CVE-2026-42504 and is rated High severity with a CVSS score of 7.5. It is not in the current CISA KEV record we collected. That does not prove exploitation has not occurred.
What to do next
Remediation and response
Microsoft Security Response Center guidance
Vendor revision: Sep 17, 2026
CBL-Mariner Releases
Vendor-listed releases
1.26.4-1
CISA KEVNot listedBased on the latest collected catalog
EPSS0.6%Estimated 30-day exploitation probability
Ransomware useNot markedCISA KEV ransomware field
Threat actors0Source-linked actor relationships
Overview
What is CVE-2026-42504?
Decoding a maliciously-crafted MIME header containing many invalid encoded-words can consume excessive CPU.