CVE-2026-20931 · Windows Telephony Service Elevation of Privilege Vulnerability

External control of file name or path in Windows Telephony Service allows an authorized attacker to elevate privileges over an adjacent network.

What privileges could be gained by an attacker who successfully exploited this vulnerability? An attacker who successfully exploited this vulnerability could gain SYSTEM privileges.

How could an attacker exploit this vulnerability? An attacker with a low‑privileged domain account could remotely send crafted RPC requests to the Telephony service, causing it to write attacker‑controlled data into files on the server. By overwriting Telephony’s configuration file, the attacker can make themselves a Telephony administrator and trigger the service to load a malicious DLL, resulting in remote code execution under a privileged service account.

According to the CVSS metric, the attack vector is adjacent (AV:A). What does that mean for this vulnerability? Successful exploitation of this vulnerability requires that an attacker needs to be in the same restricted Active Directory domain as the target system. The attack surface is not reachable from broader networks, which is why the attack vector is considered adjacent (AV:A).