Vulnerability in GNOME Libsoup
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.
Vulnerability class: HTTP Request Smuggling
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (20.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 21st percentile — 20.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: +0.000
Affected products
Weakness classification (CWE)
References
- access.redhat.com/security/cve/CVE-2026-66338 (vdb-entry, Vendor Advisory)
- bugzilla.redhat.com/show_bug.cgi (issue-tracking, Exploit, Issue Tracking, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-66338?
- CVE-2026-66338 is a medium-severity vulnerability in GNOME Libsoup, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 5.4/10. Published 2026-07-24.
- How severe is CVE-2026-66338?
- Medium severity. CVSS v3 base score is 5.4 out of 10.