wu-ftpd fb_realpath() off-by-one bug

[Janusz Niewiadomski <funkysh () isec pl>]

Synopsis:       wu-ftpd fb_realpath() off-by-one bug
Product:        wu-ftpd
Version:        2.5.0 <= 2.6.2
Vendor:         http://www.wuftpd.org/

URL:            http://isec.pl/vulnerabilities/isec-0011-wu-ftpd.txt
CVE:            http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2003-0466
Author:         Wojciech Purczynski <cliph () isec pl>
                Janusz Niewiadomski <funkysh () isec pl>
Date:           July 31, 2003 


Issue:
======

Wu-ftpd FTP server contains remotely exploitable off-by-one bug. A local
or remote attacker could exploit this vulnerability to gain root
privileges on a vulnerable system.


Details:
========

An off-by-one bug exists in fb_realpath() function. An overflow occurs
when the length of a constructed path is equal to the MAXPATHLEN+1
characters while the size of the buffer is MAXPATHLEN characters only.
The overflowed buffer lies on the stack.

The bug results from misuse of rootd variable in the calculation of
length of a concatenated string:

------8<------cut-here------8<------
    /*
     * Join the two strings together, ensuring that the right thing
     * happens if the last component is empty, or the dirname is root.
     */
    if (resolved[0] == '/' && resolved[1] == '\0')
        rootd = 1;
    else
        rootd = 0;

    if (*wbuf) {
        if (strlen(resolved) + strlen(wbuf) + rootd + 1 > MAXPATHLEN) {
            errno = ENAMETOOLONG;
            goto err1;
        }
        if (rootd == 0)
            (void) strcat(resolved, "/");
        (void) strcat(resolved, wbuf);
    }
------8<------cut-here------8<------

Since the path is constructed from current working directory and a file
name specified as an parameter to various FTP commands attacker needs to
create deep directory structure.

Following FTP commands may be used to cause buffer overflow:

        STOR
        RETR
        APPE
        DELE
        MKD
        RMD
        STOU
        RNTO

This bug may be non-exploitable if size of the buffer is greater than
MAXPATHLEN characters. This may occur for example if wu-ftpd is compiled
with some versions of Linux kernel where PATH_MAX (and MAXPATHLEN 
accordingly) is defined to be exactly 4095 characters. In such cases,
the buffer is padded with an extra byte because of variable alignment 
which is a result of code optimization.

Linux 2.2.x and some early 2.4.x kernel versions defines PATH_MAX to be 
4095 characters, thus only wu-ftpd binaries compiled on 2.0.x or later 2.4.x
kernels are affected.


Exploit:
========

We investigated and successfully exploited this vulnerability on x86 based
Linux system running 2.4.19 kernel. We believe that exploitation of other
little-endian systems is also possible.
 

Impact:
=======

Authenticated local user or anonymous FTP user with write-access could
execute arbitrary code with root privileges.


Vendor Status:
==============

June  1, 2003   security () wu-ftpd org has been notified
June  9, 2003   Request for confirmation of receipt sent to security () wu-ftpd org
June 11, 2003   Response received from Kent Landfield
July  3, 2003   Request for status update sent
July 19, 2003   vendor-sec list notified
July 31, 2003   Coordinated public disclosure


The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CAN-2003-0466 to this issue.

-- 
Janusz Niewiadomski
iSEC Security Research
http://isec.pl/