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Fencer Vulns
high 8.1

CVE-2026-52845 json

Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

published 2026-06-16 4 mo ago modified 2026-10-25 14 d ago also GHSA-f59h-q822-g45g , GO-2026-5346 , EUVD-2026-38558

cvss
8.1
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 38%
public exploits
0
none known

Summary

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.

Patch / fix

  • go github.com/caddyserver/caddy/v2 2.11.4
  • caddyserver caddy 2.11.4
  • caddy Debian 12 2.6.2-5+deb12u1
  • caddy Debian 13 2.6.2-12+deb13u1
  • caddy Debian 14 2.11.4-1

debian advisories DEBIAN-CVE-2026-52845

  • govulncheck-vulndb SLES 16.0, SLES for SAP applications 16.0 0.0.20260727T201416-160000.1.1

suse advisories SUSE-SU-2026:23216-1SUSE-SU-2026:23227-1

  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260727T201416-160000.1.1

opensuse advisories openSUSE-SU-2026:21551-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-52845 updated cvss 8.1

github.com/caddyserver/caddy: Caddy: Remote client can inject or override identity headers via header normalization

  • Red Hat Hardened Images caddy
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
total
Assessed
4 mo ago

Exploit prediction

0.45% chance of exploitation in the next 30 days, higher than 38% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
CISA ADP 3.1 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
GitHub 3.1 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Affected packages

ecosystem package affected fixed in
go github.com/caddyserver/caddy/v2
<2.11.4
2.11.4
go github.com/caddyserver/caddy
<=1.0.5
 no fix listed

Affected products

  • caddyserver caddy
  • Red Hat Red Hat Hardened Images

Classification

Weakness
  • CWE-287 Improper Authentication
  • CWE-290 Authentication Bypass by Spoofing
  • CWE-444 Inconsistent Interpretation of HTTP Requests

References

Sources

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