CVE-2026-100740: D-Link DIR-895L RCE Vulnerability

CVE-2026-100740 Overview

CVE-2026-100740 is an out-of-bounds write vulnerability in the D-Link DIR-895L router running firmware version A1_102b07. The flaw resides in the tunnel_set_params function within tunnel.c, part of the Layer 2 Tunneling Protocol (L2TP) Control Channel Parser. A remote attacker can trigger the vulnerability by sending a crafted L2TP Host Name Attribute-Value Pair (AVP) to the affected device. Public exploit code is available, increasing the likelihood of opportunistic exploitation against exposed devices. The vulnerability is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer).

Critical Impact

Remote attackers can corrupt memory on the router via malformed L2TP control traffic, potentially achieving code execution or denial of service against the network edge.

Affected Products

• D-Link DIR-895L router
• Firmware version A1_102b07
• L2TP Control Channel Parser component (tunnel.c)

Discovery Timeline

• 2026-09-27 - CVE-2026-100740 published to the National Vulnerability Database (NVD)
• 2026-09-28 - Last updated in the NVD database

Technical Details for CVE-2026-100740

Vulnerability Analysis

The vulnerability affects how the DIR-895L parses L2TP control messages during tunnel setup. The tunnel_set_params function processes Attribute-Value Pairs (AVPs) from incoming L2TP control packets, including the Host Name AVP. The function writes attacker-controlled data beyond the allocated buffer, resulting in an out-of-bounds write condition.

Successful exploitation can corrupt adjacent memory structures, including function pointers or control data on the router. This may enable arbitrary code execution in the context of the vulnerable service or cause the device to crash. Because the attack targets the network edge, a compromised router gives an attacker a persistent foothold capable of intercepting, redirecting, or manipulating all downstream traffic. Refer to the Notion Analysis on D-Link Vulnerability and VulDB CVE-2026-100740 Details for additional technical context.

Root Cause

The root cause is missing bounds validation in the tunnel_set_params routine when copying the Host Name AVP payload from an attacker-supplied L2TP control message into a fixed-size destination buffer. The parser trusts the length field or terminator supplied by the remote peer without verifying it against the destination buffer size, which produces the out-of-bounds write.

Attack Vector

The attack is initiated over the network. An attacker reaches the L2TP service on an exposed DIR-895L device and sends a crafted L2TP control packet containing a malformed Host Name AVP. No local access to the device is required. Public exploit code lowers the technical barrier for attackers targeting internet-facing or improperly segmented DIR-895L routers.

See the VulDB Vulnerability #410614 entry for additional exploitation context.

Detection Methods for CVE-2026-100740

Indicators of Compromise

• Unexpected reboots, crashes, or watchdog restarts of the DIR-895L router during or after inbound L2TP traffic.
• Inbound UDP traffic to port 1701 (L2TP) from untrusted sources, especially sessions containing abnormally large or malformed Host Name AVPs.
• New or modified administrative accounts, DNS settings, or firewall rules on the device without a corresponding change-management record.

Detection Strategies

• Inspect L2TP control traffic at perimeter firewalls or intrusion detection systems for AVPs with length fields that exceed protocol norms.
• Alert on inbound L2TP sessions sourced from the public internet to devices that are not acting as a VPN concentrator.
• Correlate router crash events with preceding L2TP control packets by capturing syslog from the DIR-895L.

Monitoring Recommendations

• Forward DIR-895L syslog to a central Security Information and Event Management (SIEM) platform and alert on repeated service restarts.
• Baseline normal L2TP traffic volume and alert on anomalous spikes toward edge routers.
• Monitor for outbound connections initiated by the router to unfamiliar hosts, which may indicate post-exploitation activity.

How to Mitigate CVE-2026-100740

Immediate Actions Required

• Block inbound UDP port 1701 at the perimeter unless L2TP termination on the DIR-895L is explicitly required.
• Restrict administrative access to the router to a trusted management network.
• Inventory DIR-895L devices on A1_102b07 firmware and prioritize them for replacement or isolation.

Patch Information

No vendor advisory or firmware update has been published at the time of writing. Administrators should monitor the D-Link Official Website for firmware updates addressing CVE-2026-100740. The DIR-895L is a legacy consumer-grade router, and end-of-life status should be verified with D-Link before planning long-term remediation.

Workarounds

• Disable the L2TP service on the DIR-895L if it is not required for operations.
• Place the router behind an upstream firewall that filters or rate-limits L2TP control traffic from untrusted networks.
• Replace end-of-life DIR-895L devices with actively supported hardware that receives security updates.

# Example: block inbound L2TP at an upstream Linux gateway
iptables -A FORWARD -p udp --dport 1701 -i eth0 -j DROP
iptables -A INPUT   -p udp --dport 1701 -i eth0 -j DROP