[UNIX] Courier-IMAP Remote Format String Vulnerability

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  Courier-IMAP Remote Format String Vulnerability
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SUMMARY

 <http://www.courier-mta.org/imap/> Courier-IMAP is "an IMAP/POP3 mail 
server popular on sites utilizing Qmail/Exim/Postfix". Remote exploitation 
of a format string vulnerability in Double Precision Inc.'s, Courier-IMAP 
daemon allows attackers to execute arbitrary code.

DETAILS

Vulnerable Systems:
 * Courier-IMAP versions 1.6.0 up to 2.2.1

Immune Systems:
 * Courier-IMAP version 3.0.7

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.

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:
 <http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2004-0777> 
CAN-2004-0777

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


ADDITIONAL INFORMATION

The information has been provided by  
<mailto:idlabs-advisories@idefense.com> iDEFENSE.
The original article can be found at:  
<http://www.idefense.com/application/poi/display?id=131&type=vulnerabilities&flashstatus=true>
 
http://www.idefense.com/application/poi/display?id=131&type=vulnerabilities&flashstatus=true



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