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CVE-2026-123339— Symfony: Inline Fragments Let Untrusted Forwarded Headers Set the Host, Scheme and Port of the URLs Generated for the Main Request

Quick assessment

Affected
sensiolabs symfony
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 Symfony HttpKernel 的以下版本中存在对转发头部（forwarded headers）信任不当的漏洞：5.4 系列低于 5.4.56、6.x 系列低于 6.4.50、7.x 系列低于 7.4.23，以及 8.x 系列低于 8.1.11。该漏洞允许远程攻击者控制请求后续阶段中生成的绝对 URL 的主机（host）、协议（scheme）和端口（port），从而污染由共享缓存存储的页面链接，以及在该页面渲染后发送的电子邮件中的链接。 当页面渲染内联片段（例如使用 Twig 的 函数，或内联渲染的 ES

CVSS 2.3 · Low
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I. Basic Information for CVE-2026-123339

Vulnerability Information
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Vulnerability Title
Symfony: Inline Fragments Let Untrusted Forwarded Headers Set the Host, Scheme and Port of the URLs Generated for the Main Request
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
使用不可信的源
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
sensiolabs symfony 0 ~ 5.4.56 -

II. Public POCs for CVE-2026-123339

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III. Intelligence Information for CVE-2026-123339

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Other References for CVE-2026-123339 (2)

Same Patch Batch · sensiolabs · 2026-11-06 · 7 CVEs total

CVE-2026-123792 8.7 HIGH Symfony: Exponential and Quadratic Parse Times in the YAML Parser Let a Single Document Ho
CVE-2026-118515 8.4 HIGH Symfony: The Serializer Builds UploadedFile Objects From Request Data, Letting a Client Po
CVE-2026-121623 8.2 HIGH Symfony: DataUriNormalizer Opens Data URIs With a Non-Lowercase Scheme as Local Files, Let
CVE-2026-122053 6.9 MEDIUM Symfony: ObjectNormalizer Dotted-Key Property-Path Traversal Bypasses #[Ignore] on Nested
CVE-2026-123746 2.3 LOW Symfony: Programmatic Logins Skip the checkPostAuth() Method of User Checkers
CVE-2026-123824 2.3 LOW Symfony: XXE (Local File Read) in Config XmlUtils::parse() via validateOnParse = true

IV. Related Vulnerabilities

Same product: symfony
Same vendor: sensiolabs
Same weakness: CWE-348