CVE-2026-42897: Microsoft Exchange Server Cross-Site Scripting Vulnerability

What it is

Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing over a network.

Exploitation evidence

First exploit evidence seen: 2026-05-15 (earliest date across the sources below).

Exploited in the wild

• CISA KEV: listed since 2026-05-15. Source

Public exploit code

GitHub proof-of-concept repositories below quality bar

1 public repositories from 1 accounts, first seen 2026-05-15.

Verify before running: public PoCs can be malicious. Read the code and only run it in an isolated lab. A PoC means exploit code is public, not that this CVE is exploited in the wild.

Sources: CISA KEV · nomi-sec/PoC-in-GitHub.

How to fix it

CISA required action

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. (source: CISA KEV)

Fixed versions

• microsoft exchange server subscription edition: upgrade to 15.02.2562.043 or later

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

• https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2026-42897 (CISA KEV notes)
• https://learn.microsoft.com/en-us/exchange/plan-and-deploy/post-installation-tasks/security-best-practices/exchange-emergency-mitigation-service (CISA KEV notes)
• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42897 (Mitigation, Vendor Advisory)

MITRE ATT&CK mitigations for this chain

• M1050 Exploit Protection (for T1189) : Security applications that look for behavior used during exploitation such as Windows Defender Exploit Guard (WDEG) and the Enhanced Mitigation Experience Toolkit (EMET) can be used to mitigate some exploitation behavior. Control flow integrity checking is another way to potentially identify and stop a software exploit
• M1051 Update Software (for T1189) : Ensuring that all browsers and plugins are kept updated can help prevent the exploit phase of this technique. Use modern browsers with security features turned on.
• M1048 Application Isolation and Sandboxing (for T1189) : Browser sandboxes can be used to mitigate some of the impact of exploitation, but sandbox escapes may still exist. Other types of virtualization and application microsegmentation may also mitigate the impact of client-side exploitation. The risks of additional exploits and weaknesses in implementation may still exist
• M1021 Restrict Web-Based Content (for T1189, T1059.007) : Adblockers can help prevent malicious code served through ads from executing in the first place. Script blocking extensions can also help to prevent the execution of JavaScript. Consider disabling browser push notifications from certain applications and browsers.
• M1017 User Training (for T1189) : Train users to be aware of access or manipulation attempts by an adversary to reduce the risk of successful spearphishing, social engineering, and other techniques that involve user interaction.
• M1040 Behavior Prevention on Endpoint (for T1059.007) : On Windows 10, enable Attack Surface Reduction (ASR) rules to prevent [JavaScript](https://attack.mitre.org/techniques/T1059/007) scripts from executing potentially malicious downloaded content .
• M1038 Execution Prevention (for T1059.007) : Denylist scripting where appropriate.
• M1042 Disable or Remove Feature or Program (for T1059.007) : Turn off or restrict access to unneeded scripting components.

Detection Generic guidance

• Initial Access: Alert on unusual requests to the affected service (odd paths, oversized payloads, exploit strings) in WAF, reverse-proxy and application logs.
• Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.

Response checklist Generic guidance

1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.