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:
------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;
Since the path is constructed from current working directory and a file name specified as a 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 that 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.
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
