# CVE-2026-66338: Libsoup: libsoup: http request smuggling via permissive chunk-size parsing in soup_body_input_stream_read_chunked()

> Attackers can smuggle HTTP requests via permissive chunk-size parsing in libsoup when used behind a strict frontend proxy.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/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:** 24 Jul 2026
- **Updated:** 27 Jul 2026
- **Also known as:** AZL-94095, CLSA-2026-1787732034, CLSA-2026-1787732250, CLSA-2026-1787751026, CLSA-2026-1787783675, DEBIAN-CVE-2026-66338, ECHO-9348-56aa-8bba, GHSA-8wm8-3499-x4v9, UBUNTU-CVE-2026-66338

## What is CVE-2026-66338?

CVE-2026-66338 is a medium-severity vulnerability in Redhat Enterprise Linux, published on 24 Jul 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 libsoup. The chunked transfer encoding parser uses a permissive parsing function for chunk sizes that silently accepts inputs violating RFC 9112, including leading whitespace, plus sign prefixes, and trailing invalid characters. When libsoup operates behind a strict frontend proxy, this parsing differential can be exploited to smuggle HTTP requests.

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

## Which versions are affected by CVE-2026-66338?

It affects 2 products.


Products: Gnome Libsoup, Redhat Enterprise Linux.

## How do I fix CVE-2026-66338?

Apply the vendor's patch.

## Is CVE-2026-66338 being exploited?

There is no known exploitation of CVE-2026-66338 in the wild and no public exploit code.

## Other vulnerabilities in Redhat Enterprise Linux

- [CVE-2026-102556](https://insights.sbomify.com/CVE-2026-102556/): Libsoup: libsoup: heap buffer overflow from websocket pong signal type confusion (high 8.6)
- [CVE-2026-102557](https://insights.sbomify.com/CVE-2026-102557/): Libsoup: libsoup: heap buffer overflow during websocket message reassembly (high 8.6)
- [CVE-2026-102558](https://insights.sbomify.com/CVE-2026-102558/): Libsoup: libsoup: heap buffer overflow during websocket receive-buffer growth (high 8.6)
- [CVE-2026-102559](https://insights.sbomify.com/CVE-2026-102559/): Libsoup: libsoup: heap buffer overflow during websocket client-frame masking (high 8.6)
- [CVE-2026-102560](https://insights.sbomify.com/CVE-2026-102560/): Libsoup: libsoup: heap buffer overflow during outgoing permessage-deflate buffer growth (high 8.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66338 and fix it if it is affected.

What it is: CVE-2026-66338 is a medium-severity vulnerability in Redhat Enterprise Linux, published on 24 Jul 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-2026-66338 in the wild and no public exploit code.

Affected product: Redhat Enterprise Linux. 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-2026-66338/ (Markdown: https://insights.sbomify.com/CVE-2026-66338/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66338
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66338
- OSV: https://osv.dev/vulnerability/CVE-2026-66338

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66338/
Summaries are generated and may contain mistakes; the official description above is the record.