Horde Groupware Under Siege: Unveiling CVE-2025-41066 User Enumeration

Overview

This article details CVE-2025-41066, a user enumeration vulnerability discovered in Horde Groupware v5.2.22. This vulnerability allows unauthenticated attackers to determine the existence of valid user accounts on a Horde Groupware instance. Successfully exploiting this issue can be a precursor to further attacks, such as password brute-forcing or targeted phishing campaigns.

Technical Details

The vulnerability lies within the /imp/attachment.php script. By sending a crafted HTTP request with specific parameters, an attacker can probe the existence of user accounts. The attack leverages the following parameters:

• id: This parameter seems to be related to attachment handling but is used in conjunction with the ‘u’ parameter for the exploit.
• u: This parameter is used to specify the username being checked.

The core of the vulnerability is the server’s response. When a valid username is provided in the u parameter, the server initiates a file download (even if it’s an empty file). However, if the username is invalid, no download is initiated. This difference in server behavior provides a distinct and reliable method for attackers to identify valid users.

The specific HTTP request structure is as follows:

GET /imp/attachment.php?id=arbitrary_id&u=username_to_check HTTP/1.1

An attacker can automate this process by iterating through a list of potential usernames and observing the server’s response for each request.

CVSS Analysis

The provided information indicates that no CVSS score is available (N/A) for CVE-2025-41066. This may be due to the vulnerability being relatively new or the scoring process being incomplete. However, even without a formal CVSS score, the inherent risk of user enumeration should not be underestimated. User enumeration is often a crucial first step in reconnaissance for more impactful attacks.

While the severity is listed as N/A, this vulnerability should be considered at least a **Medium** risk due to its potential to facilitate further attacks. A lack of a CVSS score does not negate the real-world risk posed by the vulnerability.

Possible Impact

The primary impact of CVE-2025-41066 is:

• User Enumeration: An attacker can create a list of valid usernames within the Horde Groupware system.
• Increased Attack Surface: Knowing valid usernames significantly narrows the attack surface for subsequent attacks, such as:
  • Password Brute-Forcing: Attackers can focus their brute-force attempts on valid user accounts.
  • Phishing Campaigns: Targeted phishing emails can be crafted to appear legitimate, as the attacker knows the recipient is a valid user.
  • Social Engineering: Attackers can use valid usernames to impersonate users, tricking other users into divulging sensitive information.
• Data Breach Risk: While user enumeration itself doesn’t directly lead to a data breach, it significantly increases the likelihood of a successful breach by enabling more targeted attacks.

Mitigation and Patch Steps

The most effective mitigation is to upgrade to a patched version of Horde Groupware that addresses this vulnerability. Contact the Horde Groupware developers or refer to their official website for the latest security updates and patches.

In the absence of an immediate patch, consider the following temporary mitigations:

• Web Application Firewall (WAF) Rules: Implement WAF rules to detect and block suspicious requests to /imp/attachment.php that include the id and u parameters. This is complex and might break legitimate functionality.
• Rate Limiting: Implement rate limiting on requests to /imp/attachment.php to slow down automated enumeration attempts.
• Monitor Logs: Closely monitor web server logs for unusual activity related to /imp/attachment.php.

References

• INCIBE-CERT Advisory