# CVE-2026-123339: Improper trust of forwarded headers in RouterListener::onKernelFinishRequest() in Symfony HttpKernel versions prior to 5.4.56, 6.x prior to


- **Severity:** Low 2.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:L/VA:N/SC:L/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 6 Nov 2026
- **Updated:** 6 Nov 2026
- **Also known as:** CLSA-2026-1794321812, CLSA-2026-1794321899, CLSA-2026-1794406601, CLSA-2026-1794406685, CLSA-2026-1794406774, DEBIAN-CVE-2026-123339, GHSA-wg5q-m2ww-f3c9, UBUNTU-CVE-2026-123339

## What is CVE-2026-123339?

CVE-2026-123339 is a low-severity vulnerability in sensiolabs symfony, published on 6 Nov 2026. 

### Official description

Improper trust of forwarded headers in RouterListener::onKernelFinishRequest() in Symfony HttpKernel versions prior to 5.4.56, 6.x prior to 6.4.50, 7.x prior to 7.4.23, and 8.x prior to 8.1.11 allows a remote attacker to control the host, scheme, and port of absolute URLs generated for the rest of the request, poisoning links in pages stored by a shared cache and in e-mails sent after the page was rendered.When a page renders an inline fragment (render() in Twig, or an ESI tag rendered inline), SubRequestHandler temporarily adds 127.0.0.1 to the trusted proxies, and RouterListener rebuilds the router context from the main request while that trust is still active, so the X-Forwarded-* and Forwarded headers of a request whose client address is 127.0.0.1 are trusted even though that address is not a configured trusted proxy. From that point on, every absolute URL the router generates for the rest of the main request takes its host, scheme, and port from headers sent by the client, including URLs produced by url() in Twig and by UrlGeneratorInterface::ABSOLUTE_URL.

Applications are affected when they are reached over HTTP from 127.0.0.1, for example behind a TLS terminator, reverse proxy, local HTTP cache, or sidecar proxy on the same host, and that proxy passes the client’s forwarded headers along. With the default configuration (framework.trusted_proxies unset), all forwarded headers, including X-Forwarded-Host, are trusted during that window. Applications served through php-fpm, mod_php, or a proxy on another host are not affected. When framework.trusted_hosts is configured, a forwarded host outside the list is rejected, but the scheme and port can still be changed.

Weakness: CWE-348 (Use of Less Trusted Source)

## Which versions are affected by CVE-2026-123339?


## How do I fix CVE-2026-123339?

Upgrade to a fixed version.

## Is CVE-2026-123339 being exploited?

There is no known exploitation of CVE-2026-123339 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-123339 and fix it if it is affected.

What it is: CVE-2026-123339 is a low-severity vulnerability in sensiolabs symfony, published on 6 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-123339 in the wild and no public exploit code.

Affected product: sensiolabs symfony. 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-2026-123339/ (Markdown: https://insights.sbomify.com/CVE-2026-123339/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-123339
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-123339
- OSV: https://osv.dev/vulnerability/CVE-2026-123339

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-123339/
Summaries are generated and may contain mistakes; the official description above is the record.