CVE-2026-71940 Overview
CVE-2026-71940 is a buffer overflow vulnerability affecting multiple DrayTek VigorSwitch models. The flaw resides in the acl_general_setup Edit ACE function, which copies the name field into a fixed-size buffer without validating input length [CWE-120]. An authenticated remote attacker with administrative credentials to the web management interface can send crafted input to trigger the overflow. Successful exploitation can cause a denial of service or, under favorable conditions, arbitrary command execution on the switch.
Critical Impact
Authenticated attackers can crash affected VigorSwitch devices or potentially execute arbitrary commands with elevated privileges through crafted Access Control Entry (ACE) input.
Affected Products
- Multiple DrayTek VigorSwitch series models (see vendor advisory for the complete model list)
- VigorSwitch firmware versions exposed to the affected acl_general_setup handler
- DrayTek VigorSwitch web management interface (HTTP/HTTPS)
Discovery Timeline
- 2026-08-24 - CVE-2026-71940 published to NVD
- 2026-08-26 - Last updated in NVD database
Technical Details for CVE-2026-71940
Vulnerability Analysis
The vulnerability exists in the acl_general_setup Edit ACE handler within the VigorSwitch web management interface. When an administrator edits an Access Control Entry, the handler copies the user-supplied name field into a fixed-size stack or global buffer. The routine performs no bounds check against the destination buffer size before the copy operation.
Submitting an over-long name parameter overwrites adjacent memory. This corrupts saved return addresses, function pointers, or control structures within the HTTP daemon process. The attacker interacts with the vulnerability through authenticated HTTP requests to the ACL configuration endpoint. Exploitation requires valid administrator credentials, which limits the attacker population but does not prevent post-compromise lateral use.
Root Cause
The root cause is a classic unchecked buffer copy operation [CWE-120]. The acl_general_setup function trusts client-supplied length for the ACE name field and calls an unsafe copy primitive such as strcpy or sprintf without enforcing the destination buffer boundary. Input validation and length truncation are absent at the request-parsing layer.
Attack Vector
The attack vector is network-based through the VigorSwitch web management interface. An attacker authenticates with administrator credentials, navigates to the ACL general setup page, and submits an Edit ACE request containing an oversized name value. The malformed request is delivered over HTTP or HTTPS to the switch. Impact ranges from web daemon crash and denial of service to potential arbitrary command execution if the attacker crafts input that reliably controls execution flow.
No verified proof-of-concept code has been published. See the DrayTek Security Advisory and the VulnCheck DrayTek Buffer Overflow Advisory for additional technical detail.
Detection Methods for CVE-2026-71940
Indicators of Compromise
- Unexpected reboots, watchdog resets, or crashes of the VigorSwitch web management daemon
- HTTP or HTTPS POST requests to the ACL general setup endpoint containing abnormally long name field values
- Administrator session activity from unfamiliar source IP addresses accessing the ACL configuration pages
- Configuration changes to Access Control Entries that do not correlate with a scheduled change window
Detection Strategies
- Inspect web server and management-plane logs on VigorSwitch devices for oversized parameters submitted to the acl_general_setup handler
- Deploy network IDS signatures that flag HTTP requests to VigorSwitch management endpoints containing name parameters exceeding expected length thresholds
- Correlate authentication events with subsequent ACL edit requests to identify credential misuse
Monitoring Recommendations
- Forward switch syslog and web management logs to a centralized platform such as Singularity Data Lake for correlation and long-term retention
- Alert on repeated failed administrator logins followed by ACL configuration changes on VigorSwitch devices
- Monitor for outbound connections initiated by the switch itself, which would be unusual and may indicate post-exploitation activity
How to Mitigate CVE-2026-71940
Immediate Actions Required
- Apply the firmware update referenced in the DrayTek Security Advisory to all affected VigorSwitch models
- Restrict access to the VigorSwitch web management interface to a dedicated management VLAN and trusted administrator hosts only
- Rotate administrator credentials on all VigorSwitch devices and disable any unused administrator accounts
- Audit ACL configurations for unauthorized entries added prior to patch deployment
Patch Information
DrayTek has published fixed firmware for the affected VigorSwitch models. Consult the DrayTek Security Advisory for the current list of models, fixed firmware versions, and download locations. Verify firmware integrity using the vendor-published hash before applying.
Workarounds
- Disable the web management interface on WAN and untrusted interfaces; use SSH or serial console for administration where feasible
- Enforce network-layer access control lists that restrict TCP/80 and TCP/443 to the switch only from authorized management workstations
- Require multi-factor authentication in front of the management network using a jump host or bastion
- Isolate management traffic on a dedicated out-of-band network segment
# Configuration example: restrict management access with an upstream ACL
# Permit only the management workstation subnet to reach the switch web UI
access-list 110 permit tcp 10.10.99.0 0.0.0.255 host 10.10.1.10 eq 443
access-list 110 permit tcp 10.10.99.0 0.0.0.255 host 10.10.1.10 eq 80
access-list 110 deny tcp any host 10.10.1.10 eq 443
access-list 110 deny tcp any host 10.10.1.10 eq 80
access-list 110 permit ip any any
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
