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CVE-2026-27586— Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed

Quick assessment

Affected
caddyserver caddy
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Caddy是Caddy公司的一款开源、跨平台的HTTP/Web服务器。 Caddy 2.11.1之前版本存在安全漏洞，该漏洞源于ClientAuthentication.provision函数中的两个被忽略的错误导致mTLS客户端证书身份验证静默失败，可能允许绕过预期的私有CA信任边界。

AI Predicted 9.1 Difficulty: Trivial EPSS 0.29% · P22

Possible ATT&CK Techniques 1 AI

T1199 · Trusted Relationship
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I. Basic Information for CVE-2026-27586

Vulnerability Information
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Vulnerability Title
Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed
Source: CVE Program / CVE List V5
Vulnerability Description
Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, two swallowed errors in `ClientAuthentication.provision()` cause mTLS client certificate authentication to silently fail open when a CA certificate file is missing, unreadable, or malformed. The server starts without error but accepts any client certificate signed by any system-trusted CA, completely bypassing the intended private CA trust boundary. Any deployment using `trusted_ca_cert_file` or `trusted_ca_certs_pem_files` for mTLS will silently degrade to accepting any system-trusted client certificate if the CA file becomes unavailable. This can happen due to a typo in the path, file rotation, corruption, or permission changes. The server gives no indication that mTLS is misconfigured. Version 2.11.1 fixes the vulnerability.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:P
Source: CVE Program / CVE List V5
Vulnerability Type
对异常条件的处理不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
Caddy 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Caddy是Caddy公司的一款开源、跨平台的HTTP/Web服务器。 Caddy 2.11.1之前版本存在安全漏洞，该漏洞源于ClientAuthentication.provision函数中的两个被忽略的错误导致mTLS客户端证书身份验证静默失败，可能允许绕过预期的私有CA信任边界。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
caddyserver caddy < 2.11.1 -

II. Public POCs for CVE-2026-27586

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III. Intelligence Information for CVE-2026-27586

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Vendor Pages for CVE-2026-27586 (1)

Same Patch Batch · caddyserver · 2026-02-24 · 6 CVEs total

CVE-2026-27585 Caddy's improper sanitization of glob characters in file matcher may lead to bypassing sec
CVE-2026-27587 Caddy: MatchPath %xx (escaped-path) branch skips case normalization, enabling path-based r
CVE-2026-27588 Caddy: MatchHost becomes case-sensitive for large host lists (>100), enabling host-based r
CVE-2026-27590 Caddy: Unicode case-folding length expansion causes incorrect split_path index (SCRIPT_NAM
CVE-2026-27589 Caddy vulnerable to cross-origin config application via local admin API /load (caddy)

IV. Related Vulnerabilities

Same product: caddy
Same vendor: caddyserver
Same weakness: CWE-755