CVE-2026-55877 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in symfony/ux-icons. A fix is available for symfony/ux-icons — see the affected versions and patch details below.
Symfony UX: XSS in symfony/ux-icons via unsanitized SVG content in local files and Iconify on-demand responses
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55877.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
symfony/ux-icons🐘symfony/ux-iconsReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Description
The ux_icon() Twig function is marked is_safe=['html'], so Twig never escapes its output. Icon::toHtml() inlines the SVG source verbatim into the page. Browsers execute <script> elements and on* event-handler attributes found inside inline SVG, making any unsanitized icon a vector for cross-site scripting.
Two code paths were affected. In the local file path, Icon::fromFile() only stripped <script> elements that were direct children of <svg>, leaving nested scripts and all on* attributes untouched despite a code comment claiming broader protection. In the Iconify on-demand path (enabled by default), the remote JSON body field was wrapped into an Icon object with no sanitization at all. Concrete attack vectors include a malicious SVG icon pack from a third-party theme or downloaded icon set, or a controlled Iconify endpoint configured via iconify.endpoint (including a poisoned cache).
Resolution
Introducing an IconFactory that centralizes sanitization across every icon source before an Icon object is created. The sanitizer removes script-capable elements (script, foreignObject, iframe, object, embed), SMIL animations targeting on*, href, or xlink:href attributes, CDATA sections, processing instructions, all on* attributes, and javascript:, vbscript:, and data:text/html URL schemes.
<style> elements are kept for theming but have any handlers stripped. Icons that contain none of these constructs are byte-for-byte identical after sanitization.
Credits
Symfony would like to thank Pascal Cescon for reporting the issue and Hugo Alliaume for providing the fix.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | symfony/ux-icons | ≥ 2.17.0&&< 2.36.1 | 2.36.1composer require symfony/ux-icons:^2.36.1 |
| 🐘Packagist | symfony/ux-icons | ≥ 3.0.0&&< 3.2.0 | 3.2.0composer require symfony/ux-icons:^3.2.0 |
Affected Products
uxsymfonyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for symfony/ux-icons, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update symfony/ux-icons to 2.36.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55877 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2026-55877 in your dependencies?
Find it across Packagist, including transitive dependencies.