CVE-2026-58270: Sync-in Server ReDoS DOS Vulnerability

CVE-2026-58270 Overview

CVE-2026-58270 is a Regular Expression Denial of Service (ReDoS) vulnerability in Sync-in Server, an open-source platform for file storage, sharing, collaboration, and syncing. The sync diff endpoint compiles a user-supplied string into a RegExp object without validating pattern complexity. An authenticated attacker can submit a catastrophic-backtracking pattern such as ^(a+)+b to block the Node.js event loop. The blocked event loop makes the entire server unresponsive to all users until the container is restarted. Sync-in Server version 2.4.0 patches the issue by rejecting complex regex patterns [CWE-1333].

Critical Impact
A single authenticated request to the sync diff endpoint can freeze the Node.js event loop and take the entire Sync-in Server offline for all users until the container is restarted.

Affected Products

Sync-in Server versions prior to 2.4.0
Node.js-based deployments using the affected sync diff endpoint
Containerized Sync-in deployments

Discovery Timeline

2026-09-21 - CVE-2026-58270 published to NVD
2026-09-22 - Last updated in NVD database

Technical Details for CVE-2026-58270

Vulnerability Analysis

The vulnerability resides in the sync diff endpoint of Sync-in Server. The endpoint accepts a user-supplied string and compiles it directly into a JavaScript RegExp object. No complexity validation, pattern length restriction, or execution timeout guards the compilation or matching phase.

Node.js executes JavaScript on a single-threaded event loop. When the regex engine enters catastrophic backtracking, it consumes CPU on the main thread and blocks all pending and future requests. All connected users lose access to the service until an administrator restarts the container.

The attack requires authentication but only low privileges. No user interaction is needed. Successful exploitation yields high availability impact without affecting confidentiality or integrity.

Root Cause

The root cause is unsafe regular expression handling classified as [CWE-1333], Inefficient Regular Expression Complexity. The application trusts client-supplied regex strings and passes them to the RegExp constructor. Nested quantifiers such as (a+)+ create exponential state expansion in the regex engine when matching against non-matching input.

Attack Vector

An authenticated user sends a crafted request to the sync diff endpoint containing a regex pattern with nested quantifiers. Example patterns include ^(a+)+b, ^(a|a)+$, and (.*a){20}b. When the regex engine attempts to match input that fails at the final anchor, it explores every possible partition of the input.

This exploration blocks the Node.js event loop for seconds, minutes, or indefinitely depending on input length. See the GitHub Security Advisory for technical details.

Detection Methods for CVE-2026-58270

Indicators of Compromise

Sudden unresponsiveness of the Sync-in Server across all client sessions
Sustained 100% CPU usage by the Node.js process on a single core
Requests to the sync diff endpoint containing regex metacharacters with nested quantifiers such as (a+)+, (a|a)+, or (.*)+
Container health checks failing without process crash or memory exhaustion

Detection Strategies

Log request payloads to the sync diff endpoint and inspect regex parameters for nested quantifier patterns
Monitor Node.js event loop lag using metrics libraries or APM tooling and alert when lag exceeds a defined threshold
Correlate CPU spikes on the application container with recent authenticated requests to the sync diff endpoint

Monitoring Recommendations

Track authenticated user activity for repeated or malformed requests to sync-related endpoints
Alert on container restarts that follow periods of event loop stall or CPU saturation
Feed application and container telemetry into a centralized data lake to correlate performance degradation with request patterns

How to Mitigate CVE-2026-58270

Immediate Actions Required

Upgrade Sync-in Server to version 2.4.0 or later, which contains the official patch
Audit access to the sync diff endpoint and revoke unnecessary authenticated accounts
Place the Sync-in Server behind a reverse proxy that enforces request timeouts and payload inspection
Review recent container restart events for evidence of exploitation attempts

Patch Information

Sync-in maintainers released version 2.4.0 to remediate CVE-2026-58270. The patch validates regex complexity before compilation. Refer to the GitHub Security Advisory GHSA-jx63-h26r-8cph for release details and upgrade guidance.

Workarounds

Restrict access to the sync diff endpoint at the network or reverse-proxy layer until the upgrade is applied
Enforce short request timeouts at the load balancer to limit the impact of long-running matches
Configure container orchestration to automatically restart Sync-in Server on health check failure to reduce recovery time

bash

# Example: enforce a 5-second upstream response timeout in nginx
location /sync/diff {
    proxy_pass http://sync_in_backend;
    proxy_read_timeout 5s;
    proxy_send_timeout 5s;
    client_max_body_size 16k;
}