CVE-2026-71911 Overview
CVE-2026-71911 is a buffer overflow vulnerability affecting multiple DrayTek VigorAP wireless access point models. The flaw resides in the setLan function of the device's web management interface. Missing length checks during memory copy operations on the lanVlanId0, lanIp, and lanNetmask fields allow attackers to overflow adjacent memory. Authenticated attackers with administrative credentials can trigger the condition remotely by submitting crafted input. Successful exploitation can cause denial of service or lead to arbitrary command execution on the affected device. The issue is tracked under CWE-120: Buffer Copy without Checking Size of Input.
Critical Impact
An authenticated remote attacker can crash the device or execute arbitrary commands on affected DrayTek VigorAP access points, potentially gaining persistent control over network infrastructure.
Affected Products
- Multiple DrayTek VigorAP series wireless access point models
- Firmware versions addressed by the August 2026 DrayTek security advisory
- Devices with the web management interface exposed to authenticated administrators
Discovery Timeline
- 2026-08-24 - CVE-2026-71911 published to NVD
- 2026-08-26 - Last updated in NVD database
Technical Details for CVE-2026-71911
Vulnerability Analysis
The vulnerability exists in the setLan handler responsible for processing LAN configuration submitted through the web management interface. The handler copies user-controlled values from the lanVlanId0, lanIp, and lanNetmask parameters into fixed-size stack or heap buffers. Because the copy routines do not validate input length against destination buffer size, oversized values overwrite adjacent memory. Depending on which field is abused and how memory is laid out, the corruption can crash the HTTP daemon or overwrite control data such as saved return addresses and function pointers. Attackers who chain memory corruption with precise payloads may achieve command execution as the process handling the web interface, which typically runs with elevated privileges on embedded devices.
Root Cause
The root cause is missing bounds validation in memory copy operations inside setLan. The function trusts client-supplied field lengths and copies them directly into fixed-size buffers without enforcing size limits, matching the pattern described by CWE-120.
Attack Vector
Exploitation occurs over the network against the device's web management interface. The attacker must first authenticate with valid administrative credentials. Once authenticated, the attacker submits a crafted request to the setLan endpoint with oversized values in one or more of the vulnerable fields. Technical details are documented in the DrayTek Security Advisory and the VulnCheck Advisory on DrayTek.
No public proof-of-concept exploit code is available. See the referenced advisories for further technical detail.
Detection Methods for CVE-2026-71911
Indicators of Compromise
- Unexpected restarts or crashes of the VigorAP HTTP management daemon
- Administrative logins from unusual source IP addresses followed by setLan requests
- LAN configuration changes with abnormally long lanVlanId0, lanIp, or lanNetmask values in device logs
- New outbound connections initiated by the access point to unknown hosts after configuration changes
Detection Strategies
- Inspect HTTP or HTTPS traffic destined for VigorAP management interfaces for setLan requests containing oversized field values
- Alert on administrative sessions originating from external networks or non-management VLANs
- Correlate device configuration changes with authentication events to identify anomalous change patterns
- Baseline expected LAN configuration parameters and flag deviations from allowed formats and lengths
Monitoring Recommendations
- Forward VigorAP syslog and authentication events to a centralized SIEM for retention and correlation
- Monitor for repeated failed authentication attempts against management interfaces that may precede exploitation
- Track firmware versions across the fleet to identify unpatched devices exposed to the vulnerability
How to Mitigate CVE-2026-71911
Immediate Actions Required
- Apply the firmware update referenced in the DrayTek Security Advisory to all affected VigorAP models
- Restrict web management interface access to trusted management networks only
- Rotate administrative credentials on all VigorAP devices and enforce strong, unique passwords
- Audit administrative accounts and remove unused or default accounts
Patch Information
DrayTek published fixed firmware in its August 2026 advisory covering multiple remote code execution and buffer overflow issues in the VigorAP series. Refer to the DrayTek Security Advisory for the list of affected models and corresponding firmware versions.
Workarounds
- Disable remote administration on WAN interfaces where it is not required
- Place VigorAP management interfaces behind a VPN or firewall access control list restricted to administrator source addresses
- Segment access points onto a dedicated management VLAN unreachable from general user networks
- Monitor and rate-limit access to the setLan endpoint at any upstream reverse proxy where applicable
No command-line configuration example is provided by the vendor advisory. Follow vendor documentation for firmware upgrade procedures and management interface hardening.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
