CVE-2026-123339

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in, but needs a user to act. Limited direct impact.

Exploit odds

0.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-11-06 15:17 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-11-06 21:40 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.2% in the next 30 days (EPSS).
• Severity: 2.3 LOW (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2026-11-06 15:17 UTC

Last modified

2026-11-06 21:40 UTC

Exploit probability

0.2% · EPSS, next 30 days

Weakness

CWE-348

Record status

Awaiting Analysis

Attack profile CVSS 4.0 · 2.3

Attack vector

Network

Complexity

Low

Attack requirements

Present

Privileges needed

None

User interaction

Passive

Confidentiality

None

Integrity

Low

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-123339.md and follow it. Check whether this project is affected by CVE-2026-123339. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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