Zero-Days Under Attack: Microsoft Plugs Exchange Server, Excel Holes

Microsoft on Tuesday pushed out patches for at least 55 documented security vulnerabilities in a wide range of products and called urgent attention to a pair of flaws that have already been exploited in the wild.

Microsoft said the two under-attack vulnerabilities exist in Microsoft Exchange Server and Microsoft Excel, two widely deployed products in the Windows ecosystem. 

“We are aware of limited targeted attacks in the wild using one of vulnerabilities (CVE-2021-42321), which is a post-authentication vulnerability in Exchange 2016 and 2019,” Redmond acknowledged, noting that the issue affects on-premises Microsoft Exchange Server, including servers used by customers in Exchange Hybrid mode.

Microsoft slapped a “critical” rating on the Exchange Server bug and warned that an authenticated attacker can launch code execution attacks.  The company did not share details on the pre-patch attacks beyond crediting multiple Chinese researchers for the discovery.

“Our recommendation is to install these updates immediately to protect your environment,” the company said.

The second zero-day was flagged in the popular Microsoft Excel productivity tool and is described as a “feature bypass vulnerability” that allows code execution via specially crafted spreadsheets.  Microsoft said the Excel bug —  CVE-2021-42292 — is being actively exploited but did not provide any additional details.

[ READ: Exchange Server Zero-Days Under Attack by Chinese APT Group ]

The Excel vulnerability affects Microsoft Office for macOS but patches for Apple’s platform are not yet available.

The Microsoft Patch Tuesday updates also include fixes for serious flaws affecting Azure, Microsoft Edge, Windows Defender, Visual Studio and multiple Windows components.

Six of the 55 security bulletins from Microsoft are rated “critical,” Microsoft’s highest severity rating.  Four of the 55 bugs are listed as publicly known.

The Patch Tuesday releases also included patches from software maker Adobe to cover at least four security defects that expose users to malicious hacker attacks.

The most serious of the flaw was addressed in RoboHelp Server and is rated “critical” because it exposes corporate environments to arbitrary code execution attacks.

Adobe warned that the vulnerability — CVE-2021-39858 — affects RoboHelp Server RHS2020.0.1 and earlier versions on the Microsoft Windows platform. The company said it was unaware of any exploits in the wild targeting this flaw.

Related: Adobe Patches Critical RoboHelp Server Security Flaw

Related: Microsoft Says Exchange Server Zero-Days Under Attack by Chinese APT Group

Related: Microsoft Exchange Servers in Attacker Crosshairs

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Ryan Naraine is Editor-at-Large at SecurityWeek and host of the popular Security Conversations podcast series. He is a journalist and cybersecurity strategist with more than 20 years experience covering IT security and technology trends.
Ryan has built security engagement programs at major global brands, including Intel Corp., Bishop Fox and Kaspersky GReAT. He is a co-founder of Threatpost and the global SAS conference series. Ryan’s career as a journalist includes bylines at major technology publications including Ziff Davis eWEEK, CBS Interactive’s ZDNet, PCMag and PC World.
Ryan is a director of the Security Tinkerers non-profit, and a regular speaker at security conferences around the world.
Follow Ryan on Twitter @ryanaraine.

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