CVE-2025-55129: Revive Adserver Auth Bypass Vulnerability

CVE-2025-55129 Overview

CVE-2025-55129 is a medium-severity username impersonation vulnerability affecting Revive Adserver. This vulnerability exists in the username handling functionality and allows attackers to bypass previous security fixes implemented for CVE-2025-52672. The flaw enables malicious actors to impersonate legitimate users through several alternate techniques, including homoglyph-based attacks where visually similar characters are substituted to create deceptive usernames.

Critical Impact

Attackers can impersonate legitimate users by exploiting improper unicode character handling in usernames, potentially leading to unauthorized access, social engineering attacks, and trust exploitation within the advertising platform.

Affected Products

• Revive Adserver (specific versions not disclosed)

Discovery Timeline

• 2025-12-02 - CVE-2025-55129 published to NVD
• 2025-12-02 - Last updated in NVD database

Technical Details for CVE-2025-55129

Vulnerability Analysis

This vulnerability is classified under CWE-176 (Improper Handling of Unicode Encoding) and carries a CVSS 3.0 score of 5.4 (Medium). The CVSS vector CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N indicates that the attack can be executed over the network with low complexity, requires low privileges, and does not require user interaction. The vulnerability impacts both confidentiality and integrity at a low level.

The Exploit Prediction Scoring System (EPSS) assigns this vulnerability a probability score of 0.027% with a percentile ranking of 6.798, suggesting a relatively low likelihood of exploitation in the wild based on current threat intelligence.

Root Cause

The root cause of CVE-2025-55129 lies in the incomplete fix for the earlier CVE-2025-52672 vulnerability. The username validation logic in Revive Adserver fails to properly handle Unicode encoding and character normalization. Specifically, the application does not adequately filter or normalize homoglyphs—characters from different character sets that appear visually identical or similar to standard ASCII characters.

When users register or authenticate, the system fails to canonicalize usernames before comparison, allowing attackers to create accounts with visually indistinguishable names from legitimate administrators or trusted users.

Attack Vector

The attack exploits the network-accessible username handling functionality. An authenticated attacker with low privileges can create usernames using homoglyphs (such as Cyrillic characters that visually resemble Latin letters) to impersonate other users. For example, using the Cyrillic 'а' (U+0430) instead of the Latin 'a' (U+0061) in a username like "admin" would create a visually identical but technically different username.

The vulnerability allows multiple exploitation techniques beyond homoglyphs, as reported by security researchers. These alternate methods were discovered after the initial CVE-2025-52672 fix was deployed, demonstrating that the original mitigation was insufficient.

Detection Methods for CVE-2025-55129

Indicators of Compromise

• Usernames containing Unicode characters from unexpected character sets (Cyrillic, Greek, etc.)
• Multiple accounts with visually similar or identical-appearing usernames
• Authentication logs showing login attempts for accounts with non-ASCII characters
• User reports of impersonation or suspicious account activity

Detection Strategies

Organizations can implement detection strategies focusing on Unicode anomaly detection in user registration and authentication flows. Security teams should audit existing usernames for homoglyph patterns and implement logging that captures the raw Unicode codepoints of usernames rather than just their rendered form.

Database queries can be crafted to identify accounts where username characters fall outside expected ASCII ranges. Additionally, monitoring for accounts created shortly after legitimate high-privilege accounts may indicate targeted impersonation attempts.

Monitoring Recommendations

Security teams should implement enhanced logging for user registration and authentication events, specifically capturing Unicode codepoint information. Regular audits of the user database for homoglyph patterns should be conducted. Consider deploying application-layer monitoring that alerts on username registration attempts containing mixed-script characters or known homoglyph patterns.

SentinelOne customers can leverage the platform's behavioral detection capabilities to identify suspicious authentication patterns and user impersonation activities within their environments.

How to Mitigate CVE-2025-55129

Immediate Actions Required

• Audit existing user accounts in Revive Adserver for homoglyph-based username variations
• Implement additional input validation to restrict usernames to ASCII characters only
• Review and restrict user registration capabilities until a patch is applied
• Monitor authentication logs for suspicious username patterns

Patch Information

Organizations should monitor the official Revive Adserver security advisories and the HackerOne report (https://hackerone.com/reports/3434156) for patch release information. Given that this vulnerability bypasses the previous fix for CVE-2025-52672, ensure that any new patches are applied promptly upon release.

Workarounds

Until an official patch is available, administrators should consider implementing the following workarounds:

Implement strict username validation at the application or web server level that restricts usernames to basic ASCII alphanumeric characters. This can be achieved through input validation rules that reject any characters outside the A-Z, a-z, 0-9 range and selected safe special characters.

Additionally, organizations can implement confusable detection libraries (such as ICU's confusable detection) at the application layer to identify and reject usernames that could be confused with existing accounts. Regular username audits and manual review of new account registrations for high-privilege impersonation attempts are also recommended as interim measures.