Microsoft on Thursday disclosed a zero-day vulnerability in Exchange that’s under active exploitation, but four days later customers are still awaiting a patch.
The zero-day, tracked as CVE-2026-42897, affects Exchange Outlook Web Access (OWA) and enables an unauthorized attacker to execute spoofing attacks over a network. According to Microsoft, the zero-day stems from a cross-site scripting (XSS) flawwhich is one of the most common software vulnerabilities found by security researchers, frequently making the Open Web Application Security Project’s (OWASP) Top 10 lists.
“An attacker could exploit this issue by sending a specially crafted email to a user,” Microsoft said in an advisory. “If the user opens the email in Outlook Web Access and certain interaction conditions are met, arbitrary JavaScript can be executed in the browser context.”
CVE-2026-42897 was disclosed two days after a large Patch Tuesday release last week that, ironically, contained no zero-days. The Cybersecurity and Infrastructure Security Agency (CISA) added the flaw to its Known Exploit Vulnerabilities (KEV) catalog on Friday.
Cyber-Risks to OWA Users
CVE-2026-42897 affects the on-premise versions of Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition (SE). Microsoft assigned the zero-day a CVSS score of 8.1, though the NIST’s National Vulnerability Database assigned it a medium-severity 6.1 score.
Microsoft did not provide details about the potential scope of cyberattacks, but in an advisory published on Monday, the Centre for Cybersecurity Belgium (CCB) warned that successful exploitation could give a threat actor access to a victim’s Outlook mailbox and session tokens, and also allow them to make unauthorized changes to mailbox settings or modifications to email content.
While CVE-2026-42897 is a Microsoft Exchange Server vulnerability, the risk is to OWA users’ mailboxes. In a LinkedIn postBogdan Tiron, founder of penetration testing firm Fortbridge, emphasized the impact “isn’t server compromise. It’s mailbox compromise — reading mail, sending emails as the victim, stealing session tokens, planting forwarding rules that survive password resets.” He warned that such mailbox compromises can lead to business email compromise (BEC) or ransomware attacks.
Tiron also noted that that XSS “still owns enterprise mail in 2026,” adding that while such flaws may be considered “junior” threats by the cybersecurity industry, attackers continue to exploit them for reliable initial access to victims’ networks. “The boring vulnerabilities are the ones that keep working,” he warned.
Mitigating the Microsoft Exchange Zero-Day
In a blog postMicrosoft provided two mitigations options that customers can apply while they wait for a patch to arrive. The first, which Microsoft recommended, is for organizations that have the Exchange Emergency Mitigation (EM) Service, which received a mitigation for Exchange Server 2016, 2019, and SEs instances that is enabled automatically.
Microsoft noted that the Exchange EM Service was released in 2021 and is enabled by default. “Using EM Service is the best way for your organization to mitigate this vulnerability right away. If you have EM Service currently disabled, we recommend you enable it right away,” the software giant said.
It’s unclear what percentage of Exchange customers currently have the EM service enabled. Dark Reading contacted Microsoft for comment, and a company spokesperson provided the following statement but did not elaborate further: “We have issued CVE-2026-42897 to address a spoofing vulnerability affecting Exchange Outlook Web Access (OWA). We recommend customers enable EEMS to be better protected and to follow our guidance available here.”
The second mitigation option is an updated Exchange On-premises Mitigation Tool (EOMT), which Microsoft recommended customers download and apply either on a per-server basis or by executing the script through an elevated Exchange Management Shell (EMS).
Microsoft disclosed several issues caused by the mitigation, including disruptions to OWA Print Calendar and OWA light functionality, among other hiccups. Microsoft said it is currently working on a security update for the bug and will deployed it for affected Exchange versions “in the future,” though no timetable was provided.
