medium CVSS 5.3 Actively exploited EPSS 1.1%

CVE-2025-35939·PHP vulnerability

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.

Severity
medium
Software
PHP
Fixed in
5.7.5
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 1.1% — higher than 62% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

From: 5.0.0

Until: 5.7.5

Fixed in: 5.7.5

How to fix this CVE

Update Craft CMS immediately to version 5.7.5 or later to patch the session file injection vulnerability. The issue allows unauthenticated attackers to write arbitrary content into predictable session file locations, which could be leveraged with a second vulnerability for code execution. Ensure all servers running Craft CMS 5.0.0–5.7.4 are patched without delay, as this vulnerability is actively exploited in the wild.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST /login or GET requests with crafted redirect parameters containing unusual characters or PHP code patterns; session files in /var/lib/php/sessions/ with names matching sess_[alphanumeric] containing suspicious PHP syntax or encoded payloads

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement input validation on redirect/return URL parameters to block requests containing PHP syntax (<?php, base64_encoded content, or protocol handlers). Use a WAF rule to detect and block session parameter injection patterns: (ARGS:return) contains (<?|base64|php:|eval)

How to check if you are affected

  1. Run `php --version` to check your current PHP version and confirm Craft CMS version in your application's composer.json or config file
  2. Review the Craft CMS version: `grep -r '"version"' config/cms/` or check the `/admin` dashboard for the displayed version number
  3. Search application logs for unusual session file creation patterns: `grep -r "sess_" /var/lib/php/sessions/ | head -20` and examine modification times for recent unauthenticated requests
  4. Verify the patch was applied by running `php -r 'echo phpversion();'` and confirming Craft CMS reports version 5.7.5 or later in the admin panel

FAQ

What is CVE-2025-35939?

This vulnerability in Craft CMS allows unauthenticated users to inject arbitrary content (including PHP code) into server-side session files by manipulating redirect parameters during authentication flows. Session files are written to predictable locations and could be executed if chained with another vulnerability.

Is CVE-2025-35939 being actively exploited?

Yes, this CVE is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and is actively exploited in the wild. Immediate patching is strongly recommended.

What versions of Craft CMS are affected by CVE-2025-35939?

Craft CMS versions 5.0.0 through 5.7.4 are vulnerable. Version 5.7.5 and later contain the fix. Craft CMS 4.15.3 and later also address this issue for the 4.x branch.

How do I check if my server is vulnerable to CVE-2025-35939?

Run `grep -i version /path/to/craft/cms/composer.json` or check the admin dashboard to determine your Craft CMS version. If it is below 5.7.5 (or 4.15.3 for version 4.x), your installation is vulnerable.

Does Defensia detect CVE-2025-35939?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Craft CMS is detected on a monitored server, CVE-2025-35939 will appear in your dashboard with remediation steps.

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References

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