2.9 Low

Caddy has an RFC 7230 §6.1 compliance issue / Hop-by-hop headers that leak through

CVE-2026-55458

AI Disclosure This vulnerability was discovered via deepseek-v4-pro[1m] for finding potential vulnerabilities / inconsistencies in the code base. I made the AI check if Caddy is compliant with various RFCs, incl. 7230. I also used it to help write the PoC, since it was honestly much faster than writing it by hand. I also let it write parts of this advisory text for brevity's sake, but had to rewrite half of it by hand. Summary Caddy's reverse proxy fails to strip RFC 7230 hop-by-hop headers from upstream responses when handling WebSocket upgrades (HTTP 101 Switching Protocols). Headers including Alt-Svc, Keep-Alive, Proxy-Authenticate, Transfer-Encoding, or Trailer, which Caddy actually correctly strips for normal HTTP responses, pass through unfiltered to the client during protocol upgrades. An upstream that sets these headers (intentionally or otherwise) can inject them into client responses, enabling connection hijacking via Alt-Svc, cookie injection via Set-Cookie, and proxy authentication phishing. The normal response path in finalizeResponse correctly strips all hop-by-hop headers before forwarding, but the 101 code path returns early, skipping the stripping entirely.…

Assessment

CVSS: 2.9 (v4.0)
Severity: Low
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 28 October 2026
Last updated: 28 October 2026
Affected: go · github.com/caddyserver/caddy/v2
Weakness: CWE-444

CVSS vectorCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P

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