CVE-2026-41161: Sync-in Server Auth Bypass Vulnerability

CVE-2026-41161 Overview

CVE-2026-41161 affects Sync-in Server, an open-source platform for file storage, sharing, collaboration, and syncing. The /api/auth/login endpoint contains a logic flaw that allows unauthenticated remote attackers to enumerate valid usernames by measuring the application's response time. This is an observable timing discrepancy issue classified under [CWE-208]. All versions prior to 2.2.0 are affected. The maintainers patched the vulnerability in version 2.2.0.

Critical Impact
Unauthenticated remote attackers can build a list of valid account usernames against any internet-exposed Sync-in Server, enabling targeted credential stuffing, password spraying, and phishing campaigns.

Affected Products

Sync-in Server versions prior to 2.2.0
Self-hosted Sync-in deployments exposing /api/auth/login to untrusted networks
Sync-in instances behind reverse proxies without rate limiting or response-time normalization

Discovery Timeline

2026-05-08 - CVE-2026-41161 published to NVD
2026-05-12 - Last updated in NVD database

Technical Details for CVE-2026-41161

Vulnerability Analysis

The vulnerability is a timing side-channel in the authentication path of Sync-in Server. When a client submits credentials to /api/auth/login, the server's processing time differs measurably depending on whether the submitted username exists in the user store. An attacker can issue authentication requests with candidate usernames and an arbitrary password, then compare response times. Requests for valid usernames take longer (or shorter) than requests for invalid ones, leaking account existence without ever requiring valid credentials.

This class of weakness is tracked as [CWE-208] Observable Timing Discrepancy. While the flaw does not directly disclose passwords or session tokens, it removes an attacker's uncertainty about which usernames are real, narrowing the search space for follow-on attacks.

Root Cause

The login handler performs different work paths for known and unknown accounts. A typical pattern is to skip the password hashing operation when the username lookup fails, returning an authentication error immediately. Password hashing functions such as bcrypt, scrypt, or Argon2 are intentionally expensive, so skipping them on the negative path creates a measurable timing gap. The fix shipped in version 2.2.0 normalizes the work performed for both cases so response timing no longer correlates with username validity.

Attack Vector

Exploitation requires only network reach to the /api/auth/login endpoint and no authentication or user interaction. An attacker scripts a loop that submits each candidate username from a wordlist with a fixed dummy password, records the round-trip time across multiple samples, and applies statistical analysis to classify each username as valid or invalid. The technique is described in prose only; no public proof-of-concept code is referenced in this advisory. See the GitHub Security Advisory GHSA-43fj-qp3h-hrh5 for the maintainer's description.

Detection Methods for CVE-2026-41161

Indicators of Compromise

High volumes of POST requests to /api/auth/login from a single source or distributed set of sources, each containing a different username value
Login attempts where the password field is constant or follows a trivial pattern across many distinct usernames
Sequential or alphabetical iteration through username candidates in access logs

Detection Strategies

Alert on authentication endpoints receiving more than a defined threshold of failed logins per source IP within a short window
Correlate unique-username counts per source IP against historical baselines to flag enumeration behavior
Inspect User-Agent and request-timing patterns for automated tooling signatures against /api/auth/login

Monitoring Recommendations

Forward Sync-in application logs and reverse proxy access logs to a centralized logging or SIEM platform for retention and search
Track per-IP and per-ASN request rates against authentication endpoints over rolling time windows
Monitor for follow-on credential stuffing activity targeting usernames that appeared in enumeration attempts

How to Mitigate CVE-2026-41161

Immediate Actions Required

Upgrade Sync-in Server to version 2.2.0 or later, which contains the official patch
Place authentication endpoints behind rate limiting and IP-based throttling at the reverse proxy or WAF layer
Audit recent access logs for sustained enumeration patterns against /api/auth/login and reset credentials for any accounts that appear in successful logins from suspicious sources

Patch Information

The maintainers fixed the timing discrepancy in version 2.2.0. Release notes and the upgrade artifact are available at the GitHub Release v2.2.0 page. Full vulnerability details are documented in the GitHub Security Advisory GHSA-43fj-qp3h-hrh5.

Workarounds

Restrict network access to /api/auth/login using VPN, allowlisted IP ranges, or zero-trust network access until the upgrade is applied
Enforce strict per-IP rate limits on the login endpoint at a reverse proxy such as nginx, HAProxy, or Cloudflare
Require multi-factor authentication so that enumerated usernames cannot be trivially weaponized through password spraying

bash

# Example nginx rate limit for the login endpoint
limit_req_zone $binary_remote_addr zone=syncin_login:10m rate=5r/m;

location = /api/auth/login {
    limit_req zone=syncin_login burst=3 nodelay;
    proxy_pass http://syncin_backend;
}