CVE-2026-25133: 
PHP vulnerability analysis and mitigation

CVE-2026-25133 is a stored cross-site scripting (XSS) vulnerability in the SVG sanitization logic of October CMS (the october/rain Composer package). The flaw exists in versions prior to 3.7.14 (3.x branch) and prior to 4.1.10 (4.x branch, starting from 4.0.0). It was discovered by Pentest-Tools.com, published on April 14, 2026, and patched the same day. The vulnerability carries a CVSS v3.1 base score of 4.8 (Medium) and a CVSS v4.0 base score of 4.8 (Medium) (GitHub Advisory, October Security Advisory).

The root cause is improper neutralization of user-controllable input during web page generation (CWE-79). October CMS uses a regex pattern to strip on* event handler attributes (e.g., onclick, onload) from uploaded SVG files as part of its sanitization logic; however, this pattern can be bypassed by crafting a payload that exploits how the regex matches attribute boundaries, allowing embedded JavaScript to survive the sanitization step. An authenticated backend user with media.library.create permissions can upload a malicious SVG through the Media Manager, and the payload executes when a superuser views or embeds the SVG in a page. No public proof-of-concept code has been identified at this time (GitHub Advisory, October Security Advisory).

Successful exploitation results in stored XSS execution in the context of any user — including superusers — who views or embeds the malicious SVG, leading to low confidentiality and low integrity impacts with no availability impact. The most significant risk is privilege escalation: if a superuser's session is hijacked via the injected JavaScript, an attacker with only media upload permissions could gain administrative control over the CMS. Availability is not directly affected, and there is no evidence of lateral movement beyond the CMS application boundary (GitHub Advisory, Feedly).

There is no known public proof-of-concept exploit and no evidence of in-the-wild exploitation at this time. The EPSS score is approximately 0.012% (1st percentile), indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires authenticated backend access with media upload permissions, significantly limiting the attacker pool (GitHub Advisory, Feedly).

1. Gain authenticated access: Obtain backend credentials for an October CMS account with media.library.create permissions (e.g., a compromised editor or contributor account).
2. Craft malicious SVG: Create an SVG file containing a JavaScript payload embedded in an event handler attribute using a boundary-exploiting format that bypasses the on* regex sanitization (e.g., manipulating whitespace, case, or attribute quoting around onload or onclick).
3. Upload via Media Manager: Log into the October CMS backend and upload the crafted SVG file through the Media Manager interface.
4. Trigger execution: Wait for or socially engineer a superuser to view the SVG directly or embed it in a CMS page. When the SVG is rendered in the browser, the embedded JavaScript executes in the superuser's session context.
5. Achieve privilege escalation: Use the executed JavaScript to exfiltrate the superuser's session token, perform unauthorized administrative actions, or create a new privileged account (October Security Advisory, GitHub Advisory).

• File System: Presence of SVG files in the October CMS Media Manager directory containing on* event handler attributes (e.g., onload, onclick, onerror) with obfuscated or boundary-exploiting formatting; unexpected SVG files uploaded by non-administrative accounts.
• Logs: CMS backend access logs showing SVG file uploads by accounts with media.library.create permissions, particularly from unfamiliar IP addresses; subsequent access to those SVG files by superuser accounts.
• Network: Outbound HTTP requests from the CMS server or from superuser browsers to unknown external domains shortly after SVG file views, potentially indicating session token exfiltration or beacon callbacks.
• Application: New or modified administrator accounts created without a corresponding legitimate administrative action; unexpected changes to CMS configuration or content following superuser sessions that included SVG viewing (October Security Advisory).

Upgrade October CMS to version 3.7.14 (3.x branch) or 4.1.10 (4.x branch) to remediate the vulnerability. If immediate upgrading is not possible, two workarounds are available: (1) disable SVG uploads by adding svg to the blocked extensions list in the media configuration, or (2) ensure media.clean_vectors is set to true in the configuration (this is the default setting). Additionally, restricting media upload permissions to only highly trusted users reduces the attack surface (October Security Advisory, GitHub Advisory).

The vulnerability was reported to the October CMS team by Pentest-Tools.com and was patched promptly on the same day it was publicly disclosed (April 14, 2026). The advisory was published by October CMS maintainer daftspunk and reviewed by GitHub's advisory database. No significant broader media coverage or notable community commentary beyond the official advisory has been identified (October Security Advisory).