---
id: CVE-2026-85534
title: A flaw was found in libsoup
summary: >-
  A flaw was found in libsoup. When a client sends an HTTP/2 request body from a
  non-pollable input stream, the library can buffer more data than the current
  flow-control window later allows. A malicious HTTP/2 server can shrink
  SETTINGS_I…
severity: medium
cvss: 5.9
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-617
published: '2026-09-04'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T19:08:15.590'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-85534'
references:
  - url: 'https://access.redhat.com/security/cve/CVE-2026-85534'
    label: secalert@redhat.com
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2528440'
    label: secalert@redhat.com
  - url: >-
      https://gitlab.gnome.org/GNOME/libsoup/-/blob/master/libsoup/http2/soup-client-message-io-http2.c
    label: secalert@redhat.com
  - url: 'https://gitlab.gnome.org/GNOME/libsoup/-/work_items/551'
    label: secalert@redhat.com
tags:
  - nvd
epss: 0.00315
epssPercentile: 0.24702
ingestedAt: '2026-09-08T20:10:03.164Z'
---

## Overview

A flaw was found in libsoup. When a client sends an HTTP/2 request body from a non-pollable input stream, the library can buffer more data than the current flow-control window later allows. A malicious HTTP/2 server can shrink SETTINGS_INITIAL_WINDOW_SIZE while that buffered read is still in progress. The client then copies the full buffer into a smaller DATA callback without a runtime bounds check, which can abort the process or fail the HTTP/2 session.

## Remediation

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