CVE-2026-34835: Rack :Request Accepts Invalid Host Characters, Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-g2pf-xv49-m2h5",
  "modified" : "2026-05-13T16:19:21Z",
  "published" : "2026-04-02T20:36:40Z",
  "aliases" : [ "CVE-2026-34835" ],
  "summary" : "Rack::Request accepts invalid Host characters, enabling host allowlist bypass",
  "details" : "## Summary\n\n`Rack::Request` parses the `Host` header using an `AUTHORITY` regular expression that accepts characters not permitted in RFC-compliant hostnames, including `/`, `?`, `#`, and `@`. Because `req.host` returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed.\n\nFor example, a check such as `req.host.start_with?(\"myapp.com\")` can be bypassed with `Host: myapp.com@evil.com`, and a check such as `req.host.end_with?(\"myapp.com\")` can be bypassed with `Host: evil.com/myapp.com`.\n\nThis can lead to host header poisoning in applications that use `req.host`, `req.url`, or `req.base_url` for link generation, redirects, or origin validation.\n\n## Details\n\n`Rack::Request` parses the authority component using logic equivalent to:\n\n```ruby\nAUTHORITY = /\n  \\A\n  (?<host>\n    \\[(?<address>#{ipv6})\\]\n    |\n    (?<address>[[[:graph:]&&[^\\[\\]]]]*?)\n  )\n  (:(?<port>\\d+))?\n  \\z\n/x\n```\n\nThe character class used for non-IPv6 hosts accepts nearly all printable characters except `[` and `]`. This includes reserved URI delimiters such as `@`, `/`, `?`, and `#`, which are not valid hostname characters under RFC 3986 host syntax.\n\nAs a result, values such as the following are accepted and returned through `req.host`:\n\n```text\nmyapp.com@evil.com\nevil.com/myapp.com\nevil.com#myapp.com\n```\n\nApplications that attempt to allowlist hosts using string prefix or suffix checks may therefore treat attacker-controlled hosts as trusted. For example:\n\n```ruby\nreq.host.start_with?(\"myapp.com\")\n```\n\naccepts:\n\n```text\nmyapp.com@evil.com\n```\n\nand:\n\n```ruby\nreq.host.end_with?(\"myapp.com\")\n```\n\naccepts:\n\n```text\nevil.com/myapp.com\n```\n\nWhen those values are later used to build absolute URLs or enforce origin restrictions, the application may produce attacker-controlled results.\n\n## Impact\n\nApplications that rely on `req.host`, `req.url`, or `req.base_url` may be affected if they perform naive host validation or assume Rack only returns RFC-valid hostnames.\n\nIn affected deployments, an attacker may be able to bypass host allowlists and poison generated links, redirects, or origin-dependent security decisions. This can enable attacks such as password reset link poisoning or other host header injection issues.\n\nThe practical impact depends on application behavior. If the application or reverse proxy already enforces strict host validation, exploitability may be reduced or eliminated.\n\n## Mitigation\n\n* Update to a patched version of Rack that rejects invalid authority characters in `Host`.\n* Enforce strict `Host` header validation at the reverse proxy or load balancer.\n* Do not rely on prefix or suffix string checks such as `start_with?` or `end_with?` for host allowlisting.\n* Use exact host allowlists, or exact subdomain boundary checks, after validating that the host is syntactically valid.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "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-g2pf-xv49-m2h5"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34835"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/rack/rack"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-34835.yml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1286" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-02T20:36:40Z",
    "nvd_published_at" : "2026-04-02T18:16:33Z"
  }
}