CVE-2025-41066: Horde Groupware User Enumeration Flaw

CVE-2025-41066 Overview

CVE-2025-41066 is a user enumeration vulnerability in Horde Groupware v5.2.22 that allows unauthenticated attackers to determine the existence of valid user accounts on the system. This information disclosure flaw exists in the attachment handling endpoint and can be exploited remotely without any authentication requirements.

The vulnerability is classified as CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor). By sending crafted HTTP requests to the /imp/attachment.php endpoint with specific parameters, attackers can distinguish between valid and invalid usernames based on the server's response behavior.

Critical Impact

Unauthenticated attackers can enumerate valid user accounts, enabling targeted attacks such as credential stuffing, phishing campaigns, or brute-force authentication attempts against confirmed accounts.

Affected Products

• Horde Groupware version 5.2.22

Discovery Timeline

• 2025-12-02 - CVE CVE-2025-41066 published to NVD
• 2025-12-03 - Last updated in NVD database

Technical Details for CVE-2025-41066

Vulnerability Analysis

This information disclosure vulnerability (CWE-200) affects the Horde Groupware webmail application's attachment handling functionality. The vulnerability has a CVSS 4.0 score of 6.9 (Medium) with the following vector string:

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N

Key CVSS metrics indicate:

• Attack Vector (AV): Network - exploitable remotely
• Attack Complexity (AC): Low - no special conditions required
• Privileges Required (PR): None - unauthenticated exploitation
• User Interaction (UI): None - no victim participation needed
• Confidentiality Impact (VC): Low - limited information disclosure

The EPSS (Exploit Prediction Scoring System) data shows a probability of 0.05% with a percentile ranking of 15.653, indicating a relatively low likelihood of active exploitation in the wild.

Root Cause

The root cause of CVE-2025-41066 lies in improper access control within the /imp/attachment.php endpoint. The application fails to properly validate authentication before processing user-related requests, allowing the endpoint to leak information about account validity through differential response behavior.

When processing requests with the id and u parameters, the server exhibits different behaviors based on whether the specified username exists in the system. This oracle can be leveraged by attackers to systematically enumerate valid user accounts without any authentication.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending HTTP requests to the /imp/attachment.php endpoint with the id and u parameters.

The vulnerability manifests through a timing/response oracle:

• If the specified user exists, the server returns an empty file download response
• If the specified user does not exist, no download is initiated

This binary response behavior allows attackers to definitively determine whether a username is valid on the target system. The attack can be automated to enumerate large lists of potential usernames, building a list of valid accounts for subsequent attacks.

For detailed technical information, refer to the INCIBE-CERT advisory.

Detection Methods for CVE-2025-41066

Indicators of Compromise

• High volume of HTTP requests to /imp/attachment.php from a single source
• Sequential or automated requests with varying u parameter values
• Requests to the attachment endpoint without valid session cookies
• Unusual patterns of requests with different username values in rapid succession

Detection Strategies

Web Server Log Analysis:
Monitor web server access logs for suspicious patterns targeting the /imp/attachment.php endpoint. Look for:

• Multiple requests with different u parameter values from the same IP address
• Requests lacking valid session identifiers
• Automated request patterns (consistent timing, sequential usernames)

Web Application Firewall (WAF) Rules:
Implement WAF rules to detect and rate-limit requests to the vulnerable endpoint. Alert on unusual request volumes or patterns consistent with enumeration attempts.

Network Traffic Monitoring:
SentinelOne Singularity platform can detect anomalous HTTP traffic patterns indicative of reconnaissance activities. The behavioral AI engine identifies enumeration attempts by analyzing request frequency, parameter variations, and response patterns.

Monitoring Recommendations

Organizations running Horde Groupware should implement the following monitoring measures:

1. Enable detailed logging for the /imp/attachment.php endpoint
2. Configure alerting for failed authentication attempts following enumeration patterns
3. Monitor for subsequent attack phases such as credential stuffing or brute-force attempts against enumerated accounts
4. Implement network-level monitoring to detect automated scanning tools

How to Mitigate CVE-2025-41066

Immediate Actions Required

• Review web server logs for evidence of exploitation attempts against /imp/attachment.php
• Implement IP-based rate limiting on the vulnerable endpoint
• Consider restricting access to /imp/attachment.php to authenticated users only via web server configuration
• Deploy Web Application Firewall rules to block enumeration attempts
• Monitor for vendor security updates and patches

Patch Information

As of the last modified date (2025-12-03), no vendor patch information is available in the CVE data. Organizations should:

1. Monitor Horde's official security announcements for patches
2. Check the INCIBE-CERT advisory for updated mitigation guidance
3. Consider upgrading to newer versions if patches become available

Workarounds

Until an official patch is available, organizations can implement the following workarounds to reduce exposure:

Web Server Access Restrictions:
Configure your web server to require authentication before accessing the vulnerable endpoint, or restrict access based on IP address.

# Apache configuration example - restrict access to attachment.php
<Location "/imp/attachment.php">
    Require ip 192.168.1.0/24
    # Or require authentication
    # AuthType Basic
    # AuthName "Restricted Access"
    # AuthUserFile /etc/apache2/.htpasswd
    # Require valid-user
</Location>

Rate Limiting:
Implement rate limiting to slow down enumeration attempts:

# Nginx rate limiting configuration
limit_req_zone $binary_remote_addr zone=attachment_limit:10m rate=5r/m;

location /imp/attachment.php {
    limit_req zone=attachment_limit burst=2 nodelay;
}

Network Segmentation:
If Horde Groupware is only used internally, restrict external access to the application through firewall rules or VPN requirements.