# CVE-2026-103399: libsoup HTTP request smuggling via undrained Expect: 100-continue body

Source: https://junglewise.ai/threats/cve-2026-103399-a-flaw-was-found-in-soupserver-libsoup-when-an-http-1-x-client  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-30. Updated: 2026-09-30  
License: CC BY 4.0 with a link back

CVE-2026-103399 · Severity: medium · CVSS 5.3 · Published 2026-09-30

libsoup is a library used in web servers to handle HTTP requests. When a client sends a request with the "Expect: 100-continue" header, the server can sometimes send a response before reading the full request body, leaving leftover bytes on the connection. An attacker can exploit this on reused connections to inject a hidden HTTP request that the server processes unintentionally, potentially leading to unintended actions or data exposure.

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

## Quick answers

**Is CVE-2026-103399 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-103399 being exploited in the wild.

**How severe is CVE-2026-103399?** Medium severity, with a CVSS score of 5.3, published 30 September 2026.

**Which products and versions are affected?** GNOME libsoup.

**Is there a fix for CVE-2026-103399?** 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-2026-103399 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-103399?** Check whether you run Gnome Libsoup. 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

This is an HTTP/1 request smuggling vulnerability (CWE-444) in SoupServer that occurs when an early final response is sent for an Expect: 100-continue request before the body is consumed. Undrained body bytes on a keep-alive connection are interpreted as a subsequent HTTP request, allowing a remote unauthenticated attacker to inject smuggled requests. The vulnerability requires the server to be exposed to untrusted networks and typically affects lightweight development or internal servers rather than hardened production deployments.

## Affected products

- GNOME libsoup

## Timeline

- 2026-09-30: 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-2026-103399](https://access.redhat.com/security/cve/CVE-2026-103399)

## Related threats

- [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)
- [libsoup heap buffer overflow in WebSocket handling](https://junglewise.ai/threats/cve-2026-102558-a-flaw-was-found-in-libsoup-when-max-incoming-payload-size-is)
- [libsoup out-of-bounds read in data-URI base64 decoding](https://junglewise.ai/threats/cve-2026-102555-a-flaw-was-found-in-libsoup-the-soup-uri-decode-data-uri)
- [libsoup heap corruption in WebSocket message reassembly](https://junglewise.ai/threats/cve-2026-102557-a-flaw-was-found-in-libsoup-when-reassembling-fragmented)
- [libsoup type confusion in WebSocket Pong signal](https://junglewise.ai/threats/cve-2026-102556-a-flaw-was-found-in-libsoup-when-handling-an-incoming-websocket)