WEBrick 1.3.1 in Ruby 1.8.6 through patchlevel 383, 1.8.7 through patchlevel 248, 1.8.8dev, 1.9.1 through patchlevel 376, and 1.9.2dev writes data to a log file without sanitizing non-printable characters, which might allow remote attackers to modify a window's title, or possibly execute arbitrary commands or overwrite files, via an HTTP request containing an escape sequence for a terminal emulator.
http://www.vupen.com/english/advisories/2010/0089
http://www.ruby-lang.org/en/news/2010/01/10/webrick-escape-sequence-injection
http://www.redhat.com/support/errata/RHSA-2011-0909.html