CVE-2026-126359
Deferred Deferred - Pending Action

XSS in Hono CSS Helper via Unsanitized Style Tag

Publication date: 2026-11-13

Last updated on: 2026-11-13

Assigner: GitHub, Inc.

Description
Hono is a Web application framework that provides support for any JavaScript runtime. From 3.12.0 until 4.13.16, the css helper in hono/css escapes interpolated string values separately and therefore fails to neutralize a closing style tag when < ends the template text or a preceding value and / begins the next attacker-controlled value. During server-side rendering, the combined output can close the <Style /> element early and cause the remainder of the interpolated value to be interpreted as HTML. Applications that place an untrusted string immediately after < in a css template, including adjacent untrusted interpolations, can expose viewers to attacker-controlled HTML or script and information accessible to that page. Other uses of the css helper are not affected. This issue is fixed in version 4.13.16.

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Meta Information

Published
2026-11-13
Last Modified
2026-11-13
Generated
2026-11-13
AI Q&A
2026-11-13
EPSS Evaluated
N/A
NVD
CVE-2026-126359

Affected Vendors & Products

Showing 4 associated CPEs
Vendor Product Version / Range
honojs hono From 3.12.0 (inc) to 4.13.16 (exc)
honojs hono 4.13.16
hono hono From 3.12.0 (inc) to 4.13.16 (inc)
hono hono 4.13.16

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-79 The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

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Executive Summary

This vulnerability is in the Hono web framework's CSS helper function. It occurs when untrusted strings are interpolated into CSS templates. The issue is that the framework fails to properly escape closing style tags when certain characters like < and / appear in sequence. This can cause the CSS helper to output malformed HTML that prematurely closes a style tag, allowing the remaining content to be interpreted as HTML or JavaScript instead of CSS.

Detection Guidance

Check if your Hono framework version is between 3.12.0 and 4.13.15. Run: npm list hono or grep hono package.json. If vulnerable, update to 4.13.16 or later.

Impact Analysis

If you use the affected versions of Hono (3.12.0 to 4.13.15) and interpolate untrusted strings into CSS templates where text ends with < before the interpolation, attackers could inject malicious HTML or scripts. This could lead to cross-site scripting (XSS) attacks, allowing attackers to execute arbitrary code in users' browsers, steal sensitive data, or perform actions on behalf of users.

Compliance Impact

This vulnerability could lead to data breaches by enabling XSS attacks, which may expose user data such as cookies, session tokens, or personal information. This could violate GDPR's data protection requirements and HIPAA's safeguards for protected health information, potentially resulting in legal penalties, reputational damage, and loss of trust.

Mitigation Strategies

Upgrade Hono to version 4.13.16 or later immediately. Avoid interpolating untrusted values into CSS templates. Ensure text before interpolation does not end with <.

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