{
  "canonical_id": "caddy--CVE-2026-27586",
  "system_id": "caddy",
  "display_name": "Caddy",
  "category": "servers",
  "advisory_mode": "server",
  "title": "Caddy: mTLS client authentication silently fails open when CA certificate file is missing or malformed",
  "summary": "### Summary\n\nTwo 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.\n\n### Details\n\nIn `modules/caddytls/connpolicy.go`, the `provision()` method has two `return nil` statements that should be `return err`:\n\n**Bug #1 \u2014 line 787:**\n```go\nders, err := convertPEMFilesToDER(fpath)\nif err != nil {\n    return nil  // BUG: should be \"return err\"\n}\n```\n\n**Bug #2 \u2014 line 800:**\n```go\nerr := caPool.Provision(ctx)\nif err != nil {\n    return nil  // BUG: should be \"return err\"\n}\n```\n\nCompare with line 811 which correctly returns the error:\n```go\ncaRaw, err := ctx.LoadModule(clientauth, \"CARaw\")\nif err != nil {\n    return err  // CORRECT\n}\n```\n\nWhen the error is swallowed on line 787, the chain is:\n\n1. `TrustedCACerts` remains empty (no DER data appended from the file)\n2. The `len(clientauth.TrustedCACerts) > 0` guard on line 794 is false \u2014 skipped\n3. `clientauth.CARaw` is nil \u2014 line 806 returns nil\n4. `clientauth.ca` remains nil \u2014 no CA pool was created\n5. `provision()` returns nil \u2014 caller thinks provisioning succeeded\n\nThen in `ConfigureTLSConfig()`:\n\n6. `Active()` returns true because `TrustedCACertPEMFiles` is non-empty\n7. Default mode is set to `RequireAndVerifyClientCert` (line 860)\n8. But `clientauth.ca` is nil, so `cfg.ClientCAs` is never set (line 867 skipped)\n9. Go's `crypto/tls` with `RequireAndVerifyClientCert` + nil `ClientCAs` verifies client certs against the **system root pool** instead of the intended CA\n\nThe fix is changing `return nil` to `return err` on lines 787 and 800.\n\n### PoC\n\n1. Configure Caddy with mTLS pointing to a nonexistent CA file:\n\n```\n{\n    \"apps\": {\n        \"http\": {\n            \"servers\": {\n                \"srv0\": {\n                    \"listen\": [\":443\"],\n                    \"tls_connection_policies\": [{\n                        \"client_authentication\": {\n                            \"trusted_ca_certs_pem_files\": [\"/nonexistent/ca.pem\"]\n                        }\n                    }]\n                }\n            }\n        }\n    }\n}\n```\n\n2. Start Caddy \u2014 it starts without any error or warning.\n\n3. Connect with any client certificate (even self-signed):\n```bash\nopenssl s_client -connect localhost:443 -cert client.pem -key client-key.pem\n```\n\n4. The TLS handshake succeeds despite the certificate not being signed by the intended CA.\n\nA full Go test that proves the bug end-to-end (including a successful TLS handshake with a random self-signed client cert) is here: https://gist.github.com/moscowchill/9566c79c76c0b64c57f8bd0716f97c48\n\nTest output:\n```\n=== RUN   TestSwallowedErrorMTLSFailOpen\n    BUG CONFIRMED: provision() swallowed the error from a nonexistent CA file.\n    tls.Config has RequireAndVerifyClientCert but ClientCAs is nil.\n    CRITICAL: TLS handshake succeeded with a self-signed client cert!\n    The server accepted a client certificate NOT signed by the intended CA.\n--- PASS: TestSwallowedErrorMTLSFailOpen (0.03s)\n```\n\n### Impact\n\nAny 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.",
  "published_at": "2026-02-24T20:22:53Z",
  "updated_at": "2026-02-27T19:52:41Z",
  "severity": "medium",
  "cvss_score": 4.0,
  "exploit_status": "unknown",
  "source_confidence": "official",
  "official_source_url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-hffm-g8v7-wrv7",
  "secondary_source_urls": [
    "https://nvd.nist.gov/vuln/detail/CVE-2026-27586",
    "https://github.com/caddyserver/caddy/commit/d42d39b4bc237c628f9a95363b28044cb7a7fe72",
    "https://gist.github.com/moscowchill/9566c79c76c0b64c57f8bd0716f97c48",
    "https://github.com/caddyserver/caddy",
    "https://github.com/caddyserver/caddy/releases/tag/v2.11.1",
    "https://pkg.go.dev/vuln/GO-2026-4539"
  ],
  "aliases": [
    "CVE-2026-27586",
    "GO-2026-4539",
    "GHSA-hffm-g8v7-wrv7"
  ],
  "cve_ids": [
    "CVE-2026-27586"
  ],
  "ghsa_ids": [
    "GHSA-hffm-g8v7-wrv7"
  ],
  "osv_ids": [
    "GHSA-hffm-g8v7-wrv7",
    "GO-2026-4539"
  ],
  "affected_versions": [
    "introduced=0, fixed<2.11.1"
  ],
  "fixed_versions": [
    "2.11.1"
  ],
  "package_name": "github.com/caddyserver/caddy/v2",
  "render_markdown": false,
  "case_path": null,
  "secure_code_topics": [
    "proxy-trust-boundary",
    "request-smuggling-boundary",
    "plugin-extension-trust-policy"
  ],
  "status": "generated",
  "triage_reasons": [],
  "entity_refs": [
    {
      "entity_id": "caddy",
      "entity_type": "system",
      "relation": "root-system",
      "root_system_id": "caddy",
      "official": true
    },
    {
      "entity_id": "caddy--repo--github-com-caddyserver-caddy-v2",
      "entity_type": "repo",
      "relation": "affected-component",
      "root_system_id": "caddy",
      "official": false
    }
  ],
  "affected_components": [
    {
      "name": "caddyserver / caddy / v2",
      "entity_id": "caddy--repo--github-com-caddyserver-caddy-v2",
      "scope": "repo",
      "package_name": "github.com/caddyserver/caddy/v2",
      "official": false
    }
  ],
  "affected_version_ranges": [
    "introduced=0, fixed<2.11.1"
  ],
  "fixed_version_ranges": [
    "2.11.1"
  ],
  "introduced_version": "introduced=0, fixed<2.11.1",
  "patched_version": "2.11.1",
  "version_evidence_sources": [
    "https://github.com/caddyserver/caddy/security/advisories/GHSA-hffm-g8v7-wrv7",
    "https://nvd.nist.gov/vuln/detail/CVE-2026-27586",
    "https://github.com/caddyserver/caddy/commit/d42d39b4bc237c628f9a95363b28044cb7a7fe72",
    "https://gist.github.com/moscowchill/9566c79c76c0b64c57f8bd0716f97c48",
    "https://github.com/caddyserver/caddy",
    "https://github.com/caddyserver/caddy/releases/tag/v2.11.1",
    "https://pkg.go.dev/vuln/GO-2026-4539"
  ],
  "affected_version_refs": [
    "caddy--repo--github-com-caddyserver-caddy-v2--introduced-0-fixed-2-11-1"
  ],
  "fixed_version_refs": [
    "caddy--repo--github-com-caddyserver-caddy-v2--2-11-1"
  ],
  "patched_version_refs": [
    "caddy--repo--github-com-caddyserver-caddy-v2--2-11-1"
  ],
  "version_sync_confidence": "high",
  "advisory_scope": "repo",
  "version_confidence": "high",
  "version_gap_reason": "",
  "version_resolution_needed": false,
  "workflow": {
    "workflow_id": "caddy--CVE-2026-27586--workflow",
    "vuln_family": "proxy-boundary",
    "entry_surface": "proxy-header-or-trust-boundary",
    "preconditions": [
      "\u4ec5\u5728 lab-local\u3001lab-public \u6216\u660e\u786e\u6388\u6743\u76ee\u6807\u4e2d\u6267\u884c\u3002",
      "\u786e\u8ba4\u76ee\u6807\u547d\u4e2d\u7248\u672c\u65ad\u8a00: introduced=0, fixed<2.11.1",
      "\u82e5\u5bf9\u8c61\u5c5e\u4e8e `repo`\uff0c\u5148\u786e\u8ba4\u6269\u5c55/\u4ed3\u5e93/\u5305\u5df2\u542f\u7528\u5e76\u5904\u4e8e\u53d7\u5f71\u54cd\u7248\u672c\u3002"
    ],
    "required_role": "reverse-proxy-or-edge-client",
    "affected_version_assertion": [
      "introduced=0, fixed<2.11.1"
    ],
    "trigger_vector": "\u5bf9 `proxy-boundary` \u5bb6\u65cf\u5165\u53e3\u6295\u9012\u6700\u5c0f\u5316\u3001\u53ef\u5ba1\u8ba1\u3001\u53ef\u56de\u6eda\u7684\u53d7\u63a7\u8f93\u5165\uff0c\u6bd4\u8f83\u4fee\u590d\u524d\u540e\u5dee\u5f02\u3002",
    "request_or_ui_path": [
      "/middleware",
      "/x-forwarded-* trust path"
    ],
    "input_shape": "\u63d0\u4ea4\u53d7\u63a7\u4ee3\u7406\u5934\u6216\u6765\u6e90\u5934\uff0c\u9a8c\u8bc1\u4fe1\u4efb\u8fb9\u754c\u548c\u56de\u6e90\u9274\u6743\u3002",
    "expected_unsafe_behavior": "\u4ec5\u51ed\u4ee3\u7406\u5934\u5373\u53ef\u8d8a\u8fc7\u9274\u6743\u6216\u6765\u6e90\u63a7\u5236\u3002",
    "server_evidence_points": [
      "\u5e94\u7528\u65e5\u5fd7\u4e2d\u7684\u547d\u4e2d\u8def\u5f84\u3001\u9274\u6743\u51b3\u7b56\u548c\u5f02\u5e38\u6808",
      "\u53cd\u5411\u4ee3\u7406\u6216\u8fb9\u754c\u5c42\u65e5\u5fd7\u4e2d\u7684\u8bf7\u6c42\u5934\u3001\u6765\u6e90 IP \u4e0e\u8def\u7531\u51b3\u7b56"
    ],
    "browser_evidence_points": [
      "\u57fa\u7ebf\u622a\u56fe\u4e0e\u653b\u51fb\u540e\u622a\u56fe\u7684 DOM/\u89c6\u89c9\u5dee\u5f02",
      "console\u3001network \u4e0e response metadata \u4e2d\u7684\u5f02\u5e38\u4fe1\u53f7"
    ],
    "db_or_fs_evidence_points": [
      "\u6570\u636e\u5e93\u4e2d\u65b0\u589e/\u8d8a\u6743\u8bfb\u53d6\u7684\u6d4b\u8bd5\u6570\u636e",
      "\u6587\u4ef6\u7cfb\u7edf\u4e2d\u65b0\u589e\u4e0a\u4f20\u6837\u672c\u3001\u7f13\u5b58\u6761\u76ee\u6216\u8d8a\u6743\u8bfb\u53d6\u75d5\u8ff9"
    ],
    "detection_signals": [
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      "\u5e94\u7528\u5ba1\u8ba1\u65e5\u5fd7\u4e2d\u7684\u6743\u9650\u9519\u8bef\u3001\u91cd\u5b9a\u5411\u5f02\u5e38\u3001\u6a21\u677f\u6e32\u67d3\u6216\u4e0a\u4f20\u843d\u76d8\u4e8b\u4ef6",
      "\u4e0a\u6e38\u4ee3\u7406\u4e0e\u5e94\u7528\u5c42\u5bf9 Content-Length / Transfer-Encoding / forwarded headers \u7684\u89e3\u91ca\u5dee\u5f02"
    ],
    "mitigation_summary": "\u4f18\u5148\u5347\u7ea7\u5230\u4fee\u590d\u7248\u672c\uff0c\u5e76\u540c\u65f6\u6536\u7d27\u8f93\u5165\u6821\u9a8c\u3001\u670d\u52a1\u7aef\u9274\u6743\u3001\u4ee3\u7406\u4fe1\u4efb\u8fb9\u754c\u3001\u6269\u5c55\u5b89\u88c5\u4fe1\u4efb\u548c\u5ba1\u8ba1\u65e5\u5fd7\u3002",
    "patch_validation_steps": [
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      "\u4fdd\u7559\u540c\u4e00\u7ec4\u53d7\u63a7\u8f93\u5165\uff0c\u5728\u4fee\u590d\u524d\u540e\u5206\u522b\u6267\u884c\u5e76\u6bd4\u5bf9\u54cd\u5e94\u3001\u65e5\u5fd7\u4e0e\u6d4f\u89c8\u5668\u8bc1\u636e\u3002",
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      "\u8865\u5145 `proxy-boundary` \u65cf\u81ea\u52a8\u5316\u56de\u5f52\uff0c\u907f\u514d\u540c\u7c7b\u8def\u5f84\u5728\u63d2\u4ef6\u3001\u4e3b\u9898\u6216\u4ee3\u7406\u94fe\u4e2d\u56de\u5f52\u3002"
    ],
    "lab_safety_notes": [
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      "\u5982\u9700\u6d4f\u89c8\u5668\u8bc1\u636e\uff0c\u4fdd\u7559 baseline / proof \u4e24\u4efd\u5feb\u7167\u4ee5\u53ca console / network \u8bb0\u5f55\u3002"
    ],
    "review_state": "ready"
  },
  "verification_status": "triage-manual",
  "verification_mode": "synthetic",
  "last_verified_at": null,
  "last_run_id": null,
  "evidence_bundle": null,
  "historical_status": null,
  "latest_status": null,
  "browser_evidence": {
    "required": false,
    "present": false,
    "refs": []
  },
  "repro_profile_id": "authz-bypass-generic",
  "artifact_mode": "synthetic",
  "blocked_reason": null,
  "metadata": {
    "source_names": [
      "OSV Caddy"
    ],
    "source_kinds": [
      "osv-batch"
    ],
    "candidate_count": 2,
    "entity_ref_count": 2,
    "advisory_scope": "repo",
    "version_confidence": "high",
    "workflow_id": "caddy--CVE-2026-27586--workflow"
  }
}