UW-IMAP Netmailbox Name Parsing Buffer Overflow Vulnerability 

iDEFENSE Security Advisory 10.04.05
www.idefense.com/application/poi/display?id=313&type=vulnerabilities
October 4, 2005

I. BACKGROUND

UW-IMAP is a popular free IMAP service for Linux and UNIX systems and 
is distributed with various Linux distributions. More information can 
be found at the vendor website:

	http://www.washington.edu/imap/

II. DESCRIPTION

Remote exploitation of a buffer overflow vulnerability in the University
of Washington's IMAP Server (UW-IMAP) allows attackers to execute 
arbitrary code.

The vulnerability specifically exists due to insufficient bounds
checking on user-supplied values. The mail_valid_net_parse_work() 
function in src/c-client/mail.c is responsible for obtaining and 
validating the specified mailbox name from user-supplied data. An error 
in the parsing of supplied mailbox names will continue to copy memory 
after a " character has been parsed until another " character is found 
as shown here:

long mail_valid_net_parse_work (char *name,NETMBX *mb,char *service)
{
  int i,j;
#define MAILTMPLEN 1024        /* size of a temporary buffer */
  char c,*s,*t,*v,tmp[MAILTMPLEN],arg[MAILTMPLEN];
    
   ...snip...
    
  if (t - v) {            /* any switches or port specification? */
1]  strncpy (t = tmp,v,j);    /* copy it */
    tmp[j] = '\0';        /* tie it off */

...

    if (*t == '"') {    /* quoted string? */
2]     for (v = arg,i = 0,++t; (c = *t++) != '"';) { /* Vulnerability */
                /* quote next character */
        if (c == '\\') c = *t++;
        arg[i++] = c;
      }

If an attacker supplys only one " character, the function will continue 
to copy bytes to the new pointer, overflowing the stack buffer and 
resulting in arbitrary code execution.

III. ANALYSIS

Successful exploitation of the vulnerability will result in the 
execution of arbitrary code with permissions of the IMAP server. The 
impact of this vulnerability is slightly reduced due to the requirement 
of valid credentials, however IMAP servers commonly are used for free 
webmail systems and other services which may give untrusted users valid 
credentials. Networks that restrict IMAP service access to trusted 
users are at low risk.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in Washington

University imap-2004c1.

The following vendors include susceptible UW-IMAP packages within their 
respective operating system distributions:

    * FreeBSD Project: FreeBSD 5.x
    * Gentoo Foundation Inc.: Gentoo 2005.x
    * Debian Project: Linux 3.x
    * Red Hat Linux, Inc.: Fedora Core 1, 2, and 3
    * Mandrakesoft SA: Mandriva Linux 9.x
    * Novell Inc.: SuSE Linux 9.x 
    
V. WORKAROUND

iDEFENSE is unaware of any valid workarounds for this issue. Restrict 
untrusted users from the IMAP service until the vendor releases a 
patch.

VI. VENDOR RESPONSE

"The fix is in the following patch to imap-????/src/c-client/mail.c:

------------------------------Cut Here----------------------------------
*** mail.c	2005/03/17 00:12:22	1.6
--- mail.c	2005/09/15 16:48:46
***************
*** 691,698 ****
--- 691,700 ----
         if (c == '=') {		/* parse switches which take
arguments */
   	if (*t == '"') {	/* quoted string? */
   	  for (v = arg,i = 0,++t; (c = *t++) != '"';) {
+ 	    if (!c) return NIL;	/* unterminated string */
   				/* quote next character */
   	    if (c == '\\') c = *t++;
+ 	    if (!c) return NIL;	/* can't quote NUL either */
   	    arg[i++] = c;
   	  }
   	  c = *t++;		/* remember delimiter for later */
------------------------------Cut Here----------------------------------

This fix is in UW release imap-2004g, which is available from as the
current release version on:

 	ftp://ftp.cac.washington.edu/mail/imap.tar.Z

IMPACT ANALYSIS:

The vulnerability is in the c-client library, which is used by the IMAP
server.

The main impact of a successful exploit in the IMAP server is that an
authorized user can execute arbitrary code, including gaining shell
access, on the server.  The code is executed with the authorized user's
userid.

A successful exploit in the IMAP server does NOT allow root access.

UW imapd has an optional facility for anonymous access; this feature
must be enabled specifically by the site and is rarely-enabled.  Due to
a security check specific to anonymous IMAP access, anonymous IMAP users
can NOT exploit this vulnerability.

In the absence of data to the contrary, I believe that this
vulnerability is LOW risk to servers which permit shell access to
authorized users; and is of LOW-MODERATE risk (unauthorized shell access
to authorized users) to other servers.

The vulnerability impacts all applications which use the c-client
library, even if these applications do not use IMAP.  In the IMAP server
and most MUAs, the application runs with the user's credentials which
reduces the overall risk.  If the application runs with other
credentials (e.g., webmail systems), the vulnerability may be of higher
risk."

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2005-2933 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

09/15/2005  Initial vendor notification
09/15/2005  Initial vendor response
10/04/2005  Coordinated public disclosure

IX. CREDIT

infamous41md@hotpop.com is credited with discovering this vulnerability.


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X. LEGAL NOTICES

Copyright (c) 2005 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
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
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