CVE-2026-71935 Overview
CVE-2026-71935 is a stack-based buffer overflow in the webBackupAction function of multiple DrayTek VigorSwitch models. The flaw stems from repeated string concatenation of the pathN, valueN, key, and option fields into fixed-size stack buffers without total length validation [CWE-120]. A remote authenticated attacker with administrative credentials to the web management interface can send crafted input to corrupt stack memory. Successful exploitation causes a denial of service and may lead to arbitrary command execution on the switch. The vulnerability was disclosed in the DrayTek Security Advisory and analyzed in the VulnCheck DrayTek Advisory.
Critical Impact
Authenticated remote attackers can crash affected VigorSwitch devices or execute arbitrary commands with the privileges of the web management service.
Affected Products
- Multiple DrayTek VigorSwitch series models (see vendor advisory for the complete model list)
- DrayTek VigorSwitch firmware versions prior to the August 2026 security update
- Devices exposing the web management interface to attacker-reachable networks
Discovery Timeline
- 2026-08-24 - CVE-2026-71935 published to NVD
- 2026-08-26 - Last updated in NVD database
Technical Details for CVE-2026-71935
Vulnerability Analysis
The webBackupAction handler in the VigorSwitch web management interface processes configuration backup requests containing multiple user-controlled parameters. The function iterates over indexed fields (pathN, valueN) along with key and option values and appends each into fixed-size stack buffers. Because the concatenation logic checks individual field sizes rather than the cumulative length written to the destination buffer, sufficiently long or numerous inputs overflow the buffer and overwrite adjacent stack memory. This corruption can crash the HTTP service, disrupting device management. On architectures without robust stack protections, the overflow may allow an attacker to redirect execution and run arbitrary commands with the privileges of the web service. Because the vulnerable handler resides in the administrative interface, exploitation requires valid credentials for the switch.
Root Cause
The root cause is unsafe string concatenation into fixed-size stack buffers using functions that do not enforce cumulative bounds. Each individual write may appear valid, but the running total exceeds the destination buffer capacity. The handler lacks total-length checks between concatenation steps, which is a classic classic buffer copy without size checking pattern.
Attack Vector
An attacker authenticates to the VigorSwitch web management interface with administrative credentials, then issues a crafted HTTP request to the backup endpoint. The request supplies oversized or repeated pathN, valueN, key, and option values designed to overflow the target buffer. The malformed request triggers memory corruption inside webBackupAction, producing a denial of service or, potentially, control-flow hijack.
No verified proof-of-concept code is available. Refer to the VulnCheck DrayTek Advisory for additional technical detail.
Detection Methods for CVE-2026-71935
Indicators of Compromise
- Unexpected restarts or crashes of the VigorSwitch HTTP management daemon following configuration backup requests
- HTTP POST requests to the backup action endpoint containing unusually long or repeated pathN, valueN, key, or option parameters
- Administrative logins from unfamiliar source addresses immediately preceding management-plane instability
Detection Strategies
- Inspect web server and syslog output from the switch for repeated segmentation faults or watchdog-triggered reboots of the management process
- Deploy network signatures that flag oversized parameter values submitted to the VigorSwitch backup endpoint
- Correlate administrative authentication events with subsequent management-interface errors to identify credential misuse
Monitoring Recommendations
- Forward switch management logs to a centralized log platform and alert on management-daemon crashes
- Monitor HTTP request sizes and parameter counts submitted to the VigorSwitch web interface
- Track administrative account usage and alert on logins from non-approved management subnets
How to Mitigate CVE-2026-71935
Immediate Actions Required
- Apply the firmware update referenced in the DrayTek Security Advisory as soon as it is available for your model
- Restrict access to the VigorSwitch web management interface to a dedicated management VLAN or jump host
- Rotate administrative credentials on affected switches and disable any unused administrator accounts
Patch Information
DrayTek has published fixed firmware versions for affected VigorSwitch models. Consult the vendor advisory for the exact firmware build number that applies to each model and upgrade through the standard DrayTek firmware update procedure. Verify the firmware image checksum before installation.
Workarounds
- Block external access to the switch web management interface at the perimeter firewall
- Enforce access control lists that permit management HTTP or HTTPS only from trusted administrator addresses
- Where operationally feasible, disable the HTTP management service and manage devices exclusively over SSH or an out-of-band channel
# Example ACL restricting VigorSwitch web management to a management subnet
# Adapt to your switch CLI syntax; consult DrayTek documentation for exact commands
access-list mgmt-http permit tcp 10.10.0.0 0.0.0.255 any eq 443
access-list mgmt-http deny tcp any any eq 443
access-list mgmt-http deny tcp any any eq 80
interface vlan 1
ip access-group mgmt-http in
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
