CVE-2026-34785: Rack :Static Prefix Matching Can Expose Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-h2jq-g4cq-5ppq",
  "modified" : "2026-05-13T15:20:39Z",
  "published" : "2026-04-02T18:44:25Z",
  "aliases" : [ "CVE-2026-34785" ],
  "summary" : "Rack::Static prefix matching can expose unintended files under the static root",
  "details" : "## Summary\n\n`Rack::Static` determines whether a request should be served as a static file using a simple string prefix check. When configured with URL prefixes such as `\"/css\"`, it matches any request path that begins with that string, including unrelated paths such as `\"/css-config.env\"` or `\"/css-backup.sql\"`.\n\nAs a result, files under the static root whose names merely share the configured prefix may be served unintentionally, leading to information disclosure.\n\n## Details\n\n`Rack::Static#route_file` performs static-route matching using logic equivalent to:\n\n```ruby\n@urls.any? { |url| path.index(url) == 0 }\n```\n\nThis checks only whether the request path starts with the configured prefix string. It does not require a path segment boundary after the prefix.\n\nFor example, with:\n\n```ruby\nuse Rack::Static, urls: [\"/css\", \"/js\"], root: \"public\"\n```\n\nthe following path is matched as intended:\n\n```text\n/css/style.css\n```\n\nbut these paths are also matched:\n\n```text\n/css-config.env\n/css-backup.sql\n/csssecrets.yml\n```\n\nIf such files exist under the configured static root, Rack forwards the request to the file server and serves them as static content.\n\nThis means a configuration intended to expose only directory trees such as `/css/...` and `/js/...` may also expose sibling files whose names begin with those same strings.\n\n## Impact\n\nAn attacker can request files under the configured static root whose names share a configured URL prefix and obtain their contents.\n\nIn affected deployments, this may expose configuration files, secrets, backups, environment files, or other unintended static content located under the same root directory.\n\n## Mitigation\n\n* Update to a patched version of Rack that enforces a path boundary when matching configured static URL prefixes.\n* Match only paths that are either exactly equal to the configured prefix or begin with `prefix + \"/\"`.\n* Avoid placing sensitive files under the `Rack::Static` root directory.\n* Prefer static URL mappings that cannot overlap with sensitive filenames.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.2.23"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.0.beta1"
      }, {
        "fixed" : "3.1.21"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.2.0"
      }, {
        "fixed" : "3.2.6"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/rack/rack/security/advisories/GHSA-h2jq-g4cq-5ppq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34785"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/rack/rack"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-34785.yml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-187", "CWE-200" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-02T18:44:25Z",
    "nvd_published_at" : "2026-04-02T17:16:24Z"
  }
}