CVE-2026-71935: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') in DrayTek Corporation VigorSwitch G2540xs

CVE-2026-71935 is a classic buffer overflow vulnerability in multiple DrayTek VigorSwitch models, specifically in the webBackupAction function. The flaw arises from repeated concatenation of input fields (pathN, valueN, key, option) into fixed-size stack buffers without verifying the total length, allowing a remote attacker with administrative access to trigger a buffer overflow. This can result in denial of service or potentially arbitrary code execution on the device.

An attacker with valid administrative credentials to the device's web management interface can exploit this vulnerability to cause a denial of service or execute arbitrary commands, potentially compromising the device's integrity and availability.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, restrict administrative access to trusted users and networks to reduce the risk of exploitation.