A buffer overflow exists in the UPnP AddPortMapping command on Zyxel VMG4005-B50B firmware versions up to 5.13(ABRL.5.4)C0. The flaw allows an attacker to send a carefully crafted UPnP request that overflows the server’s input buffer, causing a temporary denial‑of‑service of the device’s UPnP functionality. The overflow does not give direct code execution or persistent compromise, but it can interrupt network port‑mapping services for users who rely on UPnP to manage traffic flows.

The vulnerability affects Zyxel VMG4005-B50B routers running firmware versions through 5.13(ABRL.5.4)C0. Users of earlier firmware are not impacted, while newer builds beyond this version are presumed patched. The flaw applies specifically to the UPnP protocol implementation bundled with the device’s firmware.

With a CVSS score of 6.5, the risk is considered medium. No EPSS score is available, and the vulnerability is not listed in the CISA KEV catalog, indicating no widespread exploitation has been reported. The likely attack vector is a local or near‑network attacker able to communicate with the UPnP services, which is plausible for devices exposed to the internet or shared networks. Exploitation requires only the ability to send a malformed UPnP packet; the effect is a short‑lived denial of the UPnP interface, potentially disrupting applications that automatically configure port forwards. Given the moderate severity and the ease of mitigation, prompt action is recommended.