CVE-2025-5969: D-Link DIR-632 Buffer Overflow Vulnerability

CVE-2025-5969 Overview

CVE-2025-5969 is a stack-based buffer overflow [CWE-119] in the D-Link DIR-632 router running firmware FW103B08. The flaw resides in function FUN_00425fd8, which handles requests to the /biurl_grou endpoint inside the HTTP POST Request Handler. Authenticated attackers can send a crafted POST request over the network to corrupt stack memory and influence execution flow. Public proof-of-concept code is available, and the device family is end-of-life with no vendor patch planned.

Critical Impact
Remote attackers with low-level credentials can trigger stack memory corruption on the DIR-632, potentially leading to arbitrary code execution on an unsupported device with no available firmware fix.

Affected Products

D-Link DIR-632 hardware router
D-Link DIR-632 firmware version 103B08
End-of-life D-Link DIR-632 deployments still in production networks

Discovery Timeline

2025-06-10 - CVE-2025-5969 published to NVD
2025-07-16 - Last updated in NVD database

Technical Details for CVE-2025-5969

Vulnerability Analysis

The vulnerability exists in FUN_00425fd8, a routine invoked by the HTTP POST Request Handler when processing requests to the /biurl_grou URI. The function copies attacker-controlled POST data into a fixed-size stack buffer without enforcing length validation. Oversized input overruns the buffer and overwrites adjacent stack frames, including saved return addresses on the MIPS-based device.

Exploitation requires network reachability to the router's web management interface and low-privilege credentials. Because the DIR-632 line is no longer maintained, the vendor will not issue a firmware patch. Public exploit details are hosted in a third-party GitHub repository, increasing the likelihood of opportunistic targeting against exposed devices.

Root Cause

The root cause is improper restriction of operations within the bounds of a memory buffer [CWE-119]. FUN_00425fd8 uses an unbounded copy routine on attacker-supplied POST parameters, trusting client-supplied length values rather than enforcing the destination buffer size. Stack canaries and other modern mitigations are absent from this firmware build.

Attack Vector

An attacker authenticated to the router's web interface sends a POST request to /biurl_grou containing an oversized parameter value. The payload overflows the stack buffer inside FUN_00425fd8, overwrites the saved return address, and redirects execution. On embedded MIPS firmware without Address Space Layout Randomization (ASLR), reliable return-oriented programming chains can yield arbitrary code execution as the web server process, typically running as root.

A public proof of concept is referenced in the GitHub PoC Repository and tracked as VulDB #311845. No verified code samples are reproduced here.

Detection Methods for CVE-2025-5969

Indicators of Compromise

HTTP POST requests to /biurl_grou containing abnormally long parameter values or non-printable bytes
Unexpected reboots, watchdog resets, or httpd crashes on DIR-632 devices following inbound web traffic
Outbound connections from the router to unfamiliar hosts after receipt of crafted POST traffic
New or modified accounts and configuration changes on the router not initiated by administrators

Detection Strategies

Inspect web server and syslog output for repeated requests to /biurl_grou from external or untrusted internal sources
Apply network IDS signatures that flag oversized POST bodies or parameters targeting the DIR-632 management interface
Baseline router CPU, memory, and process behavior to identify anomalies consistent with exploitation attempts

Monitoring Recommendations

Forward router logs to a centralized logging or SIEM platform for correlation with perimeter telemetry
Alert on any administrative access to DIR-632 devices from non-management network segments
Track DNS and netflow records from router IPs for beaconing patterns indicative of post-exploitation

How to Mitigate CVE-2025-5969

Immediate Actions Required

Remove the DIR-632 from internet-facing positions and disable remote (WAN) management immediately
Replace end-of-life DIR-632 devices with a supported router model, as the vendor will not release a patch
Restrict access to the LAN-side web administration interface to a dedicated management VLAN or trusted hosts
Rotate router administrative credentials and audit existing accounts for unauthorized changes

Patch Information

No patch is available. D-Link has confirmed that the DIR-632 product line is no longer supported. Refer to the D-Link Official Site for end-of-life guidance and replacement options. Tracking details are available at VulDB #311845 (CTI).

Workarounds

Place the device behind an upstream firewall that blocks inbound HTTP/HTTPS to the router management port
Use ACLs to permit administrative POST requests only from a known management workstation
Monitor for and block requests to the /biurl_grou URI at any intermediate web application firewall

bash

# Example: block inbound access to the router web UI from untrusted networks
iptables -A INPUT -p tcp --dport 80 -s 0.0.0.0/0 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 0.0.0.0/0 -j DROP
iptables -A INPUT -p tcp --dport 80 -s 10.10.10.0/24 -j ACCEPT  # management VLAN only