Executive Summary

CVE-2026-41161 is a security vulnerability in the Sync-in Server application, specifically affecting versions 2.1.0 and below. It involves a logic flaw in the /api/auth/login endpoint that allows unauthenticated remote attackers to enumerate valid usernames by analyzing the application's response time.

The vulnerability arises because the system responds faster to invalid usernames (95-100ms) than to valid ones (350-400ms). This timing difference enables attackers to determine whether a username exists in the system, facilitating user account enumeration.

This issue is classified as a timing attack and is related to CWE-208, which involves information exposure through timing discrepancies. The vulnerability was reported and fixed in version 2.2.0 by implementing measures to prevent such timing attacks.

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Impact Analysis

This vulnerability can impact you by allowing attackers to enumerate valid usernames on your Sync-in Server instance without authentication.

• Facilitation of brute-force attacks by knowing which usernames are valid.
• Credential stuffing attacks using known valid usernames.
• Social engineering attacks targeting identified users.

Overall, this can lead to unauthorized access attempts and compromise of user accounts, increasing the risk to your system's security.

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Detection Guidance

This vulnerability can be detected by analyzing the response times of the /api/auth/login endpoint when submitting different usernames. Valid usernames cause the server to respond slower (350-400ms) compared to invalid usernames (95-100ms).

To detect this on your system or network, you can perform timing analysis by sending login requests with various usernames and measuring the response times to identify discrepancies that indicate valid usernames.

• Use curl or similar tools to send POST requests to the /api/auth/login endpoint with different usernames and measure response times.
• Example command using curl and time measurement in a Unix shell:

time curl -X POST https://your-sync-in-server/api/auth/login -d '{"username":"testuser","password":"any"}' -H 'Content-Type: application/json'

By comparing the real time output for different usernames, you can detect timing differences that reveal valid usernames.

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Mitigation Strategies

The immediate step to mitigate this vulnerability is to upgrade the Sync-in Server application to version 2.2.0 or later, where the timing attack flaw has been fixed.

This update implements measures to prevent timing attacks that reveal whether a username exists by normalizing response times.

If upgrading immediately is not possible, consider implementing temporary mitigations such as adding artificial delays to login responses or monitoring for unusual login attempts that may indicate enumeration attacks.

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Compliance Impact

The vulnerability allows unauthenticated remote attackers to enumerate valid usernames by measuring response times, which can lead to targeted attacks such as brute-force, credential stuffing, and social engineering.

Such exposure of user information can increase the risk of unauthorized access to personal data, potentially impacting compliance with data protection regulations like GDPR and HIPAA that require safeguarding user identity and access controls.

By enabling attackers to confirm valid usernames, the vulnerability weakens the security posture of the system, making it harder to meet standards that mandate protection against unauthorized data disclosure and identity enumeration.

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