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MS Patch Tuesday: NSA Reports New Critical Exchange Flaws

Just weeks after a wave of major in-the-wild zero-day attacks against Exchange Server installations globally, Microsoft is raising a fresh alarm for four new critical security flaws that expose businesses to remote code execution attacks.

Just weeks after a wave of major in-the-wild zero-day attacks against Exchange Server installations globally, Microsoft is raising a fresh alarm for four new critical security flaws that expose businesses to remote code execution attacks.

The four new Exchange Server vulnerabilities were fixed as part of this month’s Patch Tuesday bundle and because of the severity of these issues, Microsoft has joined with the U.S. National Security Agency (NSA) to urge the immediate deployment of the new fixes.

The NSA is credited with reporting two of the four Exchange Vulnerabilities — CVE-2021-28480 and CVE-2021-28481 – and the agency is warning that exploitation “could allow persistent access and control of enterprise networks.”

[ SEE: Microsoft Exchange Server Zero-Days Under Attack  ]

The two NSA-discovered bugs carry a CVSS score of 9.8 because of the risk of pre-auth code execution attacks without user interaction.  TippingPoint’s ZDI believes these bugs may be wormable between Exchange servers.

“Considering the source, and considering these bugs also receive Microsoft’s highest Exploit Index rating, assume they will eventually be exploited. Update your systems as soon as possible,” ZDI added.

Separately, Microsoft also patched an under-attack zero-day discovered by Boris Larin, a malware hunter at Kaspersky who regularly finds vulnerabilities being used in the wild. That vulnerability —  CVE-2021-28310 — exists in the Desktop Window Manager and is closely related to a similar bug used by a known APT group.

“We believe this exploit is used in the wild, potentially by several threat actors. It is an escalation of privilege (EoP) exploit that is likely used together with other browser exploits to escape sandboxes or get system privileges for further access. Unfortunately, we weren’t able to capture a full chain, so we don’t know if the exploit is used with another browser zero-day, or coupled with known, patched vulnerabilities,” Kaspersky’s researchers said in a blog post accompanying Microsoft’s patch release.

All told, Microsoft shipped patches for at least 114 documented vulnerabilities (counting by CVEs) in Microsoft Windows, the Chromium-based Edge browser,, Azure and Azure DevOps Server, Microsoft Office, SharePoint Server, Hyper-V, Team Foundation Server, Visual Studio, and Exchange Server. 

ZDI says this is the largest number of CVEs addressed in a month by Microsoft this year, and it is slightly higher than April of last year. 

Of the 114 documented bugs, 19 are rated “critical,” Microsoft’s highest severity rating. Microsoft ranks 88 as “important, and one as moderate in severity. 

Written By

Ryan Naraine is Editor-at-Large at SecurityWeek and host of the popular Security Conversations podcast series. He is a security community engagement expert who has built programs at major global brands, including Intel Corp., Bishop Fox and GReAT. Ryan is a founding-director of the Security Tinkerers non-profit, an advisor to early-stage entrepreneurs, and a regular speaker at security conferences around the world.

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