---
id: CVE-2026-66338
title: >-
  Libsoup: libsoup: http request smuggling via permissive chunk-size parsing in
  soup_body_input_stream_read_chunked()
summary: >-
  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 …
severity: medium
cvss: 5.4
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N'
cvssSource: cna
cwe:
  - CWE-444
vendor: Red Hat
product: libsoup3
affected:
  - libsoup3 (all versions)
  - libsoup (all versions)
  - libsoup (all versions)
  - libsoup (all versions)
  - libsoup (all versions)
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-07-27T14:34:59.011664Z'
published: '2026-07-24'
updated: '2026-11-06'
sourceUpdated: '2026-11-06T19:27:20.998Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-66338'
references:
  - url: 'https://access.redhat.com/security/cve/CVE-2026-66338'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2506950'
    label: RHBZ#2506950
tags:
  - cve.org
epss: 0.00291
epssPercentile: 0.21793
ingestedAt: '2026-11-06T20:03:04.275Z'
---

## Overview

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.

## Affected

- `libsoup3 (all versions)`
- `libsoup (all versions)`
- `libsoup (all versions)`
- `libsoup (all versions)`
- `libsoup (all versions)`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.