CVE-2025-29515: D-Link DSL-7740C Auth Bypass Vulnerability

CVE-2025-29515 Overview

CVE-2025-29515 is an incorrect access control vulnerability in the DELT_file.xgi endpoint of the D-Link DSL-7740C router running firmware DSL7740C.V6.TR069.20211230. The endpoint fails to enforce authentication and authorization checks. Unauthenticated remote attackers can modify arbitrary settings within the device's XML configuration database. This includes overwriting the administrator password, which grants full device takeover. The flaw is classified under [CWE-284: Improper Access Control].

Critical Impact
An unauthenticated attacker with network reachability to the router can rewrite the XML configuration database and seize administrative control of the device.

Affected Products

D-Link DSL-7740C router (hardware)
D-Link DSL-7740C firmware version DSL7740C.V6.TR069.20211230
Deployments exposing the device web management interface to untrusted networks

Discovery Timeline

2025-08-25 - CVE-2025-29515 published to NVD with public PoC referenced in a GitHub Gist by stevenyu113228
2025-09-02 - Last updated in the NVD database

Technical Details for CVE-2025-29515

Vulnerability Analysis

The DSL-7740C web management interface exposes the DELT_file.xgi endpoint as part of its CGI handler set. This endpoint accepts requests that operate on the device's internal XML configuration store. Access control checks that should restrict the endpoint to authenticated administrators are missing. As a result, any attacker who can reach the HTTP service can issue requests that mutate configuration nodes. Sensitive nodes include the administrator credential entry, WAN configuration, DNS settings, and TR-069 management parameters. Once the administrator password is rewritten, the attacker logs in with the new credentials and obtains persistent control. The device can then be repurposed for traffic interception, DNS hijacking, or recruitment into a botnet.

Root Cause

The root cause is the absence of session validation on the DELT_file.xgi handler. The handler trusts the request and routes parameters directly to XML database write operations. No authorization layer verifies that the caller holds an active administrator session.

Attack Vector

The attack is purely network-based and requires no user interaction or prior credentials. An attacker sends a crafted HTTP request to the DELT_file.xgi endpoint targeting the XML database keys that control authentication. Public proof-of-concept code demonstrating the request flow is published in the referenced GitHub Gist PoC Code. The vulnerability mechanism is described in prose because no verified exploit code is included in this advisory bundle. Refer to the D-Link Security Bulletin for vendor guidance.

Detection Methods for CVE-2025-29515

Indicators of Compromise

Unexpected HTTP requests to /DELT_file.xgi originating from external or untrusted internal addresses
Administrator password changes or new admin sessions that do not correlate with operator activity
Unsolicited modifications to DNS server entries, WAN credentials, or TR-069 ACS URLs in the device configuration
Outbound traffic from the router to unfamiliar management endpoints following configuration changes

Detection Strategies

Inspect router and upstream firewall logs for HTTP requests targeting DELT_file.xgi with query strings that reference XML configuration keys
Baseline the router's outbound DNS resolvers and alert on deviation
Capture and diff periodic configuration exports to identify unauthorized parameter changes

Monitoring Recommendations

Forward router syslog and authentication events to a centralized log platform for correlation
Monitor for failed and successful administrator logins from non-management subnets
Track firmware version and configuration hash across the fleet to flag drift

How to Mitigate CVE-2025-29515

Immediate Actions Required

Restrict access to the router web management interface to trusted management VLANs and block WAN-side administration
Audit administrator accounts and reset credentials on every DSL-7740C device running the affected firmware
Review the device XML configuration for unauthorized changes to DNS, WAN, and TR-069 settings
Consult the D-Link Security Bulletin for the latest vendor remediation status

Patch Information

At the time of NVD publication on 2025-08-25, no fixed firmware version is referenced in the advisory data. Operators should monitor the D-Link Security Bulletin for an updated firmware release addressing the DELT_file.xgi access control flaw and apply it as soon as it is available.

Workarounds

Disable remote management on the WAN interface and limit LAN management to a dedicated administrative subnet
Place the device behind an upstream firewall that drops inbound HTTP and HTTPS traffic to the router management ports
If the device has reached end-of-support without a patch, replace it with a supported model

bash

# Example upstream firewall rule to block external access to the router admin interface
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -j DROP