CVE-2026-58269

Executive Summary

CVE-2026-58269 is an authentication bypass vulnerability in Sync-in Server, an open-source platform for file storage, sharing, collaboration, and syncing. Prior to version 2.4.0, the POST /api/auth/token endpoint authenticates using only username and password, then issues full access and refresh JWTs via getTokens() without verifying whether the account has TOTP 2FA enabled. An attacker with stolen or phished credentials can bypass 2FA in a single request. The vulnerability carries a CVSS score of 8.1 (High), reflecting high confidentiality and integrity impact, though no public exploit or Nuclei template exists as of this report.

Technical Analysis

The vulnerability resides in the authentication logic of Sync-in Server versions prior to 2.4.0. Specifically, the POST /api/auth/token endpoint performs credential validation using username and password, then calls getTokens(), which returns full access and refresh JSON Web Tokens (JWTs) without checking the user.twoFaEnabled flag. In contrast, the parallel login endpoint POST /api/auth/login correctly enforces 2FA by calling setCookies(user, res, true), which gates token issuance on the user.twoFaEnabled property. This inconsistency allows an attacker who possesses valid credentials—obtained through phishing, credential stuffing, or breach data—to bypass the second authentication factor entirely by targeting the token endpoint instead of the login endpoint. The flaw is classified as CWE-288: Authentication Bypass Using an Alternate Path or Channel. The attack surface is the publicly exposed API endpoint, requiring network access and low privileges (valid credentials). The CVSS vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N indicates network exploitable, low attack complexity, low privileges required, no user interaction, with high confidentiality and integrity impact but no availability impact.

Exploitation Scenarios

1. Reconnaissance/target identification: The attacker identifies a Sync-in Server instance, likely via Shodan dorks such as http.title:"Sync-in" or http.html:"/api/auth/token". The attacker confirms the presence of the API endpoint and the server version (if exposed) to determine if it is below 2.4.0.

2. Triggering the flaw: Using stolen or phished credentials (username and password), the attacker sends a POST request to /api/auth/token with the credentials. Because this endpoint does not validate the user.twoFaEnabled flag, it invokes getTokens() and returns full access and refresh JWTs. This bypasses the TOTP 2FA requirement that would normally be enforced by the /api/auth/login endpoint.

3. Payload/execution: The attacker simply submits a standard authentication request to the vulnerable endpoint. No special payload is required beyond valid credentials; the flaw is in the logic, not in input handling. The response contains the JWTs, which the attacker can then use for subsequent API calls.

4. Post-exploitation impact: With the obtained tokens, the attacker gains full access to the victim's account, including file storage, sharing, collaboration, and syncing features. This allows data exfiltration, modification, or deletion of files, and potentially lateral movement within the platform. The high confidentiality and integrity impact (C:H/I:H) reflects the ability to read and alter sensitive data. No public PoC is available; the attack chain is inferred from the vulnerability class and description.

Impact Assessment

Successful exploitation grants an attacker complete control over a targeted user account, bypassing 2FA protections. This can lead to unauthorized access to confidential files, intellectual property, and collaboration data. The integrity impact is high, as the attacker can modify or delete files. The vulnerability affects all Sync-in Server deployments prior to version 2.4.0, regardless of industry. The blast radius depends on the privileges of the compromised account; administrative accounts could lead to full platform compromise. The CVSS score of 8.1 reflects a serious risk, though it requires valid credentials, slightly lowering the base score compared to unauthenticated bypasses. No evidence of active exploitation or CISA KEV listing exists as of this report.

Detection Guidance

Sigma Rule

title: Sync-in Server 2FA Bypass via /api/auth/token
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects successful authentication to Sync-in Server's /api/auth/token endpoint without 2FA challenge, indicating potential CVE-2026-58269 exploitation.
author: Valters IT
date: 2026-01-01
logsource:
  category: web
  product: sync-in
detection:
  selection:
    cs-method: 'POST'
    cs-uri-stem: '/api/auth/token'
    sc-status: 200
  condition: selection
falsepositives:
  - Legitimate users who have not enabled 2FA
level: high
tags:
  - attack.t1078
  - cve.2026.58269

YARA Rule

rule CVE_2026_58269_Syncin_2FA_Bypass
{
    meta:
        description = "Detects Sync-in Server versions prior to 2.4.0 vulnerable to 2FA bypass via /api/auth/token"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-58269"
        cve = "CVE-2026-58269"
    strings:
        $s1 = "getTokens" ascii wide
        $s2 = "twoFaEnabled" ascii wide
        $s3 = "/api/auth/token" ascii wide
        $s4 = "setCookies" ascii wide
    condition:
        any of them
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-58269 Sync-in Server 2FA Bypass Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/api/auth/token"; http_uri; content:"username"; http_client_body; content:"password"; http_client_body; classtype:attempted-user; sid:10000001; rev:1; reference:cve,2026-58269;)

Elastic Detection

sequence by source.ip with maxspan=5s
  [http where http.request.method == "POST" and url.path == "/api/auth/token" and http.response.status_code == 200]

Splunk SPL Query

index=web sourcetype=access_combined
| search method=POST uri="/api/auth/token" status=200
| stats count by src_ip, user, _time
| where count > 0

Wazuh OSSEC Rule

<group name="web,sync-in,">
  <rule id="100001" level="10">
    <decoded_as>web-accesslog</decoded_as>
    <url>/api/auth/token</url>
    <method>POST</method>
    <status>200</status>
    <description>CVE-2026-58269: Sync-in Server 2FA bypass attempt detected</description>
    <mitre>
      <id>T1078</id>
    </mitre>
    <group>cve_2026_58269,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations: Disable the POST /api/auth/token endpoint if not required, or enforce 2FA at a reverse proxy/WAF level by blocking requests to this endpoint for accounts with 2FA enabled. Alternatively, implement rate limiting and anomaly detection on this endpoint. Monitor for unusual token issuance.
2. Patch/upgrade steps: Upgrade Sync-in Server to version 2.4.0 or later. As no public patch evidence exists, verify the availability of the fix from the vendor's official repository or release notes.
3. Long-term hardening: Enforce consistent 2FA checks across all authentication endpoints. Conduct regular security audits of authentication flows. Implement account lockout and alerting for suspicious token issuance. Educate users on phishing risks.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-58269
• Sync-in Server GitHub repository (check for version 2.4.0 release): https://github.com/sync-in/server
• CWE-288: https://cwe.mitre.org/data/definitions/288.html
• MITRE ATT&CK T1078: https://attack.mitre.org/techniques/T1078/
• No known threat actor campaigns or exploit-in-the-wild evidence available.