CVE-2026-27588

CriticalPublic PoC

Published: 24 February 2026

Published

24 February 2026

Modified

25 February 2026

KEV Added

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Patch

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CVSS Score 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS Score 0.0006 19.3th percentile

Risk Priority 18 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-27588 is a critical-severity Improper Handling of Case Sensitivity (CWE-178) vulnerability in Caddyserver Caddy. Its CVSS base score is 9.1 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 19.3th 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 are NIST 800-53 SI-2 (Flaw Remediation) and AC-4 (Information Flow Enforcement).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190). What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly remediates the case-sensitivity flaw in Caddy's HTTP host matcher for large lists, preventing Host header manipulation to bypass routing and access controls.

AC-4 Information Flow Enforcement partial match

prevent

Enforces information flow control policies using host attributes, mitigating bypass of host-based routing via inconsistent case handling in the Host header.

SI-4 System Monitoring partial match

detect

Monitors system activity to identify exploitation attempts through anomalous Host header casing patterns in HTTP requests.

MITRE ATT&CK Enterprise TechniquesAI

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 →

Why these techniques?

Vulnerability in public-facing Caddy web server allows direct bypass of host-based routing and access controls via crafted Host header casing, enabling unauthorized access to restricted endpoints.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, Caddy's HTTP `host` request matcher is documented as case-insensitive, but when configured with a large host list (>100 entries) it becomes case-sensitive due to an…

optimized matching path. An attacker can bypass host-based routing and any access controls attached to that route by changing the casing of the `Host` header. Version 2.11.1 contains a fix for the issue.

Deeper analysisAI

CVE-2026-27588 affects Caddy, an extensible server platform that uses TLS by default, in versions prior to 2.11.1. The vulnerability stems from the HTTP `host` request matcher, which is documented as case-insensitive but becomes case-sensitive when configured with a large host list exceeding 100 entries due to an optimized matching path. This inconsistency, classified under CWE-178 (Improper Handling of Case Sensitivity), enables attackers to bypass host-based routing and associated access controls by manipulating the casing of the `Host` header. The issue carries a CVSS v3.1 base score of 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N), indicating critical severity with high impacts on confidentiality and integrity.

Any unauthenticated remote attacker with network access can exploit this vulnerability by crafting HTTP requests with altered casing in the `Host` header, such as changing "example.com" to "Example.com" when the configuration expects a specific case. This allows evasion of intended routing logic, potentially granting access to restricted routes, sensitive endpoints, or resources protected by host-specific access controls. Exploitation requires no privileges, user interaction, or special conditions beyond a large host list in the Caddy configuration, making it straightforward in misconfigured deployments.

The Caddy security advisory (GHSA-x76f-jf84-rqj8) and release notes for version 2.11.1 detail the fix, which restores consistent case-insensitive matching regardless of host list size. Security practitioners should upgrade to Caddy 2.11.1 or later to mitigate the issue, review configurations with large host lists for exposure, and monitor `Host` header patterns in access logs for signs of bypass attempts.