CVE 2020-1898
The fb_unserialize function did not impose a depth limit for nested deserialization. That meant a maliciously constructed string could cause deserialization to recurse, leading to stack exhaustion. This issue affected HHVM prior to v4.32.3, between versions 4.33.0 and 4.56.0, 4.57.0, 4.58.0, 4.58.1, 4.59.0, 4.60.0, 4.61.0, 4.62.0.
See the
CVE page on Mitre.org
for more details.