libxml2, possibly before 2.5.0, does not properly detect recursion during entity expansion, which allows context-dependent attackers to cause a denial of service (memory and CPU consumption) via a crafted XML document containing a large number of nested entity references, aka the "billion laughs attack."
https://github.com/advisories/GHSA-rvwf-54qp-4r6v
https://euvd.enisa.europa.eu/vulnerability/EUVD-2003-1554
http://www.stylusstudio.com/xmldev/200302/post20020.html
http://www.redhat.com/support/errata/RHSA-2008-0886.html
http://www.reddit.com/r/programming/comments/65843/time_to_upgrade_libxml2
http://secunia.com/advisories/31868
http://mail.gnome.org/archives/xml/2008-August/msg00034.html