Cyber Resilience

CVE-2026-52845

Auth Bypass in Caddyserver Caddy ≤ 2.11.4

Public PoCAuth Bypass
Published
23 June 2026
Modified
15 July 2026
CVSS Score v3.1 8.1
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
EPSS Score 0.0029 21th percentile
Risk Priority 59 floored blend · peak EPSS

Summary

CVE-2026-52845 is a high-severity Improper Authentication (CWE-287) vulnerability in Caddyserver Caddy. Its CVSS base score is 8.1 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique External Remote Services (T1133); ranked at the 21th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified map to IA-2 (Identification and Authentication (Organizational Users)) and IA-3 (Device Identification and Authentication) — see the control section below for these in your framework.

OWASP Top 10 for Web (2025)

A06:2025 Insecure DesignA07:2025 Authentication Failures

EU & UK References

Vulnerability Data

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…

more

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.

CWE(s)
CWE-287CWE-290CWE-444

Related Threats

MITRE ATT&CK Enterprise Techniques

T1133 External Remote Services Persistence
Adversaries may leverage external-facing remote services to initially access and/or persist within a network.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1557 Adversary-in-the-Middle Credential Access
Adversaries may attempt to position themselves between two or more networked devices using an adversary-in-the-middle (AiTM) technique to support follow-on behaviors such as [Network Sniffing](https://attack.
attack.mitre.org →
T1557.001 Name Resolution Poisoning and SMB Relay Credential Access
By responding to LLMNR/NBT-NS/mDNS network traffic, adversaries may spoof an authoritative source for name resolution to force communication with an adversary controlled system.
attack.mitre.org →
T1036 Masquerading Stealth
Adversaries may attempt to manipulate features of their artifacts to make them appear legitimate or benign to users and/or security tools.
attack.mitre.org →
T1036.001 Invalid Code Signature Stealth
Adversaries may attempt to mimic features of valid code signatures to increase the chance of deceiving a user, analyst, or tool.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-50463Same product: Caddyserver Caddy
CVE-2026-30851Same product: Caddyserver Caddy
CVE-2026-45135Same product: Caddyserver Caddy
CVE-2026-27588Same product: Caddyserver Caddy
CVE-2026-27587Same product: Caddyserver Caddy
CVE-2026-27585Same product: Caddyserver Caddy
CVE-2026-27590Same product: Caddyserver Caddy
CVE-2026-45692Same product: Caddyserver Caddy
CVE-2026-27589Same product: Caddyserver Caddy
CVE-2026-30852Same product: Caddyserver Caddy

Affected Assets

caddyserver
caddy
≤ 2.11.4

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)
  • 11 hardening rules · 5 OS baselines
Validate
Prove the fix (OWASP ASVS)
  • V4.1.3
  • V4.2.4
  • V6.4.4
  • V6.5.4

Mitigating Controls (NIST 800-53 r5) AI

IA-2 Identification and Authentication (Organizational Users) good match

Mandates unique identification and authentication of organizational users before access, directly stopping improper authentication.

IA-3 Device Identification and Authentication good match

Requires unique identification and authentication of devices before establishing connections, preventing unauthenticated device claims.

IA-8 Identification and Authentication (Non-organizational Users) good match

Mandates unique identification and authentication of non-organizational users, directly addressing the weakness for external actors.

IA-5 Authenticator Management good match

Manages authenticators with verification and secure distribution, reducing opportunities for improper authentication.

AC-3 Access Enforcement partial match

Enforces access only after approved authorizations, which presupposes correct authentication has occurred.

SC-7 Boundary Protection partial match

Boundary protection at external interfaces can enforce consistent HTTP request/response parsing rules between intermediaries and endpoints.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.AA-04 full match
prevents

Protecting, conveying, and verifying identity assertions specifically prevents the spoofing that enables authentication bypass.

PR.AA-03 mostly match
prevents

PR.AA-03 directly enforces authentication mechanisms that eliminate most improper-authentication defects, yet CWE-287 spans additional vectors (missing checks, flawed protocols, session handling) that one control does not fully close.

PR.PS-01 mostly match
prevents

Configuration management can enforce uniform HTTP parsing rules across intermediaries, directly mitigating inconsistent interpretation.

DE.CM-01 partial match
prevents

Network monitoring can detect smuggling attempts via anomalous HTTP traffic or logs, while eliminating the inconsistency directly aids detection of such events.

PR.AA-01 partial match
prevents

PR.AA-01 supplies and governs credentials/tokens that authentication relies on, removing some weak-credential cases, yet leaves verification logic, missing checks, and protocol flaws untouched.

PR.AA-02 partial match
prevents

PR.AA-02 ensures valid enrollment and unique credential binding, which reduces some improper-auth risks at issuance time but leaves runtime claim verification untouched, so each direction only partially addresses the other.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.5 Secure authentication full match
prevents

Requiring authentication methods whose strength matches the sensitivity of the data and mandating multi-factor authentication directly blocks attempts to access resources without proving identity.

A.5.17 Authentication information mostly match
prevents

Verifying user identity before issuing replacement credentials and forcing changes after compromise reduces the likelihood that authentication bypass can be achieved through stolen or weak credentials.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect spoofing vulnerabilities but does not prevent them by itself.

A.5.15 Access control partial match
prevents

Access control policy reduces spoofing opportunities but does not prescribe authentication mechanisms.

A.5.16 Identity management partial match
prevents

Mandating segregated approval and oversight for non-human identities reduces the chance that weak or orphaned credentials can be exploited for unauthorized authentication.

A.5.18 Access rights partial match
prevents

Access rights assignment limits exposure but does not enforce authentication strength.

Hardening callouts derived

Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).

Oracle Linux 8 (1 rule)
  • V-248827 OL 8 must not have the rsh-server package installed. prevents CWE-287, CWE-290
RHEL 7 (3 rules)
  • V-204425 The Red Hat Enterprise Linux operating system must be configured so that the SSH daemon does not allow authentication using an empty password. prevents CWE-287
  • V-204442 The Red Hat Enterprise Linux operating system must not have the rsh-server package installed. prevents CWE-287, CWE-290
  • V-204424 The Red Hat Enterprise Linux operating system must not allow accounts configured with blank or null passwords. prevents CWE-287
RHEL 8 (1 rule)
  • V-230492 RHEL 8 must not have the rsh-server package installed. prevents CWE-287, CWE-290
Ubuntu 22.04 (1 rule)
  • V-260470 Ubuntu 22.04 LTS, when booted, must require authentication upon booting into single-user and maintenance modes. prevents CWE-287
Ubuntu 24.04 (2 rules)
  • V-270675 Ubuntu 24.04 LTS when booted must require authentication upon booting into single-user and maintenance modes. prevents CWE-287
  • V-270714 Ubuntu 24.04 LTS must not allow accounts configured in Pluggable Authentication Modules (PAM) with blank or null passwords. prevents CWE-287

References