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medium · CVSS 5.9

CVE-2026-85534

Libsoup: libsoup: http/2 client crash in on_data_source_read_callback when settings initial_window_size shrinks during deferred body read

Published 4 Sept 2026 · Source updated 6 Nov 2026 · Retrieved 2026-11-06 01:53 UTC

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What happened?

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.

Description supplied by CVE Program. Formatting is simplified; follow the original advisory for full details.

Affected software and versions

Source-provided software and version information
SoftwareVersion informationFixed version
libsoup3Red HatDefault status: affectedSee vendor advisory
libsoupRed HatDefault status: unaffectedSee vendor advisory
libsoupRed HatDefault status: unaffectedSee vendor advisory
libsoupRed HatDefault status: unaffectedSee vendor advisory
libsoupRed HatDefault status: unaffectedSee vendor advisory

Version information comes from the source record and can include unaffected products and exceptions. Distribution packages may contain backported fixes: an older upstream version alone does not prove a server is affected.

Is it being exploited?

This CVE was not listed in the CISA catalogue at our last retrieval. This does not establish that exploitation is absent.

CISA data retrieved 2026-11-06 16:38 UTC. Check CISA ↗

How to respond

  1. Confirm the product, installed package version, and any configuration requirements in the vendor advisory.
  2. Follow the vendor’s remediation or workaround guidance in the references below. Structured fixed-version data is unavailable here; the publisher’s description may include a fix or workaround.
  3. Restart affected services if required, then verify the running version. Known exploitation may warrant investigation as well as patching.

Sources and remediation references

This public advisory does not determine whether a particular server is affected. Records are refreshed as sources change; verify urgent decisions against the original advisory.

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