What is CVE-2025-27794?

CVE-2025-27794 is a medium-severity vulnerability in Flarum, classified under Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection). CVSS score: 6.8/10. Published 2025-03-12.

How severe is CVE-2025-27794?

Medium severity. CVSS v3 base score is 6.8 out of 10.

Is CVE-2025-27794 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Flarum

CVE-2025-27794

Flarum is open-source forum software. A session hijacking vulnerability exists in versions prior to 1.8.10 when an attacker-controlled authoritative subdomain under a parent domain (e.g., `subdomain.host.com`) sets cookies scoped to the parent domain (`.host.com`). This allows session token replacement for applications hosted on sibling subdomains (e.g., `community.host.com`) if session tokens aren't rotated post-authentication. Key Constraints are that the attacker must control any subdomain under the parent domain (e.g., `evil.host.com` or `x.y.host.com`), and the parent domain must not be on the Public Suffix List. Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described. Version 1.8.10 contains a patch for the issue.

Published 2025-03-12 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N.

EPSS exploit prediction

EPSS: 0.005 (39.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 40th percentile — 39.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: +0.000

Affected products

Flarum
Flarum Framework — versions < 1.8.10

Weakness classification (CWE)

CWE-74 — Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection)

Public proof-of-concept exploits

fkie-cad/nvd-json-data-feeds

References

github.com/flarum/framework/security/advisories/GHSA-hg9j-64wp-m9px (Vendor Advisory)
github.com/flarum/framework/commit/a05aaea3ee1e0a8b870935183193cd6052f1d402 (Patch)
github.com/flarum/framework/releases/tag/v1.8.10 (Release Notes)

Frequently asked questions

What is CVE-2025-27794?: CVE-2025-27794 is a medium-severity vulnerability in Flarum, classified under Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection). CVSS score: 6.8/10. Published 2025-03-12.
How severe is CVE-2025-27794?: Medium severity. CVSS v3 base score is 6.8 out of 10.
Is CVE-2025-27794 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.