CVE-2026-66338: A flaw was found in libsoup

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MEDIUM 5.4REMOTEWATCHLISTMITIGATEMITIGATE

CVE-2026-66338 is a medium-severity flaw (CVSS 5.4) affecting libsoup, enterprise linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 79%)
CISA exploited list	No
Weakness	CWE-444
Published	2026-07-24
Last modified	2026-08-24

Publisher (CNA): Red Hat, Inc. · Vendor: gnome · redhat

Last updated 2026-08-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-24 23:16 UTCPublished CVE record published
2026-08-24 16:44 UTCUpdated Record last modified

gnome libsoup MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libsoup || rpm -q libsoup || libsoup --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

redhat enterprise linux MITIGATE

Affected: 6.0 · 7.0 · 8.0 · 9.0 · 10.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 6.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Advisories and coverage

Official advisories

Amazon Linux 2023 ALAS: ALAS2023-2026-3240 (medium): libsoup3 2026-10-24

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-66338?

libsoup: mitigate or apply the listed mitigation; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-66338 being exploited?

CVE-2026-66338 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-66338 affect?

CVE-2026-66338 affects libsoup, enterprise linux. Severity is medium (CVSS 5.4). 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 cha

How urgent is CVE-2026-66338?

Priority 39/100. Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Related CVEs

CVE-2026-2708 LOW 3.7 MITIGATE MITIGATE A request smuggling vulnerability exists in libsoup's HTTP/1 header parsing logic
CVE-2026-12548 MEDIUM 4.2 MITIGATE MITIGATE A heap out-of-bounds read flaw was found in libsoup
CVE-2026-102560 HIGH 8.6 UPGRADE MITIGATE A flaw was found in libsoup
CVE-2026-102559 HIGH 8.6 UPGRADE MITIGATE A flaw was found in libsoup
CVE-2026-102558 HIGH 8.6 UPGRADE MITIGATE A flaw was found in libsoup

References

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