Junio C Hamano [off-list ref] writes:

Current gitweb has a possible local privilege escalation bug that allows a
malicious repository owner to run a command of his choice by specifying
diff.external configuration variable in his repository and running a
crafted gitweb query.

Recent (post 1.4.3) gitweb itself never generates a link that would result
in such a query, and the safest and cleanest fix to this issue is to
simply drop the support for it.  Maintenance release v1.6.0.6, v1.5.6.6,
v1.5.5.6 and v1.5.4.7 are already available at k.org (see the announcement
for v1.6.0.6 I sent out a few minutes ago), and the master branch and
others pushed out tonight have the same fix.

From what I have found diff.external works only since v1.5.4 (see

commit cbe02100), so when gitweb started using git-diff for old
legacy links to not use $tmpdir and /usr/bin/diff -u it wasn't an
issue...

-- 
Jakub Narebski
Poland
ShadeHawk on #git