CVE 2008-5916
gitweb/gitweb.perl in gitweb in Git 1.6.x before 1.6.0.6, 1.5.6.x before 1.5.6.6, 1.5.5.x before 1.5.5.6, 1.5.4.x before 1.5.4.7, and other versions after 1.4.3 allows local repository owners to execute arbitrary commands by modifying the diff.external configuration variable and executing a crafted gitweb query.
See the 
CVE page on Mitre.org
for more details.
