CVE-2026-93981 in hono
Summary
by MITRE • 09/19/2026
hono before 4.13.7 fails to HTML-escape plain strings rendered by hono/jsx as a child or fallback of Suspense, as a string child of ErrorBoundary alongside an asynchronous sibling, as the single child of a Context.Provider, or as the root value passed to renderToString() or renderToReadableStream() from hono/jsx/dom/server. These paths stringify their input and treat the result as already-escaped markup, so an attacker who controls such a string during server-side rendering can inject arbitrary HTML and execute script under the application's origin.
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Analysis
by VulDB Data Team • 09/19/2026
The vulnerability identified in Hono versions prior to 4.13.7 represents a critical Server-Side Request Forgery (SSRF) variant known as Cross-Site Scripting via improper output encoding, specifically categorized under CWE-80: Improper Neutralization of Script-Related HTML Tags in a Web Page. This flaw stems from the internal handling of string literals within the hono/jsx module during server-side rendering operations. When developers utilize specific JSX patterns such as passing plain strings as children or fallbacks to Suspense components, using them alongside asynchronous siblings within ErrorBoundary contexts, employing them as single children for Context.Provider elements, or directly passing them as root values to renderToString and renderToReadableStream functions from the hono/jsx/dom/server module, the framework fails to perform necessary HTML entity encoding. Instead of sanitizing special characters like angle brackets, ampersands, and quotes, the library treats these string inputs as pre-escaped markup. This assumption allows an attacker who controls any part of this input stream to inject arbitrary HTML structures directly into the rendered output without interference from the framework's security mechanisms.
From a technical perspective, the root cause lies in the serialization logic that converts JavaScript objects and strings into their HTML representation during server-side rendering. In secure implementations, all dynamic content must be escaped to ensure it is treated as data rather than executable code by the browser. However, because Hono incorrectly assumes these specific string paths are safe or already sanitized, it bypasses this crucial step. Consequently, if an attacker can influence any user-supplied input that flows through one of these identified rendering pathways, they can inject malicious scripts. This is particularly dangerous in server-side rendered applications where the initial HTML payload is generated on the server and sent to the client. The injected script executes immediately upon page load within the context of the application's origin, granting the attacker full access to session cookies, local storage, and other sensitive data associated with that domain.
The operational impact of this vulnerability is severe, as it facilitates persistent or reflected cross-site scripting attacks depending on whether the malicious input is stored in a database or provided dynamically during request processing. Attackers can steal authentication tokens, hijack user sessions, perform actions on behalf of victims, or redirect users to phishing sites. Given that server-side rendering often serves as the primary entry point for many modern web applications built with Hono, this vulnerability affects a significant portion of the application's surface area. The lack of automatic escaping means that even seemingly innocuous string concatenations in JSX components can become vectors for exploitation if they incorporate untrusted data. This undermines the security model assumed by developers who rely on framework-level protections to prevent injection attacks.
To mitigate this risk, organizations must immediately upgrade Hono to version 4.13.7 or later, where these specific rendering paths have been patched to correctly escape HTML entities before outputting them to the browser. For applications that cannot be upgraded instantly due to dependency constraints, a temporary workaround involves manually escaping any user-controlled strings passed through Suspense fallbacks, ErrorBoundary siblings, Context.Provider children, and root values for server-side render functions using standard JavaScript string replacement methods or dedicated sanitization libraries like DOMPurify before they are rendered. Additionally, implementing Content Security Policy headers can help mitigate the impact of successful exploitation by restricting script execution sources, although this is a compensatory control rather than a fix for the underlying code flaw. Regular security audits and static analysis tools configured to detect missing output encoding in server-side rendering contexts should also be employed to prevent similar issues in future development cycles. This incident highlights the importance of verifying that all dynamic content paths are explicitly sanitized, even when using modern framework abstractions that may obscure low-level HTML generation details.