CVE-2019-25337: Php

Overview

A critical vulnerability has been identified in OwnCloud 8.1.8 that allows an unauthenticated, remote attacker to discover all valid usernames on the system. This flaw resides in the file-sharing functionality and can be exploited with minimal effort.

Vulnerability Explained

In simple terms, this vulnerability turns a normal search feature into an information leak. The share.php endpoint, which handles file sharing, did not properly restrict user lookup searches. An attacker can send a specific web request containing a wildcard character (like *) to this endpoint. Instead of returning an error or no results, the system responds with a list of all registered usernames and their associated details.

Potential Impact

The impact of this vulnerability is severe and serves as a critical first step for further attacks:

• Credential Attacks: With a list of valid usernames, attackers can launch highly targeted password spraying or brute-force attacks, significantly increasing their chance of success.
• Social Engineering: Knowledge of account names can be used to craft convincing phishing emails or other social engineering schemes.
• Privacy Breach: It exposes the structure of an organization’s user base, which may be sensitive information in itself.
• Pre-Attack Reconnaissance: It provides a foundational map of the system for planning more complex intrusions.

Given that this can be performed by anyone on the internet without any login credentials, it has been rated with a maximum CVSS score of 9.8 (CRITICAL).

Remediation and Mitigation

Immediate action is required for any instance running OwnCloud 8.1.8.

Primary Remediation:

1. Upgrade Immediately: This vulnerability is addressed in later versions of OwnCloud. Upgrade to a patched version as soon as possible. Consult the OwnCloud security advisories for the specific version that includes the fix.
2. Apply the Patch: If an immediate upgrade is not feasible, apply the official security patch provided by OwnCloud directly to version 8.1.8.

Temporary Mitigation (if patching is delayed):

• Restrict Access: Use a web application firewall (WAF) to block requests containing wildcard characters (*, %) in parameters sent to the /index.php/core/ajax/share.php endpoint.
• Network Controls: Consider restricting access to the OwnCloud instance to trusted IP addresses (e.g., corporate VPN ranges) until the patch can be applied. This reduces the attack surface to internal or authorized users only.

General Recommendation: After patching, encourage all users to employ strong, unique passwords and consider implementing multi-factor authentication (MFA) to add an extra layer of defense, particularly since usernames may have been exposed.