Bug 2567475 (CVE-2026-55458)

Summary: CVE-2026-55458 github.com/caddyserver/caddy: caddy: Hop-by-hop header leakage via improper protocol upgrade response handling
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in Caddy's reverse proxy module. When processing protocol upgrade responses, the proxy fails to properly remove hop-by-hop headers intended only for intermediate connections. A malicious or compromised upstream server can exploit this flaw to forward restricted connection headers across the proxy boundary. This can cause clients to process untrusted connection instructions, potentially leading to unintended connection behavior.
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Description OSIDB Bzimport 2026-11-05 22:00:56 UTC 
Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.5, the reverse proxy's finalizeResponse method in modules/caddyhttp/reverseproxy/reverseproxy.go handles an upstream 101 Switching Protocols response before running removeConnectionHeaders and the hopHeaders deletion loop used for ordinary responses. The early return invokes handleUpgradeResponse, which copies the upstream headers to the client and normalizes only WebSocket-specific header names. An attacker who controls or compromises a proxied WebSocket upstream can therefore cause hop-by-hop headers such as Alt-Svc, Keep-Alive, and Proxy-Authenticate, as well as headers named by Connection, to cross the proxy boundary. For HTTP/2 or HTTP/3 extended CONNECT, the upstream 101 response may be represented to the client as an ordinary successful response, increasing the chance that leaked headers are processed. This issue is fixed in version 2.11.5.