Detect Microsoft Exchange Server Cross-Site Scripting (XSS) Exploitation in Google Chronicle
Detects exploitation attempts targeting CVE-2026-42897, a Cross-Site Scripting (XSS) vulnerability in Microsoft Exchange Server. This KEV-listed vulnerability allows attackers to inject malicious scripts into Exchange web interfaces, potentially leading to session hijacking, credential theft, or further lateral movement within the environment. Detection focuses on anomalous HTTP requests to Exchange OWA/ECP endpoints containing XSS payloads, unexpected script execution from Exchange processes, and suspicious web request patterns indicative of active exploitation.
MITRE ATT&CK
YARA-L Detection Query
rule cve_2026_42897_exchange_xss {
meta:
author = "df00tech Detection Engineering"
description = "Detects XSS exploitation attempts against Microsoft Exchange Server (CVE-2026-42897)"
severity = "HIGH"
priority = "HIGH"
reference = "https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2026-42897"
tags = "cve-2026-42897, exchange, xss, kev"
events:
$e.metadata.event_type = "NETWORK_HTTP"
$e.target.url != ""
re.regex($e.target.url, `(?i)(/owa/|/ecp/|/ews/|/autodiscover/)`)
re.regex($e.target.url, `(?i)(<script|javascript:|onerror=|onload=|eval\(|document\.cookie|fromCharCode|%3Cscript|%6A%61%76%61)`)
condition:
$e
}Chronicle YARA-L 2.0 rule detecting HTTP requests to Microsoft Exchange endpoints containing XSS payload patterns in request URLs.
Data Sources
Required Tables
False Positives & Tuning
- Authorized vulnerability assessments generating XSS test traffic to Exchange
- Security research tooling submitting benign test payloads to Exchange endpoints
- Encoded URL parameters in legitimate Exchange client applications matching XSS patterns
- Web crawlers or bots submitting malformed URLs that resemble XSS payloads
Other platforms for CVE-2026-42897
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Basic Reflected XSS Probe Against Exchange OWA
Expected signal: IIS access log entry with HTTP GET or POST to /owa/auth/logon.aspx containing the URL-encoded XSS payload in cs-uri-query field; source IP of the test machine visible in c-ip field.
- Test 2Cookie-Stealing XSS Payload Delivery to Exchange ECP
Expected signal: IIS access log shows request to /ecp/ with onerror= and document.cookie in the query string, sc-status may be 200 or 400; network log may show outbound HTTP to attacker server if payload executes in a browser context.
- Test 3Encoded XSS Bypass Attempt Against Exchange Autodiscover
Expected signal: IIS access logs show multiple GET requests to /autodiscover/autodiscover.xml with encoded XSS strings in cs-uri-query; Windows PowerShell event log (Event ID 4104) may capture the script block execution.
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