CVE-2026-71849: Hono: Proxy Helper does not remove response headers listed in the `Connection` header
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.
Other sources
Hono is a Web application framework that provides support for any JavaScript runtime. From 4.7.0 to 4.12.33, the Proxy Helper proxy() function in hono/proxy does not remove response headers named by the origin's Connection header. 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, but the proxy() function only removed the well known hop by hop headers, including Connection itself, from origin responses. A client may therefore receive response headers that the origin intended only for its immediate peer, disclosing connection scoped or internal metadata contained in such headers, when an application proxies responses from an origin that declares additional, non standard headers as hop by hop via the Connection response header. This issue is fixed in version 4.12.34.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/honoto a version that resolves this vulnerability.Fixed in 4.12.34 - Upgrade
Upgrade
hono/proxyto a version that resolves this vulnerability.Fixed in 4.12.34
Event History
Frequently Asked Questions
What is the severity of CVE-2026-71849?
The severity of CVE-2026-71849 is categorized as low with a score of 3.7.
How do I fix CVE-2026-71849?
To fix CVE-2026-71849, ensure that the Proxy Helper in Hono is updated to the latest version where the issue is resolved.
What impact does CVE-2026-71849 have on my application?
CVE-2026-71849 may lead to leaking sensitive information through connection-scoped headers if not properly handled by your application.
What is the nature of the vulnerability described in CVE-2026-71849?
CVE-2026-71849 is an information leak vulnerability that allows connection-scoped headers to be forwarded to clients.
Which software is affected by CVE-2026-71849?
CVE-2026-71849 affects the Hono framework, specifically the Proxy Helper component within Hono.