CVE-2026-83746 in Lync
Summary
by MITRE • 10/14/2026
Absolute path traversal in Skype for Business and Microsoft Lync allows an authorized attacker to elevate privileges over a network.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/14/2026
The vulnerability described constitutes a critical absolute path traversal flaw within the software architectures of Skype for Business and its predecessor, Microsoft Lync. This security defect arises from insufficient validation of user-supplied input when processing file paths or resource requests. In typical operation, these applications handle various types of data inputs that may include references to local system resources. When an attacker provides a specially crafted absolute path rather than the expected relative or sanitized path segment, the application fails to properly restrict access to designated directories. This lack of boundary enforcement allows the software to interpret and execute commands targeting arbitrary locations on the host file system, bypassing intended security controls that are designed to isolate application data from sensitive operating system files.
From a technical perspective, this flaw represents a classic instance of improper input validation leading to unauthorized resource access. The vulnerability is categorized under CWE-22: Improper Limitation of a Pathname to a Restricted Directory within the Common Weakness Enumeration taxonomy. By leveraging an absolute path traversal technique, an attacker can manipulate internal file operations such as reading configuration files, accessing cached credentials, or potentially overwriting critical system binaries if write permissions are granted through other privilege escalation vectors. The core issue lies in the application's failure to canonicalize and verify that the resolved path remains within the expected sandboxed environment defined by its security policy. This behavior violates fundamental principles of least privilege and defense in depth, allowing a process running with limited user privileges to interact with resources outside its designated scope.
The operational impact of this vulnerability is severe due to its potential for local privilege escalation on networked systems where Skype for Business or Lync clients are installed. An authorized attacker who has gained access to the victim's machine, either through phishing, malware infection, or compromised credentials, can exploit this flaw to read sensitive information stored in protected directories. This may include cached authentication tokens, configuration settings containing connection strings or server addresses, and potentially other user-specific data that is not intended for general application use. In environments where these applications are deployed with elevated privileges or integrated deeply into enterprise identity management systems, the ability to traverse paths arbitrarily could lead to further compromise of network infrastructure by exposing critical security configurations or facilitating lateral movement through stolen credentials found in accessible files.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. Microsoft has addressed this issue through specific software updates that patch the underlying input validation logic within Skype for Business and Lync components. Administrators should ensure that all client machines are updated with the latest cumulative security patches released by Microsoft to close these path traversal gaps. Additionally, organizations should enforce strict application control policies using tools like Windows Defender Application Control or AppLocker to restrict which applications can access specific file system paths, thereby adding a layer of defense even if an attacker exploits this vulnerability. Regular auditing of installed software and removal of legacy versions such as Lync that are no longer supported further reduces the attack surface. Security teams should also monitor for unusual file access patterns originating from these application processes using endpoint detection and response solutions to detect potential exploitation attempts in real time.