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CVE-2026-42825: Use-After-Free in Windows Telephony Service

CVE-2026-42825 is a use-after-free vulnerability in the Windows Telephony Service that allows an authorized, local attacker to elevate privileges.

CVE-2026-42825 is a use-after-free vulnerability affecting the Windows Telephony Service. This vulnerability allows an attacker with local access and low privileges to potentially elevate their privileges on the system. The vulnerability arises from improper memory management within the Telephony Service, leading to a situation where a freed memory region is accessed again. Successful exploitation could lead to arbitrary code execution with elevated privileges. Microsoft has acknowledged this vulnerability and assigned it a CVSS v3.1 score of 7.0, indicating a high severity. Defenders should monitor for unusual activity related to the Telephony Service and prioritize patching to mitigate this risk.

Attack Chain

  1. Attacker gains initial local access to the target system with low privileges.
  2. Attacker identifies the Windows Telephony Service running on the system.
  3. Attacker crafts a specific input that triggers the use-after-free condition in the Telephony Service.
  4. The malicious input causes the Telephony Service to free a memory region.
  5. The attacker then causes the Telephony Service to access the freed memory region.
  6. This memory access allows the attacker to overwrite critical system data or inject malicious code.
  7. The injected code is executed with the privileges of the Telephony Service, leading to privilege elevation.
  8. Attacker leverages elevated privileges to perform unauthorized actions on the system.

Impact

Successful exploitation of CVE-2026-42825 allows a local attacker to escalate their privileges on a vulnerable Windows system. This could allow the attacker to gain complete control over the system, install malware, access sensitive data, or disrupt critical services. Given the potential for complete system compromise, organizations should prioritize patching this vulnerability.

Recommendation

  • Apply the patch released by Microsoft to address CVE-2026-42825 on all affected systems.
  • Enable Sysmon process creation logging to monitor for unusual processes spawned by the Telephony Service, facilitating the detection of potential exploitation attempts via the Sigma rules provided.
  • Monitor for unexpected modifications to system files or registry keys by the Telephony Service, using endpoint detection and response (EDR) solutions.
  • Deploy the Sigma rules in this brief to your SIEM and tune for your environment.

Detection coverage 2

Detect CVE-2026-42825 Exploitation Attempt - Telephony Service Spawning Suspicious Processes

high

Detects CVE-2026-42825 exploitation attempts — monitors for the Telephony Service (TapiSrv.exe) spawning suspicious child processes, indicative of potential code injection or privilege escalation.

sigma tactics: execution, privilege_escalation techniques: T1059.001, T1068 sources: process_creation, windows

Detect CVE-2026-42825 Exploitation Attempt - Suspicious Modules Loaded by TapiSrv

medium

Detects CVE-2026-42825 exploitation attempts — monitors for the Telephony Service loading suspicious or unusual modules, potentially indicating code injection.

sigma tactics: defense_evasion, privilege_escalation techniques: T1068, T1574.002 sources: image_load, windows

Detection queries are available on the platform. Get full rules →