Multiple Unix/Linux Vendor cURL/libcURL NTLM Authentication Buffer
Overflow Vulnerability

iDEFENSE Security Advisory 02.21.05:
www.idefense.com/application/poi/display?id=202&type=vulnerabilities
February 21, 2005

I. BACKGROUND

cURL is a command line tool for transferring files with URL syntax,
supporting FTP, FTPS, HTTP, HTTPS, GOPHER, TELNET, DICT, FILE and LDAP.
More information about cURL and libcURL is available from:

    http://curl.haxx.se/

II. DESCRIPTION

Remote exploitation of a stack-based buffer overflow in various Unix /
Linux vendors implementations of cURL could allow for arbitrary code
execution on the targeted host.

An exploitable stack-based buffer overflow condition exists when using
NT Lan Manager (NTLM) authentication. The problem specifically exists
within Curl_input_ntlm() defined in lib/http_ntlm.c. Within this
function an unsigned stack-based character array of size 256, buffer[],
is passed to the Curl_base64_decode() routine defined in lib/base64.c as
can be seen here:

    size_t size = Curl_base64_decode(header, (char *)buffer);

The Curl_base64_decode() routine relies on the calling function to
validate the decoded length. This function base64 decodes and copies
data directly from the HTTP reply of a server to the destination buffer,
in this case buffer[]. An attacker can construct a long base64 encoded
malicious payload that upon decoding will overflow the 256 byte static
buffer and overwrite the saved EIP. This in turn can lead to arbitrary
code execution.

III. ANALYSIS

Successful exploitation allows remote attackers to execute arbitrary
code
under the privileges of the target user. Exploitation requires that an
attacker either coerce or force a target to connect to a malicious
server using NTLM authentication.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in cURL
version 7.12.1. It is suspected that prior versions are affected as
well.
Any application built using a vulnerable version libcURL will also be
affected.

V. WORKAROUND

Replace the static buffer allocation on line 106 in lib/http_ntlm.c:

    unsigned char buffer[256];

With a dynamic buffer allocation:

    unsigned char *buffer = (unsigned char *)malloc(strlen(header));

and recompile cURL.

VI. VENDOR RESPONSE

No vendor response received.

VII. CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

12/21/2004  Initial vendor notification - No response
02/10/2005  Secondary vendor notification - No response
02/21/2005  Public disclosure

IX. CREDIT

infamous41md[at]hotpop.com is credited with this discovery.

Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp

X. LEGAL NOTICES

Copyright (c) 2005 iDEFENSE, Inc.

Permission is granted for the redistribution of this alert
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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.