CVE-2026-71941: DrayTek VigorSwitch Buffer Overflow Flaw

CVE-2026-71941 Overview

CVE-2026-71941 is a buffer overflow vulnerability affecting multiple DrayTek VigorSwitch models. The flaw resides in the diag_logmail function, which concatenates multiple smtpReceiver email addresses into a fixed-size buffer without validating the remaining buffer space. A remote authenticated attacker with administrative credentials can send crafted input through the web management interface to trigger the overflow. Successful exploitation causes denial of service or may enable arbitrary command execution on the switch. The vulnerability is classified under CWE-120 (Classic Buffer Overflow).

Critical Impact
Authenticated remote attackers can crash affected DrayTek VigorSwitch devices or potentially execute arbitrary commands by supplying oversized smtpReceiver values to diag_logmail.

Affected Products

Multiple DrayTek VigorSwitch models (see vendor advisory for the complete list)
DrayTek VigorSwitch web management interface component
Firmware versions prior to the August 2026 security update

Discovery Timeline

2026-08-24 - CVE-2026-71941 published to the National Vulnerability Database
2026-08-27 - Last updated in NVD database

Technical Details for CVE-2026-71941

Vulnerability Analysis

The vulnerability exists in the diag_logmail handler exposed through the VigorSwitch web administration interface. This handler processes SMTP notification settings, including one or more smtpReceiver email address fields. The function concatenates each supplied address into a fixed-size stack or heap buffer using unchecked string operations. Because the code does not track the remaining capacity of the destination buffer, supplying multiple addresses or an oversized string causes adjacent memory to be overwritten. Depending on the memory layout, the overflow can corrupt return addresses, function pointers, or control structures, leading to a device crash or arbitrary command execution in the context of the web management process.

Root Cause

The root cause is missing bounds validation during string concatenation in diag_logmail. The implementation uses a fixed-size buffer to collect multiple smtpReceiver values but omits length checks before each append operation. This pattern is a textbook CWE-120 classic buffer overflow.

Attack Vector

An attacker requires valid administrative credentials to reach the vulnerable endpoint. Once authenticated, the attacker submits a crafted request to the diagnostic mail configuration handler containing multiple long smtpReceiver values. The concatenated input exceeds the destination buffer, corrupting adjacent memory. Because the attack path is reachable over the network on the management interface, exposed or shared administrative access significantly increases risk. Refer to the DrayTek Security Advisory and the VulnCheck Advisory for additional technical context.

Detection Methods for CVE-2026-71941

Indicators of Compromise

Unexpected reboots or crashes of DrayTek VigorSwitch devices following administrative HTTP requests
HTTP POST requests to the diagnostic mail configuration endpoint containing multiple or oversized smtpReceiver parameter values
Web management process termination or watchdog-triggered restarts in device logs
New or modified SMTP notification receivers that were not authorized by administrators

Detection Strategies

Inspect network traffic to VigorSwitch management interfaces for POST bodies where smtpReceiver fields exceed typical email address lengths
Correlate administrative session activity with subsequent device availability loss to identify exploitation attempts
Alert on repeated failed or unusually large requests to the diag_logmail handler across the switch fleet

Monitoring Recommendations

Forward VigorSwitch syslog and web management access logs to a centralized log platform for retention and analysis
Baseline normal administrative traffic volumes and content sizes, and alert on statistical deviations
Track authentication events on switch management interfaces to detect credential misuse that could precede exploitation

How to Mitigate CVE-2026-71941

Immediate Actions Required

Apply the firmware update referenced in the DrayTek Security Advisory August 2026 to all affected VigorSwitch models
Restrict access to the switch web management interface to trusted management networks only
Rotate administrative credentials on affected devices and enforce strong, unique passwords
Audit configured SMTP notification settings for unauthorized smtpReceiver entries

Patch Information

DrayTek released fixed firmware as part of the August 2026 security advisory covering multiple VigorSwitch vulnerabilities. Administrators should consult the DrayTek advisory to identify the specific firmware version required for each affected model and follow the vendor upgrade procedure. Confirm firmware integrity using vendor-supplied checksums before deployment.

Workarounds

Disable remote access to the web management interface and require access only from a dedicated out-of-band management VLAN
Place VigorSwitch management interfaces behind a firewall that filters HTTP/HTTPS traffic to authorized administrator source addresses
Limit the number of accounts with administrative privileges and monitor their use until firmware is applied

bash

# Example ACL concept: restrict management access to a trusted subnet
# Consult DrayTek documentation for model-specific syntax
management access-list allow 10.10.10.0/24
management access-list deny any