# CVE-2026-85534: libsoup HTTP/2 denial of service via buffer overflow in data callback

Source: https://junglewise.ai/threats/cve-2026-85534-libsoup-http-2-denial-of-service-via-buffer-overflow-in-data  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-04. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-85534 · Severity: medium · CVSS 5.9 · Published 2026-09-04

libsoup is a networking library used by many applications to handle HTTP/2 communications. An attacker running a malicious HTTP/2 server can cause a client application using libsoup to crash or terminate its session by exploiting a buffer handling flaw during file uploads. This affects availability of any application relying on libsoup for HTTP/2 uploads from non-pollable streams.

Technologies: [Gnome Libsoup](https://junglewise.ai/threats/technologies/libsoup). Vendors: [Gnome](https://junglewise.ai/threats/vendors/gnome).

## Quick answers

**Is CVE-2026-85534 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-85534 being exploited in the wild.

**How severe is CVE-2026-85534?** Medium severity, with a CVSS score of 5.9, published 4 September 2026.

**Which products and versions are affected?** Gnome Libsoup 3.0.0 and later.

**Is there a fix for CVE-2026-85534?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-85534 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-85534?** Check whether you run Gnome Libsoup at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A buffer overflow vulnerability exists in libsoup's HTTP/2 implementation (on_data_source_read_callback) when handling request bodies from non-pollable input streams (e.g., GFileInputStream). The flaw occurs when a malicious HTTP/2 server shrinks the SETTINGS_INITIAL_WINDOW_SIZE parameter while a buffered read is in progress. The client then attempts to copy the full buffer into a smaller DATA callback without runtime bounds validation, causing an assertion failure (CWE-617). The vulnerability requires a client to initiate an HTTP/2 request body upload to an attacker-controlled server. In Red Hat builds with assertions enabled, this results in process abort rather than memory corruption. The vulnerability affects libsoup 3.0.0 and later versions.

## Affected products

- GNOME libsoup 3.0.0 and later

## Timeline

- 2026-09-04: disclosed

## References

- [https://access.redhat.com/downloads/content/package-browser/](https://access.redhat.com/downloads/content/package-browser/)
- [https://access.redhat.com/security/cve/CVE-2026-85534](https://access.redhat.com/security/cve/CVE-2026-85534)

## Related threats

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- [libsoup heap buffer overflow in permessage-deflate WebSocket extension](https://junglewise.ai/threats/cve-2026-102560-a-flaw-was-found-in-libsoup-when-the-permessage-deflate)
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- [libsoup heap corruption in WebSocket message reassembly](https://junglewise.ai/threats/cve-2026-102557-a-flaw-was-found-in-libsoup-when-reassembling-fragmented)