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Date: Thu, 9 Feb 2017 14:24:58 -0300
From: Gustavo Grieco <gustavo.grieco@...il.com>
To: oss-security@...ts.openwall.com, Agustin Mista <mista.agustin@...il.com>
Subject: Multiple DoS parsing and executing extended regex expressions in GNU libc

Hello,

We found a few extended regex expressions in GNU libc that will crash or
abort the execution of regcomp or regexec. For instance:

\a?{1,32767}

will immediately exhaust the stack calling calc_eclosure_iter in the
compilation. A small variation of this regex is:

\a?{0,32767}

will consume a very large amount of memory: it seems to eat 16GB in less
than a minute. It is also possible to exhaust the stack memory trying to
parse:

(((((((( ... repeated 15000 times

this issue is caused because regcomp will call the parse_expression,
parse_branch and parse_reg_exp functions over and over again.
Finally, the following regex will trigger an abort or invalid free when
regexec is called:

/S^^|\0|()//S^^|\0|()//S^^|\1|()/

I don't think these issues can be used to execute arbitrary code, but it
seems quite easy to produce a DoS if a remote application is parsing
untrusted regex expressions.
In fact, we asked one of our students, Agustín Mista, to create a simple PoC
to show how to crash a proFTP server if you can write a .ftpaccess file.
You can find the script attached.

These issues were tested in GNU libc 2.19 (Ubuntu 14.04) and 2.24 (ArchLinux).

I think it should affect the last version of GNU libc as well. Can someone
confirm it?

I'm investigating how to submit these issues in the new CVE form...


Regards,
Gustavo.

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