How severe is CVE-2018-6334?

CVE-2018-6334 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2018-6334 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (77th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2018-6334?

CVE-2018-6334 affects Facebook Hhvm. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2018-6334?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2018-6334 published?

CVE-2018-6334 was published on 2018-12-31 and last updated on 2026-06-17.

CVE-2018-6334

CVE-2018-6334 is a critical-severity vulnerability in Facebook Hhvm with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-20.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 2% (77th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-20
Affected product: Facebook Hhvm
Published: 2018-12-31
Last modified: 2026-06-17

Description

Multipart-file uploads call variables to be improperly registered in the global scope. In cases where variables are not declared explicitly before being used this can lead to unexpected behavior. This affects all supported versions of HHVM prior to the patch (3.25.1, 3.24.5, and 3.21.9 and below).

Frequently asked questions

What is CVE-2018-6334?: Multipart-file uploads call variables to be improperly registered in the global scope. In cases where variables are not declared explicitly before being used this can lead to unexpected behavior. This affects all supported versions of HHVM prior to the patch (3.25.1, 3.24.5, and 3.21.9 and below).
How severe is CVE-2018-6334?: CVE-2018-6334 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2018-6334 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (77th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2018-6334?: CVE-2018-6334 affects Facebook Hhvm. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2018-6334?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2018-6334 published?: CVE-2018-6334 was published on 2018-12-31 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:facebook:hhvm:*:*:*:*:*:*:*:*

More vulnerabilities in Facebook Hhvm

CVE-2022-36937 — Critical (CVSS 9.8): HHVM 4.172.0 and all prior versions use TLS 1.0 for secure connections when handling tls:// URLs in the stream…
CVE-2021-24036 — Critical (CVSS 9.8): Passing an attacker controlled size when creating an IOBuf could cause integer overflow, leading to an out of bounds…
CVE-2020-1900 — Critical (CVSS 9.8): When unserializing an object with dynamic properties HHVM needs to pre-reserve the full size of the dynamic property…
CVE-2021-24025 — Critical (CVSS 9.8): Due to incorrect string size calculations inside the preg_quote function, a large input string passed to the function…
CVE-2020-1917 — Critical (CVSS 9.8): xbuf_format_converter, used as part of exif_read_data, was appending a terminating null character to the generated…
CVE-2020-1916 — Critical (CVSS 9.8): An incorrect size calculation in ldap_escape may lead to an integer overflow when overly long input is passed in,…

All CVEs affecting Facebook Hhvm →

Other CWE-20 (Improper Input Validation) vulnerabilities

CVE-2026-48316 — Critical (CVSS 10.0): ColdFusion versions 2025.9, 2023.20 and earlier are affected by an Improper Input Validation vulnerability that could…
CVE-2026-48281 — Critical (CVSS 10.0): ColdFusion versions 2025.9, 2023.20 and earlier are affected by an Improper Input Validation vulnerability that could…
CVE-2026-48277 — Critical (CVSS 10.0): ColdFusion versions 2025.9, 2023.20 and earlier are affected by an Improper Input Validation vulnerability that could…
CVE-2026-48055 — Critical (CVSS 10.0): Streambert is a cross-platform Electron Desktop App to stream and download any video media. In versions 2.4.0 and…
CVE-2026-34910 — Critical (CVSS 10.0): A malicious actor with access to the network could exploit an Improper Input Validation vulnerability found in UniFi OS…
CVE-2026-33587 — Critical (CVSS 10.0): Lack of user input sanitisation in Open Notebook v1.8.3 allows the application user to execute Python code (and…

Browse all CWE-20 (Improper Input Validation) vulnerabilities →