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CVE-2016-4979: HTTPD webserver - X509 Client certificate based authentication can be bypassed when HTTP/2 is used [vs]
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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.
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.
All versions from 2.4.18 to 2.4.20. The issue is fixed in
version 2.4.23 (released 2016-7-5)
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 <erki@xxxxxxxxxx>
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 2016 (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 Base Score 7.5
CVSS Temporal Score 7.0
1.06 / : 2341 $
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