DoS attack, called 'beck2'. The only thing I want to show is very ineffective management of memory, CPU time and other resources. This attack is possible in two cases:

1. Attacker owns an account on a victim machine, or 2. Victim's directory structure is very deep (?).

When one of above statements is true, it's possible to perform a remote attack, even when Apache has been already patched against first version of 'beck'. More details can be deducted from sources :)

In well-configured system, any kind DoS attack should be at least ineffective (resources *required* to attack should be significally larger than resources *affected* by attack ;). Unfortunately, it's very, very easy to attack Apache servers using minimal amount of time and brain resources :) Maybe it's time to rewrite larger parts of code?

_______________________________________________________________________ Michal Zalewski lcamtuf@boss.staszic.waw.pl

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I really doubt that bugtraq is the place to continue this discussion. I suggest that you move it to the Apache development mailing list new-httpd@apache.org, subscribe via majordomo@apache.org (it's a reply-to style list so it's not useful for discussion with folks who are off the list). This is documented in the ABOUT_APACHE file. We also have a bug reporting system through which you can report these bugs. Remember we're free software, and if you can help constructively it's appreciated.

I sent this to you in private mail, but I'll post it again here. I said something like this in my post that contained a patch for your first problem but that post never seemed to make it to bugtraq.

Apache, like every other network service I'm aware of, is subject to DoS attacks that consume resources. That's it, period. We can't guarantee we'll catch every one of them. I really don't think posting them one-by-one to bugtraq is going to help anything. Perhaps you'd be better spending your time working on a more generic solution than forcing us to tweak one problem at a time.

Folks can see some of my thoughts on a related issue, and see yet another DoS attack, by visiting . That is specific to memory consumption. But there's obviously more we can do in a similar vein using resource limits. Doing all of this comes at a performance penalty.

I don't think I have to explain examples of DoS attacks against all web servers, or against smtp daemons, or IRC servers, or any number of networked services. They exist in all of them. Creating generic workarounds is an effort much more well spent than poking away at the problems one by one as they're discovered.

Performance problems like the first beck patch are things that we will notice as we profile the server. Until recently most of my effort on tuning Apache's performance has been in the syscall area. However in 1.3 there are other changes to how directories are handled that could have an impact on your beck2 attack. I know for certain that your beck2 attack is affected by the performance tuning guidelines at . Specifically the sections mentioning lstat() and stat().

But the easiest way to fix beck2 is to use mod_rewrite to lift this problem out of the filesystem. Add these to your config:

    RewriteEngine on
    RewriteRule /+[^/]+/+[^/]+/+ /yeah-whatever.html

Dean

Attack string

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Attack program source