Goahead webserver <= 2.1.8-path bypass-sensitive File Download vulnerability

2017-03-16T00:00:00
ID SSV:92789
Type seebug
Reporter
Modified 2017-03-16T00:00:00

Description

1) Introduction

Goahead webserver is an embedded OpenSource server that can be build on a lot of systems (CE, Ecos, GNU/Linux, Lynx, MacOS, NW, QNX4, VXWORKS, Win32 and others). It is supported by a lot of companies that use it for their projects and it is also used like "base" for other webservers, furthermore it has been developed for be very tiny and to run on embedded systems.

2) Bug

Goahead webserver has an internal problem that lets it to accept also HTTP requests that don't start with the slash or that contain backslashes (both \ and %5c) after or at the same place of the initial slash. So we have that a right request as "GET /file HTTP/1.0" is accepted but is also accepted a bad request as "GET file HTTP/1.0" or "GET \file HTTP/1.0", "GET /\%5cfile HTTP/1.0" and so on.

This bug leads to the bypassing of the management of the "special" directories (as cgi-bin) and to their usage as normal "unmanaged" directories.

In fact the server uses a function called websUrlHandlerDefine() letting the admin to specify how to manage each directory he wants. For example by default we have

websUrlHandlerDefine(T("/goform"), NULL, 0, websFormHandler, 0);

used to execute the built-in functions written by the same admin or

websUrlHandlerDefine(T("/cgi-bin"), NULL, 0, websCgiHandler, 0);

used just to manage the cgi-bin directory.

So if an attacker uses a HTTP request as one of those seen previously he will easily bypass the management decided by the admin for a specific directory (function websUrlHandlerRequest()).

The most common and useful effect of this bug is the download and the viewing of any file in the cgi-bin directory.

3) The Code

Some examples for reading the files in the cgi-bin directory:

   http://server/\cgi-bin/cgitest.c
   http://server/\\\cgi-bin/cgitest.c
   http://server/%5ccgi-bin/cgitest.c

   "GET cgi-bin/cgitest.c HTTP/1.0"
   "GET \cgi-bin/cgitest.c HTTP/1.0"
   "GET %5ccgi-bin/cgitest.c HTTP/1.0"

4) Fix

No official fix.

A simple patch can be the adding of the following 3 lines of code at about the line 265 of the file handler.c in the function "int websUrlHandlerRequest(webs_t wp)":

if((wp->path[0] != '/') || strchr(wp->path, '\\')) { websError(wp, 400, T("Bad request")); return(0); }