You're receiving this bug because the package in Summary has produced _FORTIFY_SOURCE related warnings indicating the presence of a sure overflow in a static buffer.
Even though this is not always an indication of a security problem it might even be. So please check this out ASAP.
By the way, _FORTIFY_SOURCE is disabled when you disable optimisation, so don't try finding out the cause using -O0.
Your friendly neighborhood tinderboxer
Created attachment 249574 [details]
The overflow is in a bundled and outdated copy of dev-libs/libcroco. Gentoo currently offers libcroco 0.6.1 and 0.6.2. Based on CVS commit messages for app-editors/cssed <http://cssed.cvs.sourceforge.net/viewvc/cssed/cssed/libcroco/parser/>, which appears to be abandoned, it is carrying a derivative of libcroco 0.6.0.
The overflow itself is simple to fix, if you do not mind continuing to use a bundled library. The function cr_om_parser_parse_paths_to_cascade has a stack variable of type X *, but does a memset of the variable for a length of X , which is wrong when sizeof(X*) < sizeof(X) [where X = CRStyleSheet].
memset (sheets, 0, sizeof (CRStyleSheet) * 3);
Change the memset to be:
memset (sheets, 0, sizeof (sheets));
Probably a candidate for removal as its upstream is dead, is using bundled libs and has multiple alternatives: