Hono's Cache Middleware ignores Vary: Authorization / Vary: Cookie leading to cross-user cache leakage

Summary

Cache Middleware does not skip caching for responses that declare per-user variance via Vary: Authorization or Vary: Cookie. As a result, a response cached for one authenticated user may be served to subsequent requests from different users.

Details

The Cache Middleware skips caching when a response carries Vary: *, certain Cache-Control directives (private, no-store, no-cache), or Set-Cookie. However, Vary: Authorization and Vary: Cookie — the standard signals defined in RFC 9110 / RFC 9111 to indicate per-user responses — are not treated as cache-skip reasons.

This issue arises when applications use the Cache Middleware on endpoints that return user-specific data and rely on Vary: Authorization or Vary: Cookie to scope the response per user, without also setting Cache-Control: private.

Impact

A user may receive a cached response that was originally generated for a different authenticated user. This may lead to:

• Disclosure of personally identifiable information or other user-specific data present in the response body
• Inconsistent or incorrect behavior in user-specific endpoints

This issue affects applications that use the Cache Middleware on endpoints whose responses vary by Authorization or Cookie and that do not also set Cache-Control: private.