CVE-2026-123339sensiolabs symfony use of a less trusted source
CVE-2026-123339 is a use of a less trusted source in sensiolabs symfony, rated low (2.3 on CVSS 4.0).
Why this priority: Low severity, with no exploitation signal.
Published · Updated · NVD status: Awaiting Analysis
What it is
The software trusts a value, such as a forwarded address, from a source an attacker controls.
How it is reached
An attacker can exploit it over the network, without an account, but someone has to act first, such as opening a file or following a link. Success also depends on conditions the attacker does not fully control.
What an attacker gains
The direct impact is limited: partial access to data or a partial outage. In practice, this kind of flaw lets an attacker act as a signed-in user, or an administrator, without valid credentials. The damage can spread past the vulnerable component into others.
Is it being exploited?
There is no public report of exploitation yet.
What to do
- No fixed version is listed yet. Find every place sensiolabs symfony runs, watch the references for a fix, and plan a replacement if it is no longer supported.
- While there is no fix, keep it off the internet or behind a VPN, allowlist or authenticating proxy.
- Warn the people who use it, and filter files and links on the way in.
Description from NVD
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.
What this means for your architecture.
A CVE names a product. Whether it matters depends on where that product sits in your systems: what reaches it, which trust boundary it guards and what it can reach once compromised. Veriom maps your components, their relationships and your trust boundaries from your code, cloud and delivery evidence, so you can see where a component like this sits. The radar itself does not look at your systems.
Components it touches
- Sign-in, session and token handling. The flaw sits between anonymous callers and authenticated sessions.
- Identity providers and SSO. The flaw sits between anonymous callers and authenticated sessions.
Trust boundaries in play
- Internet-facing entry points. No account is needed. Any instance reachable from outside your perimeter, through a load balancer, gateway, VPN or exposed admin port, is in scope.
- People who open files and links. Mail, chat and browsers are the delivery path.
- Isolation between components. The impact crosses out of the vulnerable component into others it should be isolated from.
- Between anonymous callers and authenticated sessions. An attacker who crosses it can act as a signed-in user, or an administrator, without valid credentials.
Questions to answer about your own systems
- Where does sensiolabs symfony run in your systems, and at which version?
- Which of those instances can be reached from the internet or partner networks?
- What can it reach once compromised: which data stores, secrets and services?
Read more
Sources
- CVE records and CVSS scores from the NVD CVE API. This product uses the NVD API but is not endorsed or certified by the NVD.
- Exploitation status from CISA’s Known Exploited Vulnerabilities catalog.
- Exploit likelihood from FIRST’s Exploit Prediction Scoring System (EPSS), refreshed daily.
- Explanations are written by Veriom from fixed templates over each record’s data, not by a person reviewing each CVE. Descriptions are quoted from the records as published.