wp-10-0001: Multiple Browser Wildcard Cerficate Validation Weakness

Type securityvulns
Reporter Securityvulns
Modified 2010-08-30T00:00:00


Westpoint Security Advisory

Title: Multiple Browser Wildcard Cerficate Validation Weakness Risk Rating: Low Author: Richard Moore <rich@westpoint.ltd.uk> Test Cases: Simon Ward <simon@westpoint.ltd.uk> Date: 14 July 2010 Advisory ID#: wp-10-0001 URL: http://www.westpoint.ltd.uk/advisories/wp-10-0001.txt CVE: not yet assigned


RFC 2818 covers the requirements for matching CNs and subjectAltNames in order to establish valid SSL connections. It first discusses CNs that are for hostnames, and the rules for wildcards in this case. The next paragraph in the RFC then discusses CNs that are IP addresses:

'In some cases, the URI is specified as an IP address rather than a hostname. In this case, the iPAddress subjectAltName must be present in the certificate and must exactly match the IP in the URI.'

The intention of the RFC is clearly that you should not be able to use wildcards with IP addresses (in order to avoid the ability to perform man-in-the-middle attacks). Unfortunately our testing showed that this rule is not adhered to by some browsers.

We created a certificate with the CN '*.168.3.48' this meets the various rules for wildcards in CNs, but should be treated as invalid since it is not a hostname. We then observed the errors reported by browsers when connecting to an https server using this certificate run on IP address

We imported the test CA used to sign the certifcate in order to perform the test.

The results we saw were as follows:

IE6 Regarded the IP address as matching the CN (VULNERABLE)

IE7 Regarded the IP address as matching the CN (VULNERABLE)

Firefox 3.6.6 Regarded the IP address as matching the CN (VULNERABLE)

Chrome Regarded the IP address as matching the CN (VULNERABLE)

Opera Reported the IP address did not match the CN (NOT VULNERABLE)

Safari 5 (win32) Reported the IP address did not match the CN (NOT VULNERABLE)

Qt (4.7 git development branch) Regarded the IP address as matching the CN (VULNERABLE)

Mitigating Factors

Obviously a good CA should refuse to issue a certificate with the CN as indicated, however there only need be one CA to issue one in error for this issue to result in the user getting no warning at all and being vulnerable to MITM.

The rules for hostname matching mean that only the first octet of the IP address can contain a wildcard. This means that you must be able to control a server that matches the remainder of the IP address of your target which reduces the risk of this attack being used dramatically.


If exploited then a MITM attack can be performed allowing the guarantees SSL provides to be circumvented.


14 July 2010 Limited disclosure to browser developers. 14 July 2010 Added Safari result. 15 July 2010 Disclosure to official browser security contacts. 15 July 2010 Microsoft confirm receipt. 15 July 2010 Mozilla fix ready. 18 July 2010 Google confirm that Chrome will be fixed by the fix to NSS on linux, and any fix provided by Microsoft on Windows. They will therefore not be adding a work-around to the Chrome code. 4 August 2010 Microsoft confirm the issue will be fixed in a future service pack, and that the issue is low enough risk that they are not asking the information to be withheld. 10 August 2010 Patch sent to Nokia for Qt. 27 August 2010 At the time of writing the NSS (Firefox) and Qt repositories both contain fixes for this issue that will be included in their releases.

-- Richard Moore, Principal Software Engineer, Westpoint Ltd, Albion Wharf, 19 Albion Street, Manchester, M1 5LN, England Tel: +44 161 237 1028 Fax: +44 161 237 1031