CVE-2026-59896

What it actually does

CVE-2026-59896 is a request-context isolation flaw in Hono's JSX server-side rendering implementation. In affected Hono versions from 4.11.8 before 4.12.27, context values used by createContext, useContext, jsxRenderer, or useRequestContext may be shared incorrectly across concurrent in-flight requests when an async component resumes after an await. This can cause one request to receive context data belonging to another request. The primary impact is confidentiality loss, with possible limited integrity impact if the leaked or reused context influences rendered content, authorization-dependent UI state, request metadata, tenant selection, personalization, CSRF-related values, or downstream application logic.

Why it matters to the business

Applications using affected Hono JSX server-side rendering may accidentally expose one user's or tenant's data to another user during concurrent traffic. Depending on what the application stores in request context, this could lead to disclosure of personal information, session-adjacent metadata, authorization state, internal request data, or business-sensitive content. The issue is not currently listed in CISA KEV, has low EPSS, and has no known mature public exploitation indicated, but it affects a web framework component and should be remediated promptly anywhere Hono SSR with request context is used.

How an attack unfolds

An attacker targets a web application built with an affected Hono version that performs server-side rendering with hono/jsx and uses request-scoped context through createContext, useContext, jsxRenderer, or useRequestContext. The attacker sends carefully timed concurrent HTTP requests to routes that render async JSX components containing await points. While another user's request is in flight, the attacker's request attempts to trigger rendering after context state has been overwritten or reused by a different request. If timing and application behavior align, the attacker's response may include data derived from the other request's context. Exploitation is race-condition dependent and application-specific, which increases complexity, but successful exploitation can expose sensitive rendered values without authentication if the vulnerable route is publicly reachable.

How to fix it

Upgrade Hono to version 4.12.27 or later. The vulnerable range is 4.11.8 through versions before 4.12.27. Rebuild and redeploy all applications, containers, serverless functions, and edge-worker bundles that include the affected Hono dependency. If immediate upgrade is not possible, reduce risk by avoiding request-sensitive data in JSX context, avoiding shared context usage across async SSR components, disabling or refactoring affected SSR paths, minimizing awaits inside components that rely on request context, and ensuring sensitive authorization or tenant decisions are enforced server-side outside rendered context values. Treat mitigations as temporary; the durable fix is to consume the patched Hono release.

How to confirm the fix worked

Confirm the resolved dependency version in lockfiles and deployed artifacts, not only in source manifests. Check package-lock.json, pnpm-lock.yaml, yarn.lock, bun.lockb, container SBOMs, and built deployment bundles for Hono versions from 4.11.8 before 4.12.27. Validate that runtime deployments use Hono 4.12.27 or later. For behavioral validation, create a test route that renders async JSX components using request context, issue high-concurrency requests with distinct per-request marker values, and confirm no response ever contains another request's marker. Repeat this test under realistic concurrency, latency, and upstream-delay conditions.

Typical attack path

1. Identify a Hono-based application endpoint that uses hono/jsx server-side rendering.
2. Find routes that render async JSX components and use createContext, useContext, jsxRenderer, or useRequestContext.
3. Generate concurrent requests to the vulnerable route, ideally mixing attacker-controlled requests with victim or privileged traffic.
4. Manipulate timing by causing awaits, slow upstream calls, cache misses, large responses, or repeated parallel requests.
5. Inspect returned HTML or response bodies for data that belongs to another request, user, tenant, locale, session, authorization state, or request metadata.
6. Repeat the request-racing pattern to collect additional leaked context values or infer sensitive state.

How to detect it

There is no universal exploit signature because exploitation depends on application-specific rendered content and timing. To assess possible prior exploitation, review web access logs, application logs, CDN/WAF logs, APM traces, and error-monitoring data for bursts of highly concurrent requests to SSR routes, repeated parallel requests from the same source, unusual request timing patterns, or requests designed to induce slow rendering. Correlate these events with user reports or telemetry showing mismatched personalized content, wrong tenant or account labels, unexpected authorization-dependent UI elements, or response bodies containing values from another active request. If response logging, synthetic marker telemetry, or APM custom attributes are available, search for cases where a response includes a request marker, user identifier, tenant identifier, locale, or context-derived value that does not match the request that generated it. Also review dependency timelines to determine whether affected Hono versions were deployed during periods of suspicious concurrency.

What your security team should do

• Inventory applications and packages that use Hono, especially services with hono/jsx server-side rendering.
• Prioritize upgrade to Hono 4.12.27 or later for internet-facing, multi-tenant, authenticated, or data-sensitive applications.
• Search dependency lockfiles and SBOMs for Hono versions 4.11.8 through 4.12.26 and open remediation tickets for any matches.
• Review SSR routes for use of createContext, useContext, jsxRenderer, and useRequestContext with sensitive data such as user identity, authorization flags, tenant identifiers, tokens, request headers, or personalized content.
• After patching, run concurrency tests with unique request markers to verify per-request isolation.
• Review application logs, support reports, and anomaly reports for evidence of users seeing another user's rendered data.
• If sensitive data may have been exposed, coordinate incident response, privacy/legal review, and user notification procedures as appropriate.