[UNIX] Courier-IMAP Remote Format String Vulnerability

From: SecuriTeam (support_at_securiteam.com)
Date: 08/18/04

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Date: 18 Aug 2004 19:40:45 +0200

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      Courier-IMAP Remote Format String Vulnerability
    ------------------------------------------------------------------------

    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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