CVE-2026-71942 Overview
CVE-2026-71942 is a buffer overflow vulnerability affecting multiple DrayTek VigorSwitch models. The flaw resides in the mail_mailalert function, which concatenates multiple smtpReceiver email addresses into a fixed-size buffer without validating the remaining buffer space. An authenticated remote attacker with administrative credentials on the web management interface can trigger the overflow using crafted input. Successful exploitation can cause denial of service or lead to arbitrary command execution on the affected switch. The issue is tracked under CWE-120: Buffer Copy without Checking Size of Input.
Critical Impact
An authenticated attacker can overflow a fixed-size buffer through the SMTP mail alert configuration, resulting in denial of service or potential arbitrary command execution on affected DrayTek VigorSwitch devices.
Affected Products
- Multiple DrayTek VigorSwitch series models (see vendor advisory for the full model list)
- DrayTek VigorSwitch firmware versions with the vulnerable mail_mailalert implementation
- Devices with the SMTP mail alert feature enabled via the web management interface
Discovery Timeline
- 2026-08-24 - CVE-2026-71942 published to the National Vulnerability Database
- 2026-08-26 - Last updated in the NVD database
Technical Details for CVE-2026-71942
Vulnerability Analysis
The vulnerability lives inside the mail_mailalert function, which builds an SMTP notification configuration by concatenating one or more smtpReceiver email address values into a single fixed-size stack or heap buffer. The routine does not track how much space remains in the destination buffer between successive concatenation operations. When an administrator submits a request containing enough receiver entries, or entries with sufficient length, the concatenation exceeds the destination buffer boundary. The resulting overflow corrupts adjacent memory on the switch's management plane. Depending on layout and firmware build, this leads to a service crash or hijacked control flow that enables arbitrary command execution in the context of the management process.
Root Cause
The root cause is missing bounds validation during string concatenation, classified as CWE-120. The code appends attacker-controlled smtpReceiver values into a fixed-size buffer without checking the cumulative length against the buffer capacity. No length parameter, safe-string primitive, or upstream input validation limits the total combined size of receiver addresses.
Attack Vector
The attack vector is network-based and targets the web management interface of the VigorSwitch. Exploitation requires valid administrative credentials, which raises the attack precondition. Once authenticated, an attacker submits a crafted mail alert configuration containing an oversized or high-count list of SMTP receiver addresses. The vulnerable function processes the input and overflows the target buffer. See the DrayTek Security Advisory and the VulnCheck Buffer Overflow Advisory for technical details. No public proof-of-concept code was referenced at publication time.
No verified exploit code is available. See vendor and VulnCheck advisories for technical details.
Detection Methods for CVE-2026-71942
Indicators of Compromise
- Unexpected reboots or crash logs from the VigorSwitch management process shortly after SMTP alert configuration changes
- Web management interface configuration submissions containing unusually long or numerous smtpReceiver values
- Administrative logins to the switch from unfamiliar source IPs preceding mail alert configuration edits
Detection Strategies
- Inspect switch configuration audit logs for modifications to SMTP mail alert settings that include abnormal receiver counts or oversized address strings
- Alert on repeated HTTP POST requests to the mail alert configuration endpoint of the VigorSwitch web UI from a single source
- Correlate management-plane crashes or watchdog resets on network switches with recent configuration change events
Monitoring Recommendations
- Forward VigorSwitch syslog and configuration change events to a central logging platform for correlation
- Restrict and monitor administrative access to switch web management interfaces at the network layer
- Track firmware versions across the switch fleet and flag any device still running vulnerable builds
How to Mitigate CVE-2026-71942
Immediate Actions Required
- Apply the firmware update referenced in the DrayTek Security Advisory to all affected VigorSwitch models
- Rotate administrative credentials on affected switches, especially where credentials may have been shared or reused
- Review recent configuration changes to SMTP mail alert settings for signs of tampering
Patch Information
DrayTek has released fixed firmware for the affected VigorSwitch series. Refer to the DrayTek Security Advisory for the specific model-to-version mapping and download instructions. Additional technical context is available in the VulnCheck Buffer Overflow Advisory.
Workarounds
- Restrict the switch web management interface to a dedicated management VLAN or trusted jump host network
- Disable the SMTP mail alert feature on affected switches until firmware can be updated
- Enforce strong, unique administrative passwords and enable account lockout on repeated failed logins
- Place VigorSwitch management interfaces behind an ACL that permits only authorized administrator source addresses
# Example ACL concept restricting switch management access to a trusted subnet
# (adapt to your switch's CLI syntax)
access-list MGMT permit tcp 10.10.0.0 0.0.0.255 any eq 443
access-list MGMT permit tcp 10.10.0.0 0.0.0.255 any eq 80
access-list MGMT deny ip any any log
interface vlan 100
ip access-group MGMT in
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
