Apache 2.4.20 X509 Authentication Bypass
Posted on 30 November -0001
<HTML><HEAD><TITLE>Apache 2.4.20 X509 Authentication Bypass</TITLE><META http-equiv="Content-Type" content="text/html; charset=utf-8"></HEAD><BODY> Security Advisory - Apache Software Foundation Apache HTTPD WebServer / httpd.apache.org X509 Client certificate based authentication can be bypassed when HTTP/2 is used CVE-2016-4979 / CVSS 7.5 The Apache HTTPD web server (from 2.4.18-2.4.20) did not validate a X509 client certificate correctly when experimental module for the HTTP/2 protocol is used to access a resource. The net result is that a resource that should require a valid client certificate in order to get access can be accessed without that credential. Background: - ----------- Apache can control access to resources based on various things; such as a password, IP address and so on. One of the options, when SSL or TLS is used, is gating access based on the client having access to a private-key of a X509 client certificate. These client certificates are typically held on a chipcard (e.g. the CAC card in the US, national identity, banking cards or, for example, medical-chip cards in Europe). In some cases they are 'soft tokens' - i.e. files, often called PKCS#12 files, which are loaded into the browser or the 'keychain'. Gating access based on a client certificate is done by adding a line such as SSLVerifyClient require to the httpd configuration; along with a list of trusted client certificate authorities (SSLCACertificateFile). Version 2.4.17 of the Apache HTTP Server introduced an experimental feature: mod_http2 for the HTTP/2 protocol (RFC7540, previous versions were known as Google SPDY). This module is NOT compiled in by default -and- is not enabled by default, although some distribution may have chosen to do so. It is generally needs to be enabled in the 'Protocols' line in httpd by adding 'h2' and/or 'h2c' to the 'http/1.1' only default. The default distributions of the Apache Software Foundation do not include this experimental feature. Details: - -------- - From version 2.4.18, upto and including version 2.4.20 the server failed to take the (failed/absent) client certificate validation into account when providing access to a resource over HTTP/2. This issue has been fixed in version 2.4.23 (r1750779). As a result - a resource thought to be secure and requiring a valid client certificate - would be accessible without authentication provided that the mod_http2 was loaded, h2 or h2c activated, that that the browser used the HTTP/2 protocol and it would do more than one request over a given connection. Impact: - ------- A third party can gain access to resources on the web server without the requisite credentials. This can then lead to unauthorised disclosure of information. Versions affected: - ------------------ All versions from 2.4.18 to 2.4.20. The issue is fixed in version 2.4.23 (released 2015-6-5) Resolution: - ----------- Upgrade to version 2.4.23 or newer. Mitigations and work arounds: - ----------------------------- As a temporary workaround - HTTP/2 can be disabled by changing the configuration by removing h2 and h2c from the Protocols line(s) in the configuration file. The resulting line should read: Protocols http/1.1 Credits and timeline - -------------------- The flaw was found and reported by Erki Aring <erki@example.ee> from Liewenthal Electronics Ltd on 2016-06-30. The issue was resolved by Stefan Eissing that same day and incorporated in the release of 5th of July 2015 (thus avoiding a bank holiday). Apache would like to thank all involved for their help with this. Common Vulnerability Scoring (Version 3) and vector - --------------------------------------------------- CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N/E:F/RL:O/RC:C CVSS Base Score 7.5 CVSS Temporal Score 7.0 </BODY></HTML>
