Summary

The Proxy Helper (hono/proxy) does not remove response headers named by the origin's Connection header. Headers that the origin marked as connection-scoped are therefore forwarded to clients.

Details

Per RFC 9110 Section 7.6.1, an intermediary must remove the header fields listed in a message's Connection header field before forwarding the message, in addition to the well-known hop-by-hop headers. The proxy() function removed the well-known hop-by-hop headers (including Connection itself) from origin responses, but did not remove the headers that the response's Connection header field designated as connection-scoped.

This issue arises when an application proxies responses from an origin that declares additional, non-standard headers as hop-by-hop via the Connection response header.

Impact

A client may receive response headers that the origin intended only for its immediate peer. This may lead to:

- Disclosure of connection-scoped or internal metadata contained in such headers

This issue affects applications that use the Proxy Helper (hono/proxy) to forward responses from origins that list custom header names in their Connection response header. Applications whose origins only use the standard hop-by-hop headers are not affected.

Hono before 4.12.7 allows proto key in parseBody with dot option enabled, permitting specially crafted form field names to create objects with proto properties. When parsed results are merged into regular JavaScript objects using unsafe merge patterns, attackers can exploit this to achieve prototype pollution and modify object behavior.

Hono before 4.10.2 (fixed in 4.10.3) contains a flaw in its CORS middleware: when the origin is not set to "", the middleware copies the Vary header from the incoming request into the response. Because Vary is a response header that should be managed by the server, an attacker can supply arbitrary Vary values that are reflected into the response, potentially causing cache key pollution and inconsistent CORS enforcement in environments that rely on shared caches or proxies.

Improper Authorization in Hono (JWT Audience Validation)

Hono’s JWT authentication middleware did not validate the aud (Audience) claim by default. As a result, applications using the middleware without an explicit audience check could accept tokens intended for other audiences, leading to potential cross-service access (token mix-up).

The issue is addressed by adding a new verification.aud configuration option to allow RFC 7519–compliant audience validation. This change is classified as a security hardening improvement, but the lack of validation can still be considered a vulnerability in deployments that rely on default JWT verification.

Recommended secure configuration

You can enable RFC 7519–compliant audience validation using the new verification.aud option:

ts import { Hono } from 'hono' import { jwt } from 'hono/jwt'

const app = new Hono()

app.use( '/api/', jwt({ secret: 'my-secret', verification: { // Require this API to only accept tokens with aud = 'service-a' aud: 'service-a', }, }) )

Below is the original description by the reporter. For security reasons, it does not include PoC reproduction steps, as the vulnerability can be clearly understood from the technical description.

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The original description by the reporter

Summary Hono’s JWT Auth Middleware does not provide a built-in aud (Audience) verification option, which can cause confused-deputy / token-mix-up issues: an API may accept a valid token that was issued for a different audience (e.g., another service) when multiple services share the same issuer/keys. This can lead to unintended cross-service access. Hono’s docs list verification options for iss/nbf/iat/exp only, with no aud support; RFC 7519 requires that when an aud claim is present, tokens MUST be rejected unless the processing party identifies itself in that claim.

Note: This problem likely exists in the JWK/JWKS-based middleware as well (e.g., jwk / verifyWithJwks)

Details - The middleware’s verifyOptions enumerate only iss, nbf, iat, and exp; there is no aud option. The same omission appears in the JWT Helper’s “Payload Validation” list. Developers relying on the middleware for complete standards-aligned validation therefore won’t check audience by default. - Standards requirement: RFC 7519 §4.1.3 states that each principal intended to process the JWT MUST identify itself with a value in the aud claim; if it does not, the JWT MUST be rejected (when aud is present). Lack of a first-class aud check increases the risk that tokens issued for Service B are accepted by Service A. - Real-world effect: In deployments with a single IdP/JWKS and shared keys across multiple services, a token minted for one audience can be mistakenly accepted by another audience unless developers implement a custom audience check. - For example, with Google Identity (OIDC), iss is always https://accounts.google.com (shared across apps), but aud differs per application because it is that app’s OAuth client ID; therefore, an attacker can host a separate service that supports “Sign in with Google,” obtain a valid ID token (JWT) for the victim user, and—if your API does not verify aud—use that token to access your API with the victim’s privileges.

Impact Type: Authentication/authorization weakness via token mix-up (confused-deputy).

Who is impacted: Any Hono user who: - shares an issuer/keys across multiple services (common with a single IdP/JWKS) - distinguishes tokens by intended recipient using aud.

What can happen: - Cross-service access: A token for Service B may be accepted by Service A. - Boundary erosion: ID tokens and access tokens, or separate API audiences, can be inadvertently intermixed. - This may causes unauthorized invocation of sensitive endpoints.

Recommended remediation: 1) Add verifyOptions.aud (string | string[] | RegExp) to the middleware and enforce RFC 7519 semantics: In verify method, if aud is present and does not match with specified audiences, reject. 2) Ensure equivalent aud handling exists in the JWK/JWKS flow (jwk middleware / verifyWithJwks) so users of external IdPs can enforce audience consistently.