CVE-2026-52845 | Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

Exp

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.

Published: 2026-06-23 Last update: 2026-06-29 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-52845 is rated Exploit Available (51/100): CVSS High severity, with low exploitation likelihood (EPSS 0.20%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2026-52845

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2026-52845

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-24 0.04% 0.20% +0.16%
2 2026-06-12 — 0.04% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-52845

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
8.1 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 2.8 5.2 [email protected]
8.1 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 2.8 5.2 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

Weakness enumeration for CVE-2026-52845

GitHub Security Advisory for CVE-2026-52845

GHSA-f59h-q822-g45g · Severity: high · Ecosystem: go — Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

OS Trackers for CVE-2026-52845

vendor priority summary link
debian not yet assigned CVE-2026-52845 not yet assigned priority: Debian including 1 source packages (caddy), 4 status rows across 4 suites (bookworm, forky, sid, trixie): open 4. https://security-tracker.debian.org/tracker/CVE-2026-52845
redhat high — https://access.redhat.com/security/cve/CVE-2026-52845
ubuntu medium CVE-2026-52845 medium priority: Ubuntu including 1 source packages (caddy), 5 status rows across 5 suites (jammy, noble, questing, resolute, upstream): needs-triage 4, DNE 1. https://ubuntu.com/security/CVE-2026-52845

Affected software / configurations for CVE-2026-52845

Vendor Product Version Raw CPE
caddyserver caddy < 2.11.4 cpe:2.3:a:caddyserver:caddy:*:*:*:*:*:*:*:*

References for CVE-2026-52845

cvelogic Threat Intelligence