CVE-2025-27794: Flarum Vulnerable to Session Hijacking via Authoritative Subdomain Cookie Overwrite
Description
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.
Classification
CVE ID: CVE-2025-27794
CVSS Base Severity: MEDIUM
CVSS Base Score: 6.8
CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
Problem Types
CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')Affected Products
Vendor: flarum
Product: framework
Exploit Prediction Scoring System (EPSS)
EPSS Score: 0.05% (probability of being exploited)
EPSS Percentile: 13.45% (scored less or equal to compared to others)
EPSS Date: 2025-04-10 (when was this score calculated)
References
https://nvd.nist.gov/vuln/detail/CVE-2025-27794https://github.com/flarum/framework/security/advisories/GHSA-hg9j-64wp-m9px
https://github.com/flarum/framework/commit/a05aaea3ee1e0a8b870935183193cd6052f1d402
https://github.com/flarum/framework/releases/tag/v1.8.10