CVE-2025-35939: Craft CMS External Control of Assumed-Immutable Web Parameter Vulnerability

Vulnerability summary

CVE-2025-35939 is a medium (CVSS 6.9) vulnerability in Craft CMS. It is known to be exploited: CISA lists it, first on 02-06-2025. EPSS estimates a 1.35% chance of exploitation in the next 30 days, higher than 71% of scored vulnerabilities.

Timeline

1. 07-05-2025
   PublishedVulnerability first disclosed.
2. 02-06-2025
   Added to CISA KEVCraft CMS External Control of Assumed-Immutable Web Parameter Vulnerability
3. 23-06-2025
   CISA remediation dueApply mitigations per vendor instructions.
4. 26-02-2026
   Last modifiedVulnerability information updated.

Description

Craft CMS stores arbitrary content provided by unauthenticated users in session files. This content could be accessed and executed, possibly using an independent vulnerability. Craft CMS redirects requests that require authentication to the login page and generates a session file on the server at '/var/lib/php/sessions'. Such session files are named 'sess_[session_value]', where '[session_value]' is provided to the client in a 'Set-Cookie' response header. Craft CMS stores the return URL requested by the client without sanitizing parameters. Consequently, an unauthenticated client can introduce arbitrary values, such as PHP code, to a known local file location on the server. Craft CMS versions 5.7.5 and 4.15.3 have been released to address this issue.

• CWE-472External Control of Assumed-Immutable Web ParameterThe web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.