Public Vulnerability Reports

Courier-IMAP Remote Format String Vulnerability

08.18.04

BACKGROUND

Courier-IMAP is an IMAP/POP3 mail server popular on sites utilizing
Qmail/Exim/Postfix. More information is available here:

    http://www.courier-mta.org/imap/

DESCRIPTION

Remote exploitation of a format string vulnerability in Double Precision
Inc.'s, Courier-IMAP daemon allows attackers to execute arbitrary code.

The vulnerability specifically exists within the auth_debug() function
defined in authlib/debug.c:

void auth_debug( const char *fmt, va_list ap ) {
    char    buf[DEBUG_MESSAGE_SIZE];
    int     i;
    int     len;

    // print into buffer to be able to replace control and other 
    // unwanted chars.
    vsnprintf( buf, DEBUG_MESSAGE_SIZE, fmt, ap );
    len = strlen( buf );

    // replace nonprintable chars by dot
    for( i=0 ; i<len ; i++ )
            if( !isprint(buf[i]) )
                    buf[i] = '.';

    // emit it
    fprintf( stderr, buf );   // <- Format String Vulnerability
    fprintf( stderr, "n" );
}

The 'buf' variable utilized in the fprintf() call is attacker-controlled
and can contain format string modifiers allowing an attacker to
manipulate the stack and eventually execute arbitrary code.

ANALYSIS

Successful exploitation does not require authentication thereby allowing
any remote attacker to execute arbitrary code under the privileges of
the user that the IMAP daemon runs as. The vulnerable function
auth_debug() is only called if login debugging is enabled requiring that
the 'DEBUG_LOGIN' be set to either '1' or '2' in the imapd configuration
file.

DETECTION

iDEFENSE has confirmed the existence of this vulnerability in
Courier-IMAP, v2.2.1. It is reported that all versions of Courier-IMAP
from 1.6.0 to 2.2.1 inclusive are vulnerable.

WORKAROUND

Disable the login debugging option of Courier-IMAP. This can be
accomplished by setting 'DEBUG_LOGIN' to '0' in the configuration file
usually located at /usr/lib/courier-imap/etc/imapd.

VENDOR RESPONSE

This issue has been resolved in the latest version of Courier IMAP
(v3.0.7). As well, the default setting of 'DEBUG_LOGIN' is '0'.

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2004-0777 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

DISCLOSURE TIMELINE

08/10/2004    Initial vendor contact
08/10/2004    iDEFENSE clients notified
08/11/2004    Initial vendor response
08/18/2004    Public disclosure

CREDIT

An anonymous contributor is credited with discovering this
vulnerability.

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LEGAL NOTICES

Copyright © 2004 Verisign, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
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email customerservice@idefense.com for permission.

Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an as is condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.