CVE-2025-3674: Totolink A3700r Auth Bypass Vulnerability

CVE-2025-3674 Overview

CVE-2025-3674 is an improper access control vulnerability in the TOTOLINK A3700R router running firmware version 9.1.2u.5822_B20200513. The flaw resides in the setUrlFilterRules function of the /cgi-bin/cstecgi.cgi endpoint. Unauthenticated remote attackers can manipulate URL filter rules on affected devices because the function fails to enforce proper authorization checks. The vulnerability has been publicly disclosed and technical details are available online, increasing the risk of opportunistic exploitation against exposed devices.

Critical Impact
Remote attackers can modify URL filtering configuration on affected TOTOLINK A3700R routers without authentication, degrading integrity of network filtering policies. [CWE-266]

Affected Products

TOTOLINK A3700R router hardware
TOTOLINK A3700R firmware 9.1.2u.5822_B20200513
Deployments exposing the /cgi-bin/cstecgi.cgi interface to untrusted networks

Discovery Timeline

2025-04-16 - CVE-2025-3674 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-3674

Vulnerability Analysis

The TOTOLINK A3700R exposes a Common Gateway Interface (CGI) binary at /cgi-bin/cstecgi.cgi that dispatches administrative actions based on request parameters. The setUrlFilterRules handler processes requests to add or modify URL filtering rules used by the router's content-filtering feature. This handler does not verify the caller's authorization state before applying changes. As a result, an attacker who can reach the HTTP interface can invoke the function directly and alter filter policy.

The issue is classified as improper privilege management under CWE-266. Filter rule tampering can be used to unblock previously restricted destinations or redirect filtering behavior against legitimate users. Because the attack requires no credentials and no user interaction, exploitation can be automated across internet-exposed devices.

Root Cause

The root cause is a missing authorization check inside the setUrlFilterRules code path of cstecgi.cgi. Administrative handlers in this binary are expected to validate a session token or authentication cookie before applying configuration changes. For setUrlFilterRules, that validation is absent or incorrectly scoped, allowing anonymous callers to reach privileged logic.

Attack Vector

Exploitation occurs over the network by sending a crafted HTTP request to /cgi-bin/cstecgi.cgi targeting the setUrlFilterRules action. No credentials, tokens, or user interaction are required. Attackers scanning for exposed TOTOLINK management interfaces can send the request directly and receive a successful configuration change response. Detailed request structure is documented in the Notion Guide for TOTOLINK A3700R and the VulDB entry #304963.

No verified proof-of-concept code is included here. Refer to the public advisory for request-level technical details.

Detection Methods for CVE-2025-3674

Indicators of Compromise

Unexpected HTTP POST requests to /cgi-bin/cstecgi.cgi referencing the setUrlFilterRules topicName or action parameter.
New or modified URL filter rules on the router that were not created by an administrator.
Requests to the router management interface originating from external or non-administrative source IPs.

Detection Strategies

Inspect router HTTP access logs, where available, for anonymous requests invoking cstecgi.cgi handlers such as setUrlFilterRules.
Correlate configuration change events with authenticated administrative sessions and flag rule changes that lack a preceding login.
Monitor network traffic for HTTP requests to router management ports from unexpected internal or external hosts.

Monitoring Recommendations

Baseline the current URL filter rule set and alert on any change to the filter configuration.
Enable syslog forwarding from the router, if supported, to a central log store for retention and analytics.
Track outbound traffic patterns from clients behind the router to detect filtering policy tampering.

How to Mitigate CVE-2025-3674

Immediate Actions Required

Restrict access to the router's HTTP administrative interface to trusted management networks only.
Disable remote (WAN-side) management on the TOTOLINK A3700R until a fixed firmware is available.
Audit current URL filter rules and reset them to a known-good configuration if tampering is suspected.
Rotate administrator credentials after confirming the device configuration is clean.

Patch Information

At the time of publication, no vendor security advisory or patched firmware release for CVE-2025-3674 is listed in the referenced sources. Consult the TOTOLINK Official Website for firmware updates, and the VulDB entry for status changes. Replacement with a currently supported router model should be considered where no vendor fix is provided.

Workarounds

Place the router management interface behind a firewall rule that only permits access from designated administrator IP addresses.
Segment the router management VLAN from user and guest networks to reduce internal exposure.
Where the A3700R is deployed in production, plan migration to a supported device that receives active security maintenance.

bash

# Example: block external access to the router management interface at an upstream firewall
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -m iprange ! --src-range <admin_ip_start>-<admin_ip_end> -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -m iprange ! --src-range <admin_ip_start>-<admin_ip_end> -j DROP