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Apache HTTP Server mod_tcl set_var Format String Vulnerability



The mod_tcl module for the Apache httpd v2.x is a scripting module that
allows a TCL developer to create server side script pages in TCL. It is
available as a contrib package for several distributions. For more
information, see http://tcl.apache.org/.


Remote exploitation of a format string vulnerability in the mod_tcl
module for the Apache httpd v2.x could allow attackers to execute
arbitrary code in the context of the httpd.

Due to programmer error, user supplied data is passed as the format
string specifier to several calls to an internally defined variable
argument function. The function 'set_var' is declared as follows:

 mod_tcl.h:117:void set_var(Tcl_Interp *interp, char *var1, char *var2, const char *fmt, ...); 
Several insecure calls to this function are made through out the code,
as seen below:
 tcl_cmds.c:437: set_var(interp, nm_var, (char*) key, (char*) val); tcl_cmds.c:2231: set_var(interp, nm_env, env[i], sptr + 1); tcl_core.c:650: set_var(interp, namespc, vl[i].var2, vl[i].var3); 


Successful exploitation allows remote attackers to gain local access to
the vulnerable system in the context of the affected httpd. To exploit
this vulnerability, the attacker must know the location of at least one
tcl server script that is configured to use this module for processing.


iDefense has confirmed the existence of this vulnerability in version
1.0 of mod_tcl for Apache 2.x.


iDefense is currently unaware of any workarounds for this issue.


The Apache mod_tcl team have addressed this vulnerability with mod_tcl
version 1.0.1. It is available from http://tcl.apache.org/mod_tcl/


The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2006-4154 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.


08/16/2006 Initial vendor notification
10/11/2006 Initial vendor response
10/13/2006 Coordinated public disclosure


This vulnerability was discovered by Sparfell.

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Copyright © 2006 Verisign, Inc.

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