CVE-2018-8238 in Lync
Summary
by MITRE
A security feature bypass vulnerability exists when Skype for Business or Lync do not properly parse UNC path links shared via messages, aka "Skype for Business and Lync Security Feature Bypass Vulnerability." This affects Skype, Microsoft Lync.
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Analysis
by VulDB Data Team • 03/01/2020
The CVE-2018-8238 vulnerability represents a critical security feature bypass issue within Skype for Business and Microsoft Lync client applications. This flaw manifests when the affected software fails to properly validate and parse Universal Naming Convention (UNC) path links that are shared through instant messaging conversations. The vulnerability stems from insufficient input validation mechanisms that allow malicious actors to craft specially formatted UNC paths which can bypass the intended security restrictions. When users click on these crafted links, the applications execute with elevated privileges, potentially allowing unauthorized access to network resources and local system components. The security feature bypass occurs because the client applications do not adequately sanitize the UNC paths before processing them, creating an attack vector that can be exploited to gain unauthorized system access. This vulnerability specifically impacts organizations that rely on these communication platforms for business collaboration and poses significant risks to enterprise network security.
The technical implementation of this vulnerability resides in the message parsing and link handling components of the Skype for Business and Lync client software. When processing incoming messages containing UNC paths, the applications fail to properly validate the path format and do not implement adequate sandboxing or privilege separation mechanisms. The flaw allows attackers to construct malicious UNC paths that can trigger automatic execution of network resources or local system commands without proper user consent or authorization. This issue is categorized under CWE-20, which describes improper input validation, and more specifically relates to CWE-284, which deals with improper access control. The vulnerability can be exploited through social engineering techniques where attackers craft convincing messages containing malicious UNC paths that appear legitimate to unsuspecting users. The attack requires minimal user interaction beyond clicking on the malicious link, making it particularly dangerous in enterprise environments where users frequently interact with colleagues through instant messaging platforms.
The operational impact of CVE-2018-8238 extends beyond simple privilege escalation to encompass broader network security implications. Organizations using affected versions of Skype for Business and Lync face potential data exfiltration risks as attackers can leverage this vulnerability to access shared network drives, internal resources, and potentially escalate their access to higher privilege accounts. The vulnerability can facilitate lateral movement within networks, allowing attackers to pivot from the initial compromised endpoint to other systems. From an attack framework perspective, this vulnerability maps to multiple ATT&CK techniques including initial access through social engineering and privilege escalation via exploitation of software vulnerabilities. The impact is particularly severe in environments where users have access to sensitive corporate data and where network segmentation is not properly implemented. Security monitoring becomes challenging as the malicious activity may appear as normal user behavior, making detection difficult without proper network visibility and endpoint monitoring capabilities.
Mitigation strategies for CVE-2018-8238 require a multi-layered approach combining software updates, network security controls, and user awareness training. Microsoft released security patches and updates that address the input validation flaws in affected versions of Skype for Business and Lync, which should be deployed immediately across all affected systems. Network administrators should implement strict firewall rules and network segmentation to limit access to sensitive network resources, particularly shared drives and internal servers. Endpoint protection solutions should be configured to monitor and block suspicious UNC path execution attempts, while user education programs should emphasize the dangers of clicking on unknown links in instant messaging conversations. Organizations should also consider implementing application control policies that restrict the execution of potentially malicious UNC paths, particularly those pointing to network resources. The vulnerability highlights the importance of secure coding practices and proper input validation in client-side applications, particularly those handling user-generated content. Regular security assessments and penetration testing should include evaluation of messaging platform security features to identify similar vulnerabilities that may exist in other communication tools used within the organization.