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Not in CISA KEV
HIGH severity

CVE-2021-28931 — forkcms

HIGHFix: forkcms/forkcms#3351

CVE-2021-28931 is a high-severity (CVSS 8.8) Unrestricted File Upload vulnerability in forkcms/forkcms. A fix is available for forkcms/forkcms — see the affected versions and patch details below.

Arbitrary file upload in Fork CMS

Also known asGHSA-748f-wv76-x9hg
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 4, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.2%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs67th percentile — riskier than 67% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-28931 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐘forkcms/forkcms

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

Arbitrary file upload vulnerability in Fork CMS 5.9.2 allows attackers to create or replace arbitrary files in the /themes directory via a crafted zip file uploaded to the Themes panel.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐘Packagistforkcms/forkcmsall versions5.9.3composer require forkcms/forkcms:^5.9.3

Affected Products

1 product · 1 configurations
Application
fork cmsfork-cms
1 version
5.9.2

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for forkcms/forkcms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update forkcms/forkcms to 5.9.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-28931 is resolved across your whole dependency graph.

  3. Workarounds

    Treat uploaded files as untrusted until proven otherwise: validate the actual content type rather than the supplied extension, store uploads outside the web root on a volume mounted without execute permission, and rename them to server-generated identifiers so an attacker cannot choose the path a request will later resolve.

Frequently Asked Questions

Arbitrary file upload vulnerability in Fork CMS 5.9.2 allows attackers to create or replace arbitrary files in the /themes directory via a crafted zip file uploaded to the Themes panel.
O3 Security · Impact-Aware SCA

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CVE-2021-28931: forkcms — Fixed in 5.9.3