Feb 15, 2024NewsroomThreat Intelligence / Vulnerability

Microsoft on Wednesday acknowledged that a newly disclosed critical security flaw in Exchange Server has been actively exploited in the wild, a day after it released fixes for the vulnerability as part of its Patch Tuesday updates.

Tracked as CVE-2024-21410 (CVSS score: 9.8), the issue has been described as a case of privilege escalation impacting the Exchange Server.

“An attacker could target an NTLM client such as Outlook with an NTLM credentials-leaking type vulnerability,” the company said in an advisory published this week.

“The leaked credentials can then be relayed against the Exchange server to gain privileges as the victim client and to perform operations on the Exchange server on the victim’s behalf.”


Successful exploitation of the flaw could permit an attacker to relay a user’s leaked Net-NTLMv2 hash against a susceptible Exchange Server and authenticate as the user, Redmond added.

The tech giant, in an update to its bulletin, revised its Exploitability Assessment to “Exploitation Detected,” noting that it has now enabled Extended Protection for Authentication (EPA) by default with the Exchange Server 2019 Cumulative Update 14 (CU14) update.

Details about the nature of the exploitation and the identity of the threat actors that may be abusing the flaw are currently unknown. However, Russian state-affiliated hacking crews such as APT28 (aka Forest Blizzard) have a history of exploiting flaws in Microsoft Outlook to stage NTLM relay attacks.

Earlier this month, Trend Micro implicated the adversary to NTLM relay attacks targeting high-value entities at least since April 2022. The intrusions targeted organizations dealing with foreign affairs, energy, defense, and transportation, as well as those involved with labor, social welfare, finance, parenthood, and local city councils.

Critical Exchange Server Flaw

CVE-2024-21410 adds to two other Windows flaws – CVE-2024-21351 (CVSS score: 7.6) and CVE-2024-21412 (CVSS score: 8.1) – that have been patched by Microsoft this week and actively weaponized in real-world attacks.

The exploitation of CVE-2024-21412, a bug that enables a bypass of Windows SmartScreen protections, has been attributed to an advanced persistent threat dubbed Water Hydra (aka DarkCasino), which has previously leveraged zero-days in WinRAR to deploy the DarkMe trojan.

“The group used internet shortcuts disguised as a JPEG image that, when selected by the user, allows the threat actor to exploit CVE-2024-21412,” Trend Micro said. “The group can then bypass Microsoft Defender SmartScreen and fully compromise the Windows host as part of its attack chain.”


Microsoft’s Patch Tuesday update also addresses CVE-2024-21413, another critical shortcoming affecting the Outlook email software that could result in remote code execution by trivially circumventing security measures such as Protected View.

Codenamed MonikerLink by Check Point, the issue “allows for a wide and serious impact, varying from leaking of local NTLM credential information to arbitrary code execution.”

The vulnerability stems from the incorrect parsing of “file://” hyperlinks by adding an exclamation mark to URLs pointing to arbitrary payloads hosted on attacker-controlled servers (e.g., “file:///!something”).

“The bug not only allows the leaking of the local NTLM information, but it may also allow remote code execution and more as an attack vector,” the cybersecurity firm said. “It could also bypass the Office Protected View when it’s used as an attack vector to target other Office applications.”

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