[VulnWatch] wu-ftpd fb_realpath() off-by-one bug

From: Janusz Niewiadomski (funkysh_at_isec.pl)
Date: 07/31/03

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    Date: Thu, 31 Jul 2003 18:16:03 +0200 (CEST)
    To: vulnwatch@vulnwatch.org, <bugtraq@securityfocus.com>

    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


    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.


    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:

         * 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;
            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);

    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:


    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.


    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.


    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

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