[UNIX] pServ Directory Traversal Vulnerability

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To: list@securiteam.com
Date: 5 Jan 2004 16:51:51 +0200

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  pServ Directory Traversal Vulnerability
------------------------------------------------------------------------

SUMMARY

The aim of <http://sourceforge.net/projects/pserv> pServ (pico Server) is
"to create a portable, small web server. Coded in portable C with UNIX
being the main reference platform, but porting is encouraged. Portability
and small footprint should enable the use of pServ on a workstation as
well as". A directory traversal vulnerability in the product allows remote
attackers to view files that reside outside the bound HTTP root directory.

DETAILS

Vulnerable systems:
 * pServ version 3.0b2

The program, by default, has an anti-directory traversal check, but this
check can be easily bypassed using the double slash ("//") into the HTTP
requests.

Exploit:
To test the pServ's vulnerability simply send to the web server an HTTP
request string, like that:
GET //../ HTTP/1.0\r\n\r\n

Or generally:
GET //../MY_PATH HTTP/1.0\r\n\r\n
GET /SOME_DIRECTORY//..//../ HTTP/1.0\r\n\r\n
                        
Therefore, the web server will allow you to go outside of the
documentsPath assigned to the web server, and navigate through the system.

Solution:
To fix the bug simply go on the pServ's official website,
<http://sourceforge.net/projects/pserv>
http://sourceforge.net/projects/pserv, and download the latest version of
pServ (see in CVS).

Or, if you want, you can use my following little patch, that should fix
the bug for the version 3.0b2 of pServ:
--- main.c 2003-09-22 10:39:24.000000000 +0200
+++ patch.c 2003-12-19 12:40:47.000000000 +0100
@@ -455,6 +455,11 @@
                     dirName[1] = req.documentAddress[2];
                     dirName[2] = req.documentAddress[3];
                     dirName[3] ='\0';
+ if (dirName[0] == '/')
+ {
+ sayError(sock, FORBIDDEN, req.documentAddress,
req);
+ return -1;
+ }
                     if (!strcmp(dirName, "../"))
                     {
                         sayError(sock, FORBIDDEN, req.documentAddress,
req); @@ -462,6 +467,15 @@
                     }
                 }
                 j = 0;
+ for(i = 1; i < sL; i++) {
+ if(req.documentAddress[i] == '/')
+ if(req.documentAddress[i+1] == '/')
+ {
+ sayError(sock, FORBIDDEN,
req.documentAddress, req);
+ return -1;
+ }
+
+ }
                 for (i = 1; i < sL; i++) {
                     if (req.documentAddress[i] == '/')
                     {

ADDITIONAL INFORMATION

The information has been provided by <mailto:fdonato@autistici.org>
Donato Ferrante.

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