Published Jul 24, 2026Sources checked Oct 28, 2026
7.2HIGHCVSS out of 10
What this means
Review the available evidence
CVE-2026-66338 and is rated High severity with a CVSS score of 7.2. 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 24, 2026
Release Notes
CISA KEVNot listedBased on the latest collected catalog
EPSS0.3%Estimated 30-day exploitation probability
Ransomware useNot markedCISA KEV ransomware field
Threat actors0Source-linked actor relationships
Overview
What is CVE-2026-66338?
A flaw was found in libsoup. The chunked transfer encoding parser uses a permissive parsing function for chunk sizes that silently accepts inputs violating RFC 9112, including leading whitespace, plus sign prefixes, and trailing invalid characters.
When libsoup operates behind a strict frontend proxy, this parsing differential can be exploited to smuggle HTTP requests.