Last Updated: 2016-07-05 19:52:55 UTC
by Johannes Ullrich (Version: 1)
Apache released an important update today to fix a vulnerability that affects servers that have http/2 enabled and use TLS client certificates for authentication.
Apache 2.4.18-20 are vulnerable if:
- TLS certificates are used for authenticating clients (look for the "SSLVerifyClient require" directive in your configuration file)
- http/2 is enabled. (see if the "Protocols" line includes h2 and/or h2c).
Only access over http/2 is affected. Access via http/1.1 is still properly controlled even if http/2 is enabled. Over TLS, clients that suport http/2 will likely use it over http/1.1.
http/2 is not enabled by default in any currently shipping version of Apache.
To quickly check your network traffic for http/2 use, you can use this tshark line:
tshark -Y 'ssl.handshake.extensions_alpn_str == "h2"' -n -i en0 \
-T fields -e ip.src -e ip.dst -e ssl.handshake.type -e ssl.handshake.extensions_server_name \
It will list the client requests as well as the server responses that contain http/2 including the host name that the client is trying to reach. For example:
10.5.1.12 18.104.22.168 1 cm.g.doubleclick.net h2,spdy/3.1,http/1.1
22.214.171.124 10.5.1.12 2 h2
In this handshake, the client offers http/2, spdy/3.1 as well as http/1.1 to cm.g.doubleclick.net . The server then selects http/2 (h2).