full-disclosure - re: webmin remote format string bug

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Date: Thu Dec  1 19:05:50 2005
From: giarc at freenet.de (giarc@...enet.de)
Subject: re: webmin remote format string bug

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Hello!
I succeeded in crashing webmin 1.230 with:

username %n
password aaaa

after klicking 4 times on "Login" webmin was dead.
There were no logs at all, and no error was shown in the web interface...
Any idea if it's really exploitable (executing code I mean)? Is anyone working on a POC?

giarc@...eet.de

Original message: ---------------------------------------------------------

To: full-disclosure@...ts.grok.org.uk
Date: Tue, 29 Nov 2005 11:15:20 -0600

On Tuesday 29 November 2005 04:07, advisory@...dsecurity.com wrote:
> [snip ] so so if remote code execution is successful, it would
> lead to a full remote root compromise in a standard configuration. 

> DESCRIPTION. ?The username parameter of the login form is logged via
> the perl `syslog' facility in an unsafe manner during a unknown user
> login attempt. the perl syslog facility passes the username on to the
> variable argument function sprintf that will treat any format
> specifiers and process them accordingly.
>
> DETAILS. ?The vectors for a simple DoS of the web server are to use the
> %n and %0(large number)d inside of the username parameter, with the
> former causing a write protection fault within perl leading to script
> abortion, and the latter causing a large amount of memory to be
> allocated inside of the perl process.

Sys::Syslog calls sprintf($format, @_). I tried testing this on perl 5.8.7 
and don't see how this can be exploitable. ?The %n specifier results in 
the following error message:

$ perl -e 'sprintf("%n")'
Modification of a read-only value attempted at -e line 1.

Using a thousand %p's results in the same address (presumably of the 
temporary char *) over and over again

It is possible to memory starve webmin with a long %9999999999d string, 
but arbitrary memory writes seem to be out of the question.

What version of perl was used by the third-party to exploit this?