CVE-2026-3958: Woahai321 ListSync SSRF Vulnerability

CVE-2026-3958 Overview

A Server-Side Request Forgery (SSRF) vulnerability has been identified in Woahai321 ListSync versions up to and including 0.6.6. This security flaw affects the requests.post function within the list-sync-main/api_server.py file, specifically in the JSON Handler component. The vulnerability allows remote attackers to manipulate server-side requests, potentially enabling unauthorized access to internal resources or services.

Critical Impact
Remote attackers can exploit the SSRF vulnerability to force the server to make arbitrary HTTP requests, potentially accessing internal services, cloud metadata endpoints, or sensitive internal network resources that should not be publicly accessible.

Affected Products

Woahai321 ListSync versions up to 0.6.6
Applications utilizing the api_server.py JSON Handler component

Discovery Timeline

2026-03-11 - CVE-2026-3958 published to NVD
2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-3958

Vulnerability Analysis

This SSRF vulnerability (CWE-918) resides in the JSON Handler component of ListSync's API server. The flaw exists because the application fails to properly validate or sanitize user-supplied URLs before making server-side HTTP requests via the requests.post function. An authenticated attacker with low privileges can exploit this weakness to redirect the server's outbound requests to arbitrary destinations.

The vulnerability enables attackers to bypass network segmentation, access internal APIs, probe internal network infrastructure, or potentially retrieve sensitive information from cloud provider metadata services. The exploit has been publicly disclosed, increasing the risk of active exploitation despite the project maintainers not yet responding to the initial vulnerability report.

Root Cause

The root cause of this vulnerability is improper input validation in the api_server.py file. When processing JSON requests, the application passes user-controlled URL parameters directly to the requests.post function without adequate validation, sanitization, or allowlist filtering. This allows attackers to specify arbitrary target URLs, including internal network addresses, localhost references, and cloud metadata endpoints.

Attack Vector

The attack is carried out remotely over the network. An attacker with low-privilege access to the application can craft malicious JSON payloads containing attacker-controlled URLs. When the vulnerable requests.post function processes these payloads, the server initiates HTTP requests to the specified destinations, effectively acting as a proxy for the attacker.

Typical exploitation scenarios include:

Accessing cloud metadata services (e.g., http://169.254.169.254/) to retrieve instance credentials
Scanning internal network services and ports
Accessing internal APIs and administrative interfaces
Bypassing firewall restrictions by routing requests through the vulnerable server

For technical details and exploitation specifics, refer to the GitHub Issue Discussion where the vulnerability was disclosed.

Detection Methods for CVE-2026-3958

Indicators of Compromise

Unusual outbound HTTP requests originating from the ListSync API server to internal IP ranges (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
Requests to cloud metadata endpoints (169.254.169.254) from the application server
Unexpected network connections to localhost (127.0.0.1) services from the API server process
Anomalous JSON payloads containing URL parameters pointing to non-standard destinations

Detection Strategies

Monitor outbound network traffic from ListSync servers for connections to internal network ranges or metadata services
Implement web application firewall (WAF) rules to detect SSRF payload patterns in JSON requests
Review application logs for unusual URL parameters being processed by the JSON Handler component
Deploy network segmentation monitoring to detect unauthorized cross-zone communications

Monitoring Recommendations

Enable detailed logging for all outbound HTTP requests made by the api_server.py component
Configure network monitoring tools to alert on connections from the ListSync server to sensitive internal resources
Implement anomaly detection for unusual patterns in JSON request payloads
Monitor for DNS lookups to internal hostnames originating from the affected application

How to Mitigate CVE-2026-3958

Immediate Actions Required

Restrict network access from ListSync servers to only required external endpoints using firewall rules
Implement allowlist-based URL validation for any user-supplied URLs processed by the application
Consider temporarily disabling the affected JSON Handler functionality if not critical to operations
Review and audit all API server configurations for additional input validation weaknesses

Patch Information

As of the last update on 2026-03-12, no official patch has been released by the project maintainers. The vulnerability was reported through a GitHub issue, but the project has not yet responded. Users should monitor the project repository for security updates and consider implementing the workarounds below until an official fix is available.

Additional vulnerability information is available through VulDB #350388.

Workarounds

Implement network-level egress filtering to prevent the server from making requests to internal IP ranges and cloud metadata services
Deploy a reverse proxy or WAF in front of the API server to filter and validate incoming JSON requests
Modify the api_server.py file to add URL validation that restricts outbound requests to an allowlist of approved domains
Isolate the ListSync server in a network segment with restricted access to sensitive internal resources

bash

# Example iptables rules to block common SSRF targets
# Block access to cloud metadata services
iptables -A OUTPUT -d 169.254.169.254 -j DROP

# Block access to common internal ranges from the application server
iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP
iptables -A OUTPUT -d 127.0.0.0/8 -j DROP