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 \
-e ssl.handshake.extensions_alpn_str  

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    216.58.192.66    1    cm.g.doubleclick.net    h2,spdy/3.1,http/1.1
216.58.192.66    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).

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Johannes B. Ullrich, Ph.D.
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