Vulnerability-Lookup

CVE-2026-34835 (GCVE-0-2026-34835)

Vulnerability from cvelistv5 – Published: 2026-04-02 17:09 – Updated: 2026-04-02 17:44

Title

Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.

Summary

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, 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. This 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. This issue has been patched in versions 3.1.21 and 3.2.6.

Severity ?

4.8 (Medium)


                        
                          CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

CWE

CWE-1286 - Improper Validation of Syntactic Correctness of Input

Assigner

GitHub_M

References

URL

Tags

https://github.com/rack/rack/security/advisories/…

x_refsource_CONFIRM

Impacted products

	Vendor	Product	Version
	rack	rack	Affected: >= 3.0.0.beta1, < 3.1.21 Affected: >= 3.2.0, < 3.2.6

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-34835

Vulnerability from fkie_nvd - Published: 2026-04-02 18:16 - Updated: 2026-04-03 19:32

Severity ?

4.8 (Medium) - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
6.5 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5	Vendor Advisory, Exploit, Mitigation

Impacted products

	Vendor	Product	Version
	rack	rack	*
	rack	rack	*

JSON

To clipboard

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    {
      "nodes": [
        {
          "cpeMatch": [
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              "criteria": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*",
              "matchCriteriaId": "6948AAA6-873D-46BA-AA22-4C81138128E1",
              "versionEndExcluding": "3.1.21",
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    }
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  "lastModified": "2026-04-03T19:32:26.663",
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GHSA-G2PF-XV49-M2H5

Vulnerability from github – Published: 2026-04-02 20:36 – Updated: 2026-04-02 20:36

Summary

Rack::Request accepts invalid Host characters, enabling host allowlist bypass

Details

Summary

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.

For 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.

This 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.

Details

Rack::Request parses the authority component using logic equivalent to:

AUTHORITY = /
  \A
  (?<host>
    \[(?<address>#{ipv6})\]
    |
    (?<address>[[[:graph:]&&[^\[\]]]]*?)
  )
  (:(?<port>\d+))?
  \z
/x

The 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.

As a result, values such as the following are accepted and returned through req.host:

myapp.com@evil.com
evil.com/myapp.com
evil.com#myapp.com

Applications that attempt to allowlist hosts using string prefix or suffix checks may therefore treat attacker-controlled hosts as trusted. For example:

req.host.start_with?("myapp.com")

accepts:

myapp.com@evil.com

and:

req.host.end_with?("myapp.com")

accepts:

evil.com/myapp.com

When those values are later used to build absolute URLs or enforce origin restrictions, the application may produce attacker-controlled results.

Impact

Applications 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.

In 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.

The practical impact depends on application behavior. If the application or reverse proxy already enforces strict host validation, exploitability may be reduced or eliminated.

Mitigation

Update to a patched version of Rack that rejects invalid authority characters in Host.
Enforce strict Host header validation at the reverse proxy or load balancer.
Do not rely on prefix or suffix string checks such as start_with? or end_with? for host allowlisting.
Use exact host allowlists, or exact subdomain boundary checks, after validating that the host is syntactically valid.

Severity ?

4.8 (Medium)


                  
                    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0.beta1"
            },
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            }
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        "ecosystem": "RubyGems",
        "name": "rack"
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              "fixed": "3.2.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-34835"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1286"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-02T20:36:40Z",
    "nvd_published_at": "2026-04-02T18:16:33Z",
    "severity": "MODERATE"
  },
  "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  (?\u003chost\u003e\n    \\[(?\u003caddress\u003e#{ipv6})\\]\n    |\n    (?\u003caddress\u003e[[[:graph:]\u0026\u0026[^\\[\\]]]]*?)\n  )\n  (:(?\u003cport\u003e\\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.",
  "id": "GHSA-g2pf-xv49-m2h5",
  "modified": "2026-04-02T20:36:40Z",
  "published": "2026-04-02T20:36:40Z",
  "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"
    }
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  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Rack::Request accepts invalid Host characters, enabling host allowlist bypass"
}

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-34835 (GCVE-0-2026-34835)

FKIE_CVE-2026-34835

GHSA-G2PF-XV49-M2H5

Summary

Details

Impact

Mitigation

Tags

Sightings

Nomenclature