CVE-2020-36946: Flexense Syncbreeze DOS Vulnerability

CVE-2020-36946 Overview

CVE-2020-36946 is a denial of service vulnerability in Flexense SyncBreeze Enterprise 10.0.28. The flaw resides in the application's login endpoint, where the service fails to enforce limits on the size of incoming authentication payloads. Remote attackers can send an oversized payload to the login interface to crash the SyncBreeze service without authentication. The issue is tracked under [CWE-770: Allocation of Resources Without Limits or Throttling]. Successful exploitation disrupts file synchronization availability for legitimate users.

Critical Impact
Unauthenticated remote attackers can crash the SyncBreeze service over the network, halting file synchronization operations and impacting business continuity.

Affected Products

Flexense SyncBreeze Enterprise 10.0.28
CPE: cpe:2.3:a:flexense:syncbreeze:10.0.28:*:*:*:enterprise:*:*:*
Deployments exposing the SyncBreeze login interface to untrusted networks

Discovery Timeline

2026-01-27 - CVE-2020-36946 published to the National Vulnerability Database (NVD)
2026-03-24 - Last updated in NVD database

Technical Details for CVE-2020-36946

Vulnerability Analysis

The vulnerability targets the authentication handler of SyncBreeze 10.0.28. The login endpoint accepts client-supplied input without enforcing a maximum length on credential fields. When the application processes an oversized payload, internal resource handling fails and the service terminates. Because authentication is not required to reach the vulnerable code path, any attacker with network access to the service can trigger the condition.

The impact is limited to availability. The flaw does not provide read or write access to data, nor does it enable code execution based on available advisory information. However, SyncBreeze deployments often serve as the backbone of scheduled data synchronization, so service outages can cascade into broader operational disruption.

Root Cause

The root cause is missing input length validation on the login request, classified as [CWE-770]. The application allocates resources to parse the supplied credentials without throttling or bounding the request size. An oversized payload exhausts these resources and forces the process to terminate.

Attack Vector

The attack vector is network based and requires no privileges or user interaction. An attacker sends a single crafted HTTP request to the SyncBreeze login endpoint containing an oversized field. The service crashes immediately upon parsing the payload. Public proof-of-concept code is available through Exploit-DB #49291, which lowers the barrier for opportunistic exploitation.

No verified code examples are reproduced here. Refer to the VulnCheck Security Advisory and the published Exploit-DB entry for technical reproduction details.

Detection Methods for CVE-2020-36946

Indicators of Compromise

Unexpected termination or restart of the syncbreeze service process on affected hosts
HTTP POST requests to the SyncBreeze login endpoint containing abnormally large credential fields
Spikes in inbound traffic volume to the SyncBreeze management interface from a single source
Gaps in scheduled synchronization job logs that correlate with service crashes

Detection Strategies

Monitor SyncBreeze application and Windows event logs for repeated service crashes or unexpected restarts
Inspect web server and reverse proxy logs for POST requests to login paths with Content-Length values exceeding normal authentication payload sizes
Deploy network intrusion detection signatures that flag oversized HTTP request bodies directed at the SyncBreeze management port

Monitoring Recommendations

Centralize SyncBreeze host telemetry, process events, and network logs into a SIEM for correlation
Alert on rapid successive crashes of the SyncBreeze service, which indicate active exploitation attempts
Track source IP addresses generating oversized requests and block repeat offenders at the perimeter

How to Mitigate CVE-2020-36946

Immediate Actions Required

Restrict network access to the SyncBreeze management interface using firewall rules or a VPN
Upgrade SyncBreeze Enterprise to a version newer than 10.0.28 if a fixed release is available from Flexense
Place the SyncBreeze login endpoint behind a reverse proxy that enforces request size limits
Monitor the SyncBreeze Official Site for vendor updates and patch announcements

Patch Information

No vendor advisory or patch is referenced in the available CVE data. Administrators should contact Flexense directly to confirm whether a fixed build is available and validate the SyncBreeze release notes for security fixes addressing the login endpoint.

Workarounds

Configure a reverse proxy such as NGINX or IIS to cap request body size on the login URL to a few kilobytes
Limit access to the SyncBreeze web interface to trusted management subnets only
Disable remote access to the login endpoint when administrative tasks are not in progress
Implement rate limiting on the login endpoint to slow automated exploitation attempts

bash

# Example NGINX reverse proxy configuration enforcing a request size cap
server {
    listen 443 ssl;
    server_name syncbreeze.internal.example.com;

    client_max_body_size 8k;

    location /login {
        limit_req zone=login_zone burst=5 nodelay;
        proxy_pass http://127.0.0.1:80;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }

    allow 10.0.0.0/8;
    deny  all;
}