CVE-2026-56761: Hono HTML Injection in Server-Side JSX Rendering

Technical summary

CVE-2026-56761 is an HTML injection vulnerability (CWE-79) in Hono's server-side JSX rendering implementation. The vulnerability exists because the framework does not properly sanitize or validate attribute names before rendering them into HTML. An attacker can exploit this by constructing JSX props with malicious attribute names containing metacharacters (quotes, angle brackets, etc.) that, when rendered server-side, break HTML tag boundaries. This allows injection of arbitrary attributes, script event handlers, or entirely new HTML elements. The vulnerability is exploitable over the network without authentication, but requires user interaction to trigger the malicious payload, limiting its severity to CVSS 4.3 (Medium).

Business impact

This vulnerability primarily affects the integrity of web applications built with Hono. Attackers can inject unauthorized HTML or attributes into rendered pages, potentially leading to defacement, unauthorized form modifications, or injection of scripts that execute in the context of the user's browser. Organizations using Hono in production environments should assess whether user-supplied data can flow into JSX prop attributes, as this creates the attack vector. The requirement for user interaction (UI:R) reduces the likelihood of automated exploitation, but social engineering or direct phishing could be used to trigger malicious payloads. Reputational and compliance impacts may arise if injected content violates regulatory requirements or damages user trust.

Affected systems

Hono versions prior to 4.12.14 are affected. Any application using vulnerable Hono versions for server-side rendering (SSR) of JSX is at risk, particularly those that incorporate unsanitized or minimally validated user input into JSX component properties. This includes web applications, APIs serving HTML templates, and microservices using Hono as a rendering layer. Verify your Hono version using npm (npm list hono or check package.json) and compare against the patched version 4.12.14 and later.

Exploitability

Exploitation requires network access and user interaction, limiting its practical attack surface. An attacker would need to control or influence JSX prop values with malformed attribute names—typically through form submissions, URL parameters, or other user input mechanisms. The attack is not self-propagating and does not allow remote code execution on the server; impact is limited to client-side HTML injection. No public exploit code is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, suggesting limited active exploitation in the wild. However, proof-of-concept demonstrations are likely to emerge post-disclosure.

Remediation

Upgrade Hono to version 4.12.14 or later as soon as possible. Review your application's data flow to identify any JSX components that accept user-controlled attribute names or values, and implement input validation or output encoding where feasible. As a defensive measure, consider implementing Content Security Policy (CSP) headers to mitigate the impact of injected HTML or inline scripts. Organizations should also audit recent logs for suspicious attribute patterns in requests to identify potential exploitation attempts.

Patch guidance

Update Hono using npm: 'npm install hono@latest' or specify the minimum safe version in your package.json: '"hono": "^4.12.14"'. After updating, run your full test suite to ensure compatibility, particularly regression tests covering JSX server-side rendering. If you maintain private forks or customizations of Hono, apply equivalent sanitization fixes to attribute name rendering. Verify the update by running 'npm list hono' to confirm the installed version is 4.12.14 or newer.

Detection guidance

Monitor application logs for HTTP requests containing unusual characters in parameter values that might be crafted attribute payloads (quotation marks, angle brackets, or HTML entity patterns in expected input fields). In Web Application Firewall (WAF) logs, look for requests attempting to inject quote or bracket characters into JSX-rendered contexts. Implement endpoint monitoring to detect unexpected HTML or attribute injection patterns in server responses. Code review tools can flag JSX expressions that render user-controlled data directly into attribute positions without sanitization. Consider adding application instrumentation to log cases where attribute names contain unexpected special characters.

Why prioritize this

This vulnerability should be prioritized for patching, but not at the highest emergency level. While the CVSS score is moderate (4.3), the requirement for user interaction significantly reduces real-world risk in most deployments. However, organizations with public-facing Hono applications that accept user input should expedite patching within their standard update cycles (typically 1-2 weeks). Applications with strict input validation or Content Security Policies in place face lower immediate risk. Prioritize systems where untrusted user input directly influences JSX rendering properties.

Risk score, explained

The CVSS 4.3 (Medium) score reflects limited attack scope and the need for user interaction. The vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N indicates: network-accessible vulnerability (AV:N), low attack complexity (AC:L), no privilege requirement (PR:N), user interaction required (UI:R), unchanged scope (S:U), no confidentiality impact (C:N), low integrity impact (I:L), and no availability impact (A:N). The score appropriately reflects that this is an HTML injection issue affecting content integrity rather than a critical remote code execution or authentication bypass.

Frequently asked questions