Description

Microsoft Outlook recently faced a now-patched security vulnerability, tracked as CVE-2023-35636, with a CVSS score of 6.5, that could potentially allow threat actors to access NT LAN Manager (NTLM) v2 hashed passwords. The flaw was addressed in Microsoft's Patch Tuesday updates for December 2023. The vulnerability, rooted in the calendar-sharing function of Outlook, could be exploited through a specially crafted file sent via email or hosted on a compromised website. In an email attack scenario, users could be tricked into opening the malicious file, exposing their NTLM v2 hashed passwords. Similarly, in a web-based attack scenario, threat actors could host a website containing the crafted file to exploit the vulnerability. The issue arises from two headers, "Content-Class" and "x-sharing-config-url," inserted with manipulated values in a malicious email message. The discovery and reporting credit go to Varonis security researcher Dolev Taler. Taler noted that NTLM hashes could be leaked using Windows Performance Analyzer (WPA) and Windows File Explorer, and these two attack vectors remain unpatched. Notably, NTLM v2 hash passing through the open internet is susceptible to relay and offline brute-force attacks. This revelation aligns with Microsoft's decision in October 2023 to discontinue NTLM in Windows 11 in favor of Kerberos for enhanced security, as NTLM lacks support for cryptographic methods and is vulnerable to relay attacks. The disclosure adds to recent findings by Check Point regarding "forced authentication," potentially leading to the leakage of a Windows user's NTLM tokens through deceptive means involving a rogue Microsoft Access file.