CVE-2026-58271: Sync-in Server Auth Bypass Vulnerability

CVE-2026-58271 Overview

CVE-2026-58271 affects Sync-in Server, an open-source platform for file storage, sharing, collaboration, and syncing. The vulnerability exists in versions prior to 2.4.0 and stems from a broken lockout mechanism in the desktop sync client registration endpoint. When a user submits an invalid time-based one-time password (TOTP) to POST /api/app/sync/register, the server fails to increment the passwordAttempts counter. Attackers with valid credentials can brute-force the six-digit TOTP without triggering account lockout. A successful guess yields sync client tokens that convert into a full JSON Web Token (JWT) and allow permanent multi-factor authentication (MFA) removal.

Critical Impact
Attackers holding valid user credentials can bypass MFA through unrestricted TOTP brute-forcing and permanently disable 2FA on the compromised account.

Affected Products

Sync-in Server versions prior to 2.4.0
Desktop sync client registration endpoint (/api/app/sync/register)
2FA management endpoint (/api/auth/2fa/disable)

Discovery Timeline

2026-09-21 - CVE-2026-58271 published to the National Vulnerability Database (NVD)
2026-09-21 - Last updated in NVD database

Technical Details for CVE-2026-58271

Vulnerability Analysis

The flaw is an Improper Restriction of Excessive Authentication Attempts weakness [CWE-307] in the sync client registration flow. The endpoint POST /api/app/sync/register accepts a username, password, and TOTP code to enroll a new desktop sync client. On a failed TOTP verification, SyncClientsManager.register() invokes updateAccesses(user, ip, false). That call routes through a freeze branch that writes the existing passwordAttempts value back unchanged instead of incrementing it. Because the counter never reaches the USER_MAX_PASSWORD_ATTEMPTS threshold of 10, the lockout gate never activates.

Once an attacker guesses the correct six-digit TOTP within its validity window, the server returns a {clientId, clientToken} pair. The clientToken is exchangeable at POST /api/app/sync/auth/cookie for a full JWT session. With that JWT, the attacker can invoke POST /api/auth/2fa/disable to permanently remove MFA from the account, converting a temporary bypass into persistent single-factor access.

Root Cause

The root cause is a logic error in updateAccesses(). The freeze branch executed on failed TOTP validation does not increment the failure counter before persisting it, defeating the intended rate-limiting control. Attack surface expands because the sync registration endpoint reuses the primary authentication path without honoring the same lockout policy.

Attack Vector

Exploitation requires network access to the Sync-in Server and valid primary credentials for a target account. The attacker repeatedly submits registration requests cycling through TOTP values. Because each attempt is treated as non-consuming for lockout purposes, the search space of one million codes is bounded only by TOTP window rotation and server throughput. See the GitHub Security Advisory GHSA-274f-6w77-8qm9 for technical details.

Detection Methods for CVE-2026-58271

Indicators of Compromise

High-volume POST /api/app/sync/register requests from a single client IP or user agent targeting one account.
Unexpected creation of new sync client records followed by calls to /api/app/sync/auth/cookie.
Successful POST /api/auth/2fa/disable events on accounts that previously had MFA enabled.
Sync client registrations from geographic locations or ASNs inconsistent with the account owner.

Detection Strategies

Aggregate authentication telemetry from Sync-in application logs and alert when registration attempts to the same account exceed a baseline threshold.
Correlate failed TOTP submissions with subsequent 2FA disablement events within short time windows.
Baseline normal sync client enrollment cadence per user and flag statistical outliers.

Monitoring Recommendations

Forward Sync-in access logs and application logs to a centralized logging or SIEM platform for retention and correlation.
Enable alerting on any state change to the MFA configuration of privileged or administrative accounts.
Monitor JWT issuance events tied to sync client tokens and validate against expected device inventory.

How to Mitigate CVE-2026-58271

Immediate Actions Required

Upgrade Sync-in Server to version 2.4.0 or later, which corrects the counter-increment logic in updateAccesses().
Audit recent registrations at /api/app/sync/register and revoke any sync client tokens that cannot be attributed to a legitimate user action.
Review recent /api/auth/2fa/disable events and re-enroll MFA for any accounts affected without user consent.
Rotate credentials for accounts that exhibit anomalous sync registration activity.

Patch Information

The maintainers released Sync-in Server version 2.4.0, which patches the flaw by ensuring passwordAttempts is correctly incremented on failed TOTP verification. Details are published in the Sync-in GitHub Security Advisory.

Workarounds

Restrict network exposure of the Sync-in Server API to trusted networks or a VPN until patching is complete.
Deploy a reverse proxy or web application firewall (WAF) rule to rate-limit POST /api/app/sync/register per source IP and per target username.
Temporarily disable the desktop sync client registration endpoint if it is not required in the environment.

bash

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

location = /api/app/sync/register {
    limit_req zone=syncreg burst=5 nodelay;
    proxy_pass http://sync_in_backend;
}