The unserialize() function supported a type code, "S", which was meant to be supported only for APC serialization.

published 2021-03-11 5 y ago modified 2026-06-17 4 mo ago also EUVD-2020-12725

cvss: 7.5; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.2%; EPSS 1.2% chance of exploitation in 30 days, percentile 68%
public exploits: 0; none known

The unserialize() function supported a type code, "S", which was meant to be supported only for APC serialization. This type code allowed arbitrary memory addresses to be accessed as if they were static StringData objects. 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.

Patch / fix

fixed in 1 links 1

Facebook HHVM 4.32.3

github.com/facebook/hhvm@1107228 commit

Exploit prediction

1.22% chance of exploitation in the next 30 days, higher than 68% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
NVD	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
NVD	2.0	5.0	AV:N/AC:L/Au:N/C:P/I:N/A:N

Affected products

Facebook HHVM

Classification

Weakness

CWE-822 Untrusted Pointer Dereference
CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer

References

github.com /facebook/hhvm/commit/1107228a5128d3ca1c4add8ac1635d933cbbe2e9
patch, third-party-advisory
hhvm.com /blog/2020/06/30/security-update.html
vendor-advisory

Sources

CVE List CVE-2020-1899
NVD CVE-2020-1899

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary