CVE-2025-30349: Horde IMP XSS Vulnerability

CVE-2025-30349 Overview

CVE-2025-30349 is a Cross-Site Scripting (XSS) vulnerability affecting Horde IMP through version 6.2.27 when used with Horde Application Framework through version 5.2.23. This vulnerability allows attackers to achieve account takeover via specially crafted HTML email messages that exploit insufficient sanitization of the onerror attribute. The attack vector leverages base64-encoded JavaScript code embedded within malicious email content, and has been confirmed as exploited in the wild in March 2025.

Horde IMP is a popular open-source webmail application used by organizations worldwide for managing email communications. The vulnerability's network-based attack vector combined with its ability to compromise user sessions makes this a significant threat to enterprise email environments.

Critical Impact

This XSS vulnerability enables complete account takeover through malicious email messages, with confirmed in-the-wild exploitation in March 2025. Attackers can steal session tokens, credentials, and execute arbitrary actions on behalf of compromised users.

Affected Products

• Horde IMP through version 6.2.27
• Horde Application Framework through version 5.2.23
• Horde Webmail version 5.2.22 and earlier

Discovery Timeline

• 2025-03-21 - CVE-2025-30349 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-30349

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw exists in how Horde IMP processes HTML email content before rendering it to users.

When a user views a malicious email in Horde IMP, the application fails to properly sanitize certain HTML attributes, specifically the onerror event handler. An attacker can craft an email containing HTML elements with malicious JavaScript code encoded in base64 format within the onerror attribute. When the email is rendered in the victim's browser, the JavaScript executes in the context of the authenticated user session.

The attack requires no user interaction beyond viewing the malicious email, making it particularly dangerous. Since the JavaScript executes within the authenticated session context, attackers can steal session cookies, capture credentials, modify account settings, send emails on behalf of the victim, or perform any action the compromised user is authorized to perform.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the HTML email rendering pipeline. Specifically, the Horde IMP application does not adequately sanitize HTML attributes that can contain JavaScript event handlers. The onerror attribute on HTML elements (such as <img> tags) can trigger JavaScript execution when an error condition occurs, and the application fails to strip or encode these potentially dangerous attributes before rendering email content to users.

The use of base64 encoding allows attackers to obfuscate their malicious payloads, potentially bypassing pattern-based security filters that might detect common XSS attack strings.

Attack Vector

The attack is executed through specially crafted email messages sent to victims using Horde IMP webmail. The attacker constructs an HTML email containing elements with onerror event handlers that decode and execute base64-encoded JavaScript.

When the victim opens or previews the malicious email in their Horde IMP interface, the browser attempts to render the HTML content. If the element triggers an error condition (such as an image failing to load), the malicious JavaScript in the onerror handler executes with full access to the user's authenticated session. This allows the attacker to exfiltrate session tokens to an external server, effectively taking over the victim's account without requiring their credentials.

The vulnerability is exploitable via network access without requiring any privileges, and no user interaction beyond viewing the email is necessary for exploitation.

Detection Methods for CVE-2025-30349

Indicators of Compromise

• Unusual base64-encoded strings within HTML email content, particularly in event handler attributes
• Network requests to unfamiliar external domains originating from webmail sessions
• Unauthorized account activity such as unexpected email sends, setting changes, or forwarding rules
• JavaScript execution errors in browser console logs during email viewing
• Session tokens being transmitted to external endpoints

Detection Strategies

• Implement email content scanning to detect suspicious onerror, onload, and other JavaScript event handlers in incoming mail
• Deploy web application firewalls (WAF) with XSS detection rules specifically monitoring webmail traffic
• Monitor for anomalous session behavior such as rapid geographic changes or simultaneous sessions from disparate locations
• Enable Content Security Policy (CSP) headers to restrict inline script execution and report violations
• Review web server logs for requests containing base64-encoded JavaScript patterns

Monitoring Recommendations

• Configure SIEM alerts for suspicious JavaScript execution patterns in webmail application logs
• Monitor outbound network connections from the webmail server for data exfiltration attempts
• Implement user behavior analytics to detect account activities inconsistent with normal user patterns
• Enable browser security logging and collect CSP violation reports
• Regularly audit email forwarding rules and account recovery settings for unauthorized modifications

How to Mitigate CVE-2025-30349

Immediate Actions Required

• Update Horde IMP to a version newer than 6.2.27 that includes the security patch
• Update Horde Application Framework to a version newer than 5.2.23
• Review user accounts for signs of compromise including unexpected forwarding rules or account setting changes
• Force session invalidation for all active webmail sessions as a precautionary measure
• Implement Content Security Policy headers to mitigate XSS impact while preparing for updates

Patch Information

Organizations should apply security updates as documented in the official Horde project resources. For detailed patch information, refer to the Horde IMP Release v6.2.27, Horde Base Release v5.2.23, and the Debian LTS Security Announcement. Additional technical details and discussion can be found in the Horde IMP Mailing List Archive and the GitHub CVE-2025-30349 Report.

Workarounds

• Configure mail transport agents to strip potentially dangerous HTML attributes from incoming emails before delivery
• Implement network-level email filtering to quarantine messages containing suspicious HTML patterns
• Consider temporarily disabling HTML email rendering and forcing plaintext display until patches can be applied
• Deploy a reverse proxy with XSS filtering capabilities in front of the Horde webmail application
• Restrict webmail access to trusted networks via VPN while remediation is in progress

# Example: Apache Content Security Policy header configuration
# Add to Horde virtual host configuration to mitigate XSS impact
<IfModule mod_headers.c>
    Header always set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';"
    Header always set X-Content-Type-Options "nosniff"
    Header always set X-XSS-Protection "1; mode=block"
</IfModule>