Caddy: RFC 7230 §6.1 compliance issue / Hop-by-hop headers that leak through
- Exploited
- No known exploitation
- Fix
- Fixed in 2.11.5
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 2.11.5.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 30/100 · provisional
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
Caddy, an extensible web server platform, has a flaw in its reverse proxy where hop-by-hop headers from a proxied WebSocket upstream are passed through to the client. A remote attacker who controls or compromises that upstream, without logging in, can cause headers such as Alt-Svc, Keep-Alive, and Proxy-Authenticate to cross the proxy boundary.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed per the CVSS vector, reachable over the network without a login · Unknown: No advisory context yet
- Reach10/20Affects Go packages
- Severity2.9/10CVSS 2.9
- No patch0/5No signal
What would change the score
- Would score about 75 if added to CISA KEV
- Would score about 70 if CISA's SSVC rates exploitation active
- Would score about 35 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 2.11.5.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- caddyserver caddy
fixed in 2.11.5
- github.com/caddyserver/caddy/v2Go
fixed in 2.11.5
Evidence 3
- Advisory
- Repository
- Repository
Corrections
None so far.
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Questions about CVE-2026-55458
Is CVE-2026-55458 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by CVE-2026-55458?
caddy: before 2.11.5. github.com/caddyserver/caddy/v2 (Go): before 2.11.5.Source: cve.now feed data
How do I fix CVE-2026-55458?
Upgrade to a fixed version: 2.11.5.Source: cve.now feed data