CVE-2025-2975: GFI Kerio Connect XSS Vulnerability

CVE-2025-2975 Overview

CVE-2025-2975 is a stored cross-site scripting (XSS) vulnerability in GFI Kerio Connect 10.0.6. The flaw resides in the Signature Handler component, specifically within the Settings/Email/Signature/EditHtmlSource processing path. An authenticated attacker can inject malicious script content into an email signature, which executes in the browser of any user who renders the affected content. The vulnerability was publicly disclosed, and according to the submitter the vendor did not respond to disclosure attempts. The weakness is tracked under CWE-79.

Critical Impact
Authenticated attackers can store malicious JavaScript in email signatures, enabling session theft, phishing, and account takeover against recipients using the Kerio Connect webmail interface.

Affected Products

GFI Kerio Connect 10.0.6
Signature Handler component (Settings/Email/Signature/EditHtmlSource)
Webmail interface rendering stored signatures

Discovery Timeline

2025-03-31 - CVE-2025-2975 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-2975

Vulnerability Analysis

The vulnerability exists in how Kerio Connect handles HTML content submitted through the signature editor. The EditHtmlSource endpoint accepts raw HTML markup to allow signature formatting, but fails to sanitize or neutralize executable script constructs before persisting the content. When a user composes, previews, or reads an email that references the stored signature, the browser interprets the attacker-controlled payload within the trusted application origin. Because the payload is persisted server-side, exploitation does not require re-delivery per victim. The attack requires low-privilege authentication and user interaction, aligning with the CWE-79 classification for improper neutralization of input during web page generation.

Root Cause

The root cause is missing output encoding and inadequate input validation in the signature HTML source editor. The application preserves attacker-supplied markup, including <script> elements or event handler attributes such as onerror and onload, and renders it directly in the Document Object Model (DOM) of other users. No Content Security Policy (CSP) enforcement blocks inline script execution in the affected view.

Attack Vector

An authenticated attacker navigates to the signature settings and submits a crafted HTML signature containing JavaScript. The payload is stored in the user profile and executed whenever the signature renders in the webmail client. A proof-of-concept has been published to a public GitHub repository and referenced in VulDB entry 302027. Successful exploitation enables cookie theft, forced actions on behalf of the victim, and credential harvesting through injected forms.

No verified exploit code is reproduced here. Refer to the public proof-of-concept for technical specifics.

Detection Methods for CVE-2025-2975

Indicators of Compromise

Stored signature records containing <script> tags, javascript: URIs, or event handler attributes such as onerror, onload, or onclick.
Outbound HTTP requests from webmail sessions to unfamiliar domains correlated with signature rendering.
Unexpected session cookie exfiltration or anomalous authenticated API calls from Kerio Connect users.

Detection Strategies

Inspect persisted signature data in the Kerio Connect user store for HTML containing executable constructs or encoded script payloads.
Review webmail HTTP access logs for POST requests to Settings/Email/Signature/EditHtmlSource paired with payloads exceeding typical signature length or containing script keywords.
Deploy browser-side telemetry or a Content Security Policy report endpoint to capture inline script violations originating from the webmail origin.

Monitoring Recommendations

Alert on administrative or privileged accounts rendering signatures that trigger CSP violations.
Monitor for lateral authentication events immediately following webmail sessions involving the affected signature view.
Correlate signature modification events with downstream credential changes or mailbox rule creation.

How to Mitigate CVE-2025-2975

Immediate Actions Required

Restrict access to the Kerio Connect webmail interface to trusted networks until a vendor patch is available.
Audit all user signature records and remove entries containing HTML script constructs or suspicious event handlers.
Rotate session tokens and credentials for any users whose signatures contained unauthorized content.

Patch Information

No vendor patch or security advisory was referenced in the available data at the time of publication. The disclosure record indicates the vendor was contacted but did not respond. Monitor the GFI Kerio Connect release notes and the VulDB entry for updates.

Workarounds

Disable or restrict the HTML source signature editor and enforce plain-text signatures where operationally feasible.
Deploy an upstream reverse proxy with a strict Content Security Policy that blocks inline scripts for the webmail origin.
Implement web application firewall (WAF) rules that strip or reject script tags and dangerous event handlers submitted to the EditHtmlSource endpoint.

bash

# Example ModSecurity rule to block script constructs in the signature editor
SecRule REQUEST_URI "@contains Settings/Email/Signature/EditHtmlSource" \
    "id:1002975,phase:2,deny,status:403,\
     chain,msg:'CVE-2025-2975 Kerio Connect XSS payload blocked'"
    SecRule REQUEST_BODY "@rx (?i)(<script|onerror=|onload=|javascript:)" \
        "t:none,t:urlDecodeUni,t:htmlEntityDecode"