CVE-2026-117645: Phoenix.HTML vulnerable to attribute injection via unvalidated attribute names in attributes_escape/1

Severity: medium (CVSS 5.3 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-30 · updated 2026-10-30
Weakness: CWE-79

Description

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') vulnerability in phoenixframework phoenix_html allows an attacker who controls attribute names to inject attributes and script into rendered HTML via Phoenix.HTML.attributes_escape/1. attributes_escape/1 in lib/phoenix_html.ex escapes attribute values but not attribute names. key_escape/1 passes a binary key through html_escape, which neutralizes quotes and angle brackets but not whitespace, so a key such as x onmouseover ends the attribute and starts a new one. An atom key is only dasherized, so any character passes, including quotes and >. Nested data, aria and phx keys take the same paths. An application is affected when it builds attribute keys from untrusted input and renders them through HEEx <tag {@attrs}>, content_tag/3, or a direct call. Before 3.0.0 the same code lived in Phoenix.HTML.Tag. This issue affects phoenix_html: from 1.0.0 before 3.3.5 and from 4.0.0 before 4.4.0.

Affected products

phoenixframework Phoenix Html: from 1.0.0 before 3.3.5; from 4.0.0 before 4.4.0 — fixed in 3.3.5, 4.4.0

How to fix

Upgrade to a fixed version: phoenixframework Phoenix Html 3.3.5, 4.4.0. Check the vendor advisory for mitigations.

References

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