CVSS 4.0
2.9 lowCVSS: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:PCVE-2026-55458
Low (2.9) in caddyserver caddy. EPSS 0.5% estimated probability of exploitation activity in the next 30 days. Fixed in github.com/caddyserver/caddy/v2 2.11.5.
On this page
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.
Severity
- EPSS
- 0.5% estimated probability of exploitation activity in the next 30 days; 43rd percentile. Scored 2026-11-10. A forecast for this CVE across the internet, not for your own systems.
- CISA's assessment (SSVC)
- Exploitation: PoC · Automatable: no · Technical impact: partial, assessed 2026-11-09. From CISA's Vulnrichment in the CVE List: whether it is exploited, whether exploiting it can be automated, and how much control it gives.
- CWE
- CWE-444 Inconsistent Interpretation of HTTP Requests (HTTP Request Smuggling)
- OWASP Top 10
- A06:2025 Insecure Design
- Published
- Modified
- NVD analysis
- Awaiting analysis
- Record from
- the NVD, synced
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Explain CVE-2026-55458 for an engineer deciding whether to act. Use https://cve.bz/api/public/cves/CVE-2026-55458?via=ask-ai-copy and https://cve.bz/cve/CVE-2026-55458?via=ask-ai-copy as the source of truth: what is affected and which versions fix it, whether it is in CISA KEV, public exploits, and its EPSS (an internet-wide 30-day estimate of exploitation activity, not a forecast for my systems). If they give no fixed versions, use the CVE record https://cveawg.mitre.org/api/cve/CVE-2026-55458 or the vendor advisory, not third-party mirrors. Tell me how to check whether I am affected and how to verify a fix. Cite the sources and say where they are uncertain.Opens a third-party assistant; its answer is its own, not cve.bz's. The prompt holds only this CVE's id and public links.
Affected
Versions
From the CNA's record in the CVE List.
| Product | Affected |
|---|---|
| caddyserver caddy | < 2.11.5 |
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-8qx7-j2v2-x6q5 | Go | github.com/caddyserver/caddy/v2 | <2.11.5 | 2.11.5 |
| GO-2026-6874 | Go | github.com/caddyserver/caddy/v2 | <2.11.5 | 2.11.5 |
Linux distributions
| Distribution | Release | Package | Fixed in | Advisory |
|---|---|---|---|---|
| Debian | 12, 13, 14 | caddy | no fix listed | DEBIAN-CVE-2026-55458 |
| Ubuntu | 26.10, Pro:24.04:LTS, Pro:26.04:LTS | caddy | no fix listed | UBUNTU-CVE-2026-55458 |
From the distributions' data via OSV.dev: the Debian Security Tracker; Ubuntu Security Notices © Canonical Ltd., CC BY-SA 4.0. Modified: reformatted and linked to CVE records. Credits. On the distributions' own pages: Debian · Ubuntu.
Red Hat
Red Hat rates this Low.
This vulnerability is classified as having a Low impact because exploitation depends on an attacker already possessing administrative control or compromise over an upstream backend service. In typical enterprise architectures, reverse proxies route traffic to trusted internal services, which significantly reduces the likelihood of encountering rogue protocol upgrade responses. Moreover, the scope of the vulnerability is limited to the propagation of unstripped transport headers to connecting clients, without directly allowing remote command execution, privilege elevation, or access to server-side memory.
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Contains Red Hat Security Data © Red Hat, Inc., licensed under CC BY 4.0. Source: https://access.redhat.com/security/data. Modified: reformatted. Red Hat's page for CVE-2026-55458.
Attack patterns (CAPEC)
- CAPEC-33 HTTP Request Smuggling Likelihood: Medium · Severity: High
- CAPEC-273 HTTP Response Smuggling Likelihood: Medium · Severity: High
The attack patterns MITRE's CAPEC relates to this record's weaknesses, as MITRE names them. Credits.