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AUSCERT External Security Bulletin Redistribution
X509 Client certificate based authentication can be bypassed
when HTTP/2 is used
6 July 2016
AusCERT Security Bulletin Summary
Product: Apache HTTPD WebServer
Publisher: The Apache Software Foundation
Operating System: Windows
UNIX variants (UNIX, Linux, OSX)
Impact/Access: Access Privileged Data -- Remote/Unauthenticated
CVE Names: CVE-2016-4979
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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.
- - -----------
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
to the httpd configuration; along with a list of trusted client certificate
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
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.
- - --------
- - 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.
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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.
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All versions from 2.4.18 to 2.4.20. The issue is fixed in
version 2.4.23 (released 2015-6-5)
- - -----------
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:
Credits and timeline
- - --------------------
The flaw was found and reported by Erki Aring <email@example.com>
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
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CVSS Base Score 7.5
CVSS Temporal Score 7.0
1.05 / : 2339 $
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