high CVSS 8.1

CVE-2026-52845·PHP vulnerability

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.

Severity
high
Software
PHP
Fixed in
2.11.4
Published
2026-06-23

Affected versions

Until: 2.11.4

Fixed in: 2.11.4

How to fix this CVE

Update Caddy to version 2.11.4 or later to patch the header normalization vulnerability in the forward_auth module. This vulnerability allows attackers to bypass identity header validation when PHP-FastCGI is used behind Caddy by exploiting HTTP header name normalization (dash-to-underscore conversion). Ensure your web server configuration is reloaded after updating.

sudo dnf update caddy && sudo systemctl restart caddy

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET /.*\s HTTP/1\.1.*\n(.*\n)*X_[A-Za-z_-]*:\s.*(identity|user|group|role|uid).*

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Block or normalize HTTP requests containing underscore-prefixed header aliases that correspond to known identity headers (e.g., X_FORWARDED_USER, X_REMOTE_USER, X_FORWARDED_GROUPS). Implement request header validation rules that reject requests with both dash and underscore variants of the same header name.

How to check if you are affected

  1. Step 1: Check Caddy version with `caddy version` and confirm it is below 2.11.4
  2. Step 2: Review your Caddyfile configuration for forward_auth directives combined with php_fastcgi handlers
  3. Step 3: Search access logs for suspicious underscore-prefixed HTTP headers (e.g., X_FORWARDED_USER, X_REMOTE_USER) that correspond to dash-based identity headers
  4. Step 4: After updating, verify the patched version with `caddy version` and test header handling with a sample request containing both dash and underscore variants

FAQ

What is CVE-2026-52845?

This vulnerability in Caddy allows unauthenticated clients to inject or override identity headers when forward_auth is used with PHP-FastCGI. The vulnerability stems from Caddy normalizing HTTP header names by converting dashes to underscores, which allows an attacker to bypass the forward_auth header deletion mechanism by supplying an underscore variant of a sensitive header.

Is CVE-2026-52845 being actively exploited?

No, this CVE is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, and no public exploit code is available. However, the attack is trivial to execute and should be remediated promptly.

What versions of Caddy are affected by CVE-2026-52845?

All versions of Caddy prior to 2.11.4 are affected. Users running Caddy 2.11.4 and later are protected.

How do I check if my server is vulnerable to CVE-2026-52845?

Run `caddy version` to determine your installed version. If it shows a version below 2.11.4 and your Caddyfile contains both `forward_auth` and `php_fastcgi` directives, your deployment is vulnerable.

Does Defensia detect CVE-2026-52845?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Caddy is installed on a monitored server running a version below 2.11.4, CVE-2026-52845 will appear in your dashboard with remediation steps.

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References

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