Caddy: mTLS client authentication silently fails open when CA certificate file is missing or malformed

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.8 · 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 · score source

EPSS

• CVE-2026-27586: 0.29% estimated exploitation activity in the next 30 days; percentile 22.18 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Details In modules/caddytls/connpolicy.go, the provision() method has two return nil statements that should be return err: Bug #1 — line 787: Bug #2 — line 800: Compare with line 811 which correctly returns the error: When the error is swallowed on line 787, the chain is: 1. TrustedCACerts remains empty (no DER data appended from the file) 2. The len(clientauth.TrustedCACerts) 0 guard on line 794 is false — skipped 3. clientauth.CARaw is nil — line 806 returns nil 4. clientauth.ca remains nil — no CA pool was created 5. provision() returns nil — caller thinks provisioning succeeded Then in ConfigureTLSConfig(): 6. Active() returns…

Published by the source

24 Feb 2026, 20:22 UTC

Last modified by the source

27 Feb 2026, 19:52 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-hffm-g8v7-wrv7

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# Patch Next advisory brief: CVE-2026-27586

Advisory: Caddy: mTLS client authentication silently fails open when CA certificate file is missing or malformed
IDs: CVE-2026-27586, GHSA-hffm-g8v7-wrv7
Record: https://patchnext.com/advisories/cve-2026-27586 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/caddyserver/caddy/v2 2.11.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/caddyserver/caddy/v2: affected < 2.11.1; fixed in 2.11.1

## Suggested inspection (read-only)
- Go github.com/caddyserver/caddy/v2: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/caddyserver/caddy/v2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-hffm-g8v7-wrv7 (GitHub-reviewed advisory GHSA-hffm-g8v7-wrv7 lists affected ranges: Go github.com/caddyserver/caddy/v2 >=0 <2.11.1)

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