Summary
symfony/ux-live-component: XSS via attacker-controlled child component tag
Description
Symfony\UX\LiveComponent\Util\ChildComponentPartialRenderer::createHtml() interpolates the $childTag argument directly into the HTML output as a tag name, without escaping or validation. The value originates from client-controlled JSON (children[id].tag) parsed by LiveComponentSubscriber and propagated through InterceptChildComponentRenderSubscriber, so an attacker who can reach the Live Component endpoint can inject arbitrary HTML, including <script> tags, on any re-render of a Live Component that contains at least one child component.
In the default configuration, the Live Component endpoint is gated by an Accept: application/vnd.live-component+html request-header check that cannot be set cross-origin without a CORS preflight, so the issue is primarily a defense-in-depth gap. It becomes directly exploitable on applications that have relaxed CORS to allow this header from untrusted origins, or that have been pivoted from another same-origin XSS.
Resolution
ChildComponentPartialRenderer now validates $childTag against a strict HTML tag-name regex before interpolating it, and rejects any value that doesn't match. Anything that wouldn't be a valid HTML tag is dropped before reaching the response.
The patch for this issue is available here for branch 2.x (and forward-ported to 3.x).
Credits
Symfony would like to thank Pascal Cescon for reporting the issue and Hugo Alliaume for providing the fix.
Impact
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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symfony/ux-live-component to 2.36.0 or later; symfony/ux-live-component to 3.1.0 or later
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Frequently Asked Questions
- What is CVE-2026-49210? CVE-2026-49210 is a medium-severity cross-site scripting (XSS) vulnerability in symfony/ux-live-component (composer), affecting versions >= 2.8.0, < 2.36.0. It is fixed in 2.36.0, 3.1.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of symfony/ux-live-component are affected by CVE-2026-49210? symfony/ux-live-component (composer) versions >= 2.8.0, < 2.36.0 is affected.
- Is there a fix for CVE-2026-49210? Yes. CVE-2026-49210 is fixed in 2.36.0, 3.1.0. Upgrade to this version or later.
- Is CVE-2026-49210 exploitable, and should I be worried? Whether CVE-2026-49210 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-49210 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-49210?
- Upgrade
symfony/ux-live-componentto 2.36.0 or later - Upgrade
symfony/ux-live-componentto 3.1.0 or later
- Upgrade