Exim dns_buld_reverse() Buffer Overflow Vulnerability 

iDEFENSE Security Advisory 01.14.05
www.idefense.com/application/poi/display?id=183&type=vulnerabilities
January 14, 2005

I. BACKGROUND

Exim is a mail transfer agent (MTA) for Unix systems similar to
sendmail. More information is available at the following URL:

 http://www.exim.org/

II. DESCRIPTION

Local exploitation of a buffer overflow vulnerability in Exim 4.41 may 
allow execution of arbitrary commands with elevated privileges.

The problem specifically exists in the dns_build_reverse() function. The

function fails to check the length of a string which it copies into a 
fixed length buffer. This string is user controlled and passed into the 
program from a command line option. 

The following example demonstrates an input that will crash Exim:

/usr/bin/exim -bh ::%A`perl -e 'print pack('L',0xdeadbeef') x 256'`

III. ANALYSIS

Exploitation of this vulnerability will give an attacker access to the 
mailer uid. (The exim mailer is setuid root, but drops privileges before

the vulnerable code is reached). Having the mailer uid may allow access 
to sensitive information in email messages, or possibly further 
elevation.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in Exim 
versions 4.40 and 4.41. A source audit of version 4.42 suggests that it 
is also vulnerable. It is suspected that earlier versions are also 
vulnerable.

V. WORKAROUND

iDEFENSE is currently unaware of any effective workarounds for this 
vulnerability.

VI. VENDOR RESPONSE

A patch for Exim release 4.43 which addresses this vulnerability is
available at:

   http://www.exim.org/mail-archives/exim-announce/2005/msg00000.html

The patch will be incorporated into a future Exim release (4.50).

VII. CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

09/30/2004  Initial vendor notification
09/30/2004  Initial vendor response
01/14/2005  Public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp

X. LEGAL NOTICES

Copyright (c) 2004 iDEFENSE, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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There are no warranties with regard to this information. Neither the
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