USN-1367-1 fixed vulnerabilities in libpng. This provides the corresponding
update for Thunderbird.
Original advisory details:
Jueri Aedla discovered that libpng did not properly verify the size used
when allocating memory during chunk decompression. If a user or automated
system using libpng were tricked into opening a specially crafted image,
an attacker could exploit this to cause a denial of service or execute
code with the privileges of the user invoking the program. (CVE-2011-3026)