How severe is CVE-2021-21283?

CVE-2021-21283 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21283?

CVE-2021-21283 affects the following packages: flarum/sticky (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-21283?

Update flarum/sticky to 0.1.0-beta.15.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21283 is resolved across your whole dependency graph.

How do I detect CVE-2021-21283 in my Packagist dependencies?

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

How do I mitigate CVE-2021-21283 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-21283 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21283 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21283?

CVE-2021-21283 has an EPSS (Exploit Prediction Scoring System) score of 0.8%, placing it in the 55th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21283?

CVE-2021-21283 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-21283 published, and has it been updated?

CVE-2021-21283 was published on January 26, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

MEDIUM severity

CVE-2021-21283 — sticky

MEDIUMFix: flarum/sticky@7ebd304

CVE-2021-21283 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in flarum/sticky. A fix is available for flarum/sticky — see the affected versions and patch details below.

XSS in Flarum Sticky extension

Also known asGHSA-h3gg-7wx2-cq3h

Published

Jan 26, 2021

Updated

Jul 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs55th percentile — riskier than 55% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-21283 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

🐘flarum/sticky

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

Impact

A change in release beta 14 of the Sticky extension caused the plain text content of the first post of a pinned discussion to be injected as HTML on the discussion list. The issue was discovered following an internal audit.

Any HTML would be injected through Mithril's m.trust() helper. This resulted in an HTML injection where <script> tags would not be executed. However it was possible to run javascript from other HTML attributes, enabling a cross-site scripting (XSS) attack to be performed.

Since the exploit only happens with the first post of a pinned discussion, an attacker would need the ability to pin their own discussion, or be able to edit a discussion that was previously pinned.

On forums where all pinned posts are authored by your staff, you can be relatively certain the vulnerability has not been exploited.

Forums where some user-created discussions were pinned can look at the first post edit date to find whether the vulnerability might have been exploited. Because Flarum doesn't store the post content history, you cannot be certain if a malicious edit was reverted.

Patches

The fix will be available in version v0.1.0-beta.16 with Flarum beta 16. The fix has already been back-ported to Flarum beta 15 as version v0.1.0-beta.15.1 of the Sticky extension.

Workarounds

Forum administrators can disable the Sticky extension until they are able to apply the update. The vulnerability cannot be exploited while the extension is disabled.

References

For more information

If you have any questions or comments about this advisory, please start a new discussion on our support forum.

If you discover a security vulnerability within Flarum, please send an e-mail to [email protected]. All security vulnerabilities will be promptly addressed. More details can be found in our security policy.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`flarum/sticky`	≥ 0.1.0-beta.14&&< 0.1.0-beta.15.1	0.1.0-beta.15.1`composer require flarum/sticky:^0.1.0-beta.15.1`

Affected Products

1 product · 2 configurations

Application

stickyflarum

1 version

0.1.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flarum/sticky, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update flarum/sticky to 0.1.0-beta.15.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21283 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Impact A change in release beta 14 of the Sticky extension caused the plain text content of the first post of a pinned discussion to be injected as HTML on the discussion list. The issue was discovered following an internal audit. Any HTML would be injected through Mithril's `m.trust()` helper. This resulted in an HTML injection where `<script>` tags would not be executed. However it was possible to run javascript from other HTML attributes, enabling a cross-site scripting (XSS) attack to be performed. Since the exploit only happens with the first post of a pinned discussion, an attacker