[UNIX] Wu-FTPd SKEY Stack Overflow Vulnerability

From: SecuriTeam (support_at_securiteam.com)
Date: 10/26/03

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Date: 26 Oct 2003 16:18:33 +0200

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      Wu-FTPd SKEY Stack Overflow Vulnerability
    ------------------------------------------------------------------------

    SUMMARY

    The Washington University FTP daemon (hereafter referred to as "wuftpd")
    is a replacement FTP server for POSIX systems. Wu-FTPd supports SKEY
    authentication to provide secure logins. However, the code that 'handles'
    this has an exploitable stack based buffer overflow. By providing
    specially crafted authentication credentials, it is possible to crash the
    daemon or execute user-supplied code, running with root privileges.

    DETAILS

    Vulnerable systems:
     * Wu-FTPd version 2.6.2 (with SKEY support enabled)

    A statically allocated buffer is filled using the sprintf() function in
    the skey_challenge() function (src/ftpd.c).

     char *skey_challenge(char *name, struct passwd *pwd, int pwok)
         $
     {
         $
         static char buf[128];
         ...
         if (pwd == NULL || skeychallenge(&skey, pwd->pw_name, sbuf))
         $
             sprintf(buf, "Password required for %s.", name);
         else
       sprintf(buf, "%s %s for %s.", sbuf,
         pwok ? "allowed" : "required", name);
         return (buf);
     }

    The variable *name is never subject to any boundaries checking. It is
    possible to write beyond the buf[] array, overwriting the return address
    of the function, modifying the path of execution flow.

    Fix/Workaround:
    To protect you from this vulnerability, disable SKEY support, or apply the
    following patch:
     % diff -u ftpd.c fixed-ftpd.c
     --- ftpd.c 2001-11-29 17:56:11.000000000 +0100
     +++ fixed-ftpd.c 2003-10-20 20:43:58.000000000 +0200
     @@ -1662,9 +1662,9 @@
          /* Display s/key challenge where appropriate. */
      
          if (pwd == NULL || skeychallenge(&skey, pwd->pw_name, sbuf))
     - sprintf(buf, "Password required for %s.", name);
     + snprintf(buf, 128-1, "Password required for %s.", name);
          else
     - sprintf(buf, "%s %s for %s.", sbuf,
     + snprintf(buf, 128-1, "%s %s for %s.", sbuf,
                     pwok ? "allowed" : "required", name);
          return (buf);
      }
     %

    Vendor status:
    Michael Hendrickx found this vulnerability in Wu-FTPd two weeks ago, and
    has been waiting for a response from the Wu-FTPd development team without
    any luck.

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:michael@scanit.be> Michael
    Hendrickx.

    ========================================

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