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

Janusz Niewiadomski

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.