{
  "canonical_id": "caddy--CVE-2026-27590",
  "system_id": "caddy",
  "display_name": "Caddy",
  "category": "servers",
  "advisory_mode": "server",
  "title": "Caddy: Unicode case-folding length expansion causes incorrect split_path index in FastCGI transport",
  "summary": "### Summary\n\nCaddy's FastCGI path splitting logic computes the split index on a lowercased copy of the request path and then uses that byte index to slice the original path. This is unsafe for Unicode because `strings.ToLower()` can change UTF-8 byte length for some characters. As a result, Caddy can derive an incorrect `SCRIPT_NAME`/`SCRIPT_FILENAME` and `PATH_INFO`, potentially causing a request that contains `.php` to execute a different on-disk file than intended (path confusion). In setups where an attacker can control file contents (e.g., upload features), this can lead to unintended PHP execution of non-.php files (potential RCE depending on deployment).\n\n### Details\n\nThe issue is in `github.com/caddyserver/caddy/modules/caddyhttp/fastcgi.Trasnport.splitPos()` (and the subsequent slicing in `buildEnv()`):\n\n```\nlowerPath := strings.ToLower(path)\nidx := strings.Index(lowerPath, strings.ToLower(split))\nreturn idx + len(split)\n```\n\nThe returned index is computed in the byte space of lowerPath, but `buildEnv()` applies it to the original path:\n\n- `docURI = path[:splitPos]`\n- `pathInfo = path[splitPos:]`\n- `scriptName = strings.TrimSuffix(path, fc.pathInfo)`\n- `scriptFilename = caddyhttp.SanitizedPathJoin(fc.documentRoot, fc.scriptName)`\n\nThis assumes `lowerPath` and `path` have identical byte lengths and identical byte offsets, which is not true for some Unicode case mappings. Certain characters expand when lowercased (UTF-8 byte length increases), shifting the computed index. This creates a mismatch where `.php` is found in the lowercased string at an offset that does not correspond to the same position in the original string, causing the split point to land later/earlier than intended.\n\n### PoC\n\nCreate a small Go program that reproduces Caddy's `splitPos()` behavior (compute the `.php` split point on a lowercased path, then use that byte index on the original path):\n\n1. Save this as `poc.go`:\n\n```go\npackage main\n\nimport (\n\t\"fmt\"\n\t\"strings\"\n)\n\nfunc splitPos(path string, split string) int {\n\tlowerPath := strings.ToLower(path)\n\tidx := strings.Index(lowerPath, strings.ToLower(split))\n\tif idx < 0 {\n\t\treturn -1\n\t}\n\treturn idx + len(split)\n}\n\nfunc main() {\n\t// U+023A: \u023a (UTF-8: C8 BA). Lowercase is \u2c65 (UTF-8: E2 B1 A5), longer in bytes.\n\tpath := \"/\u023a\u023a\u023a\u023ashell.php.txt.php\"\n\tsplit := \".php\"\n\n\tpos := splitPos(path, split)\n\n\tfmt.Printf(\"orig bytes=%d\\n\", len(path))\n\tfmt.Printf(\"lower bytes=%d\\n\", len(strings.ToLower(path)))\n\tfmt.Printf(\"splitPos=%d\\n\", pos)\n\n\tfmt.Printf(\"orig[:pos]=%q\\n\", path[:pos])\n\tfmt.Printf(\"orig[pos:]=%q\\n\", path[pos:])\n\n\t// Expected split: right after the first \".php\" in the original string\n\twant := strings.Index(path, split) + len(split)\n\tfmt.Printf(\"expected splitPos=%d\\n\", want)\n\tfmt.Printf(\"expected orig[:]=%q\\n\", path[:want])\n}\n```\n\n2. Run it:\n\n```console\ngo run poc.go\n```\n\nOutput on my side:\n\n```\norig bytes=26\nlower bytes=30\nsplitPos=22\norig[:pos]=\"/\u023a\u023a\u023a\u023ashell.php.txt\"\norig[pos:]=\".php\"\nexpected splitPos=18\nexpected orig[:]=\"/\u023a\u023a\u023a\u023ashell.php\"\n```\n\nExpected split is right after the first `.php` (`/\u023a\u023a\u023a\u023ashell.php`). Instead, the computed split lands later and cuts the original path after `shell.php.txt`, leaving `.php` as the remainder.\n\n### Impact\n\nSecurity boundary bypass/path confusion in script resolution.\nIn typical deployments, `.php` extension boundaries are relied on to decide what is executed by PHP. This bug can cause Caddy/FPM to execute a different file than intended by confusing `SCRIPT_NAME`/`SCRIPT_FILENAME`. If an attacker can place attacker-controlled content into a file that can be resolved as `SCRIPT_FILENAME` (common in web apps with uploads or writable directories), this can lead to unintended PHP execution of non-.php files and potentially remote code execution. Severity depends on deployment and presence of attacker-controlled file writes, but the primitive itself is remotely triggerable via crafted URLs.\n\nThis vulnerability was initially reported to FrankenPHP (https://github.com/php/frankenphp/security/advisories/GHSA-g966-83w7-6w38) by @AbdrrahimDahmani. The affected code has been copied/adapted from Caddy, which, according to research, is also affected.\n\nThe patch is a port of the FrankenPHP patch.",
  "published_at": "2026-02-24T20:39:08Z",
  "updated_at": "2026-02-27T19:55:10Z",
  "severity": "medium",
  "cvss_score": 4.0,
  "exploit_status": "unknown",
  "source_confidence": "official",
  "official_source_url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-5r3v-vc8m-m96g",
  "secondary_source_urls": [
    "https://github.com/php/frankenphp/security/advisories/GHSA-g966-83w7-6w38",
    "https://nvd.nist.gov/vuln/detail/CVE-2026-27590",
    "https://github.com/caddyserver/caddy",
    "https://github.com/caddyserver/caddy/releases/tag/v2.11.1",
    "https://pkg.go.dev/vuln/GO-2026-4536"
  ],
  "aliases": [
    "CVE-2026-27590",
    "GO-2026-4536",
    "GHSA-5r3v-vc8m-m96g"
  ],
  "cve_ids": [
    "CVE-2026-27590"
  ],
  "ghsa_ids": [
    "GHSA-5r3v-vc8m-m96g"
  ],
  "osv_ids": [
    "GHSA-5r3v-vc8m-m96g",
    "GO-2026-4536"
  ],
  "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",
    "file-upload-validation",
    "plugin-extension-trust-policy",
    "dependency-upgrade-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--extension--github-com-caddyserver-caddy-v2",
      "entity_type": "extension",
      "relation": "affected-component",
      "root_system_id": "caddy",
      "official": false
    }
  ],
  "affected_components": [
    {
      "name": "caddyserver / caddy / v2",
      "entity_id": "caddy--extension--github-com-caddyserver-caddy-v2",
      "scope": "extension",
      "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-5r3v-vc8m-m96g",
    "https://github.com/php/frankenphp/security/advisories/GHSA-g966-83w7-6w38",
    "https://nvd.nist.gov/vuln/detail/CVE-2026-27590",
    "https://github.com/caddyserver/caddy",
    "https://github.com/caddyserver/caddy/releases/tag/v2.11.1",
    "https://pkg.go.dev/vuln/GO-2026-4536"
  ],
  "affected_version_refs": [
    "caddy--extension--github-com-caddyserver-caddy-v2--introduced-0-fixed-2-11-1"
  ],
  "fixed_version_refs": [
    "caddy--extension--github-com-caddyserver-caddy-v2--2-11-1"
  ],
  "patched_version_refs": [
    "caddy--extension--github-com-caddyserver-caddy-v2--2-11-1"
  ],
  "version_sync_confidence": "high",
  "advisory_scope": "extension",
  "version_confidence": "high",
  "version_gap_reason": "",
  "version_resolution_needed": false,
  "workflow": {
    "workflow_id": "caddy--CVE-2026-27590--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 `extension`\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",
      "\u63d2\u4ef6/\u6269\u5c55\u7ba1\u7406\u65e5\u5fd7\u3001\u5b89\u88c5\u65e5\u5fd7\u4e0e\u7248\u672c\u6e05\u5355"
    ],
    "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",
      "\u63d2\u4ef6\u76ee\u5f55\u3001\u4e3b\u9898\u76ee\u5f55\u6216\u6269\u5c55\u914d\u7f6e\u8868\u4e2d\u7684\u6d4b\u8bd5\u6837\u672c"
    ],
    "detection_signals": [
      "WAF / reverse proxy \u5f02\u5e38\u65e5\u5fd7\u3001\u8bbf\u95ee\u65e5\u5fd7\u548c\u544a\u8b66",
      "\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": [
      "\u786e\u8ba4\u76ee\u6807\u7248\u672c\u4ece `introduced=0, fixed<2.11.1` \u5347\u7ea7\u6216\u56de\u79fb\u5230 `2.11.1`\u3002",
      "\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",
      "\u786e\u8ba4\u4fee\u590d\u540e\u4ec5\u4fdd\u7559\u9884\u671f\u4e1a\u52a1\u884c\u4e3a\uff0c\u4e0d\u518d\u89e6\u53d1\u8d8a\u6743\u3001\u56de\u663e\u3001\u5f02\u5e38\u6e32\u67d3\u6216\u9519\u8bef\u8bf7\u6c42\u3002",
      "\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": [
      "\u53ea\u4f7f\u7528\u56de\u73af\u5730\u5740\u3001\u54e8\u5175\u76ee\u6807\u3001\u65e0\u5bb3\u6837\u672c\u6216\u53ef\u56de\u6eda\u6d4b\u8bd5\u6570\u636e\u3002",
      "\u7981\u6b62\u9020\u6210\u6301\u4e45\u7834\u574f\u3001\u8d8a\u6743\u4e0b\u8f7d\u771f\u5b9e\u6570\u636e\u6216\u4e0d\u53ef\u56de\u6eda side effect\u3002",
      "\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": "proxy-boundary-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": "extension",
    "version_confidence": "high",
    "workflow_id": "caddy--CVE-2026-27590--workflow"
  }
}