When unserializing an object with dynamic properties HHVM needs to pre-reserve the full size of the dynamic property array before inserting anything into it. Otherwise the array might resize, invalidating previously stored references. This pre-reservation was not occurring in 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.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-1900
• facebook/hhvm@c1c4bb0
• https://hhvm.com/blog/2020/06/30/security-update.html