# CVE-2026-103399: Libsoup: soupserver: http/1 request smuggling via undrained expect: 100-continue body

> Remote attackers can smuggle HTTP requests via undrained bodies in SoupServer, causing unintended request processing on keep-alive connections.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 30 Sep 2026
- **Updated:** 30 Sep 2026
- **Also known as:** AZL-105543, DEBIAN-CVE-2026-103399, ECHO-0353-a267-f75e, GHSA-mfgq-q7gx-p678, UBUNTU-CVE-2026-103399

## What is CVE-2026-103399?

CVE-2026-103399 is a medium-severity vulnerability in Red Hat Enterprise Linux 10, published on 30 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A flaw was found in SoupServer (libsoup). When an HTTP/1.x client sends a request with Expect: 100-continue and a request body, and SoupServer returns an early final (non-1xx) response before the body is read, the server neither drains the declared body bytes nor closes the connection. On a keep-alive connection, those leftover bytes are interpreted as a subsequent HTTP request. A remote, unauthenticated attacker can place a complete HTTP request in the body and cause SoupServer to process that smuggled request, leading to unintended request handling.

Weakness: CWE-444 (Inconsistent Interpretation of HTTP Requests)

## Which versions are affected by CVE-2026-103399?


## How do I fix CVE-2026-103399?

Apply the vendor's patch.

## Is CVE-2026-103399 being exploited?

There is no known exploitation of CVE-2026-103399 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-103399 and fix it if it is affected.

What it is: CVE-2026-103399 is a medium-severity vulnerability in Red Hat Enterprise Linux 10, published on 30 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-103399 in the wild and no public exploit code.

Affected product: Red Hat Enterprise Linux 10. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-103399/ (Markdown: https://insights.sbomify.com/CVE-2026-103399/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-103399
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-103399
- OSV: https://osv.dev/vulnerability/CVE-2026-103399

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-103399/
Summaries are generated and may contain mistakes; the official description above is the record.