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

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-1286 - Improper Validation of Syntactic Correctness of Input

Assigner

GitHub_M

References

1 reference

URL	Tags
https://github.com/rack/rack/security/advisories/…	x_refsource_CONFIRM

Impacted products

1 product

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

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-34835",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-04-02T17:43:54.517566Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-04-02T17:44:03.453Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "rack",
          "vendor": "rack",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 3.0.0.beta1, \u003c 3.1.21"
            },
            {
              "status": "affected",
              "version": "\u003e= 3.2.0, \u003c 3.2.6"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "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."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "HIGH",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 4.8,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "LOW",
            "integrityImpact": "LOW",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-1286",
              "description": "CWE-1286: Improper Validation of Syntactic Correctness of Input",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-04-02T17:09:07.047Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5"
        }
      ],
      "source": {
        "advisory": "GHSA-g2pf-xv49-m2h5",
        "discovery": "UNKNOWN"
      },
      "title": "Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass."
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-34835",
    "datePublished": "2026-04-02T17:09:07.047Z",
    "dateReserved": "2026-03-30T20:52:53.284Z",
    "dateUpdated": "2026-04-02T17:44:03.453Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-34835",
      "date": "2026-07-24",
      "epss": "0.00192",
      "percentile": "0.09151"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-34835\",\"sourceIdentifier\":\"security-advisories@github.com\",\"published\":\"2026-04-02T18:16:33.967\",\"lastModified\":\"2026-07-24T21:10:00.143\",\"vulnStatus\":\"Analyzed\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"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.\"},{\"lang\":\"es\",\"value\":\"Rack es una interfaz modular de servidor web Ruby. Desde las versiones 3.0.0.beta1 hasta antes de la 3.1.21, y de la 3.2.0 hasta antes de la 3.2.6, Rack::Request analiza el encabezado Host utilizando una expresi\u00f3n regular AUTHORITY que acepta caracteres no permitidos en nombres de host conformes a RFC, incluyendo /, ?, # y @. Debido a que req.host devuelve el valor analizado completo, las aplicaciones que validan hosts utilizando comprobaciones ingenuas de prefijo o sufijo pueden ser eludidas. Esto puede llevar a un envenenamiento del encabezado Host en aplicaciones que utilizan req.host, req.url o req.base_url para la generaci\u00f3n de enlaces, redirecciones o validaci\u00f3n de origen. Este problema ha sido parcheado en las versiones 3.1.21 y 3.2.6.\"}],\"affected\":[{\"source\":\"security-advisories@github.com\",\"affectedData\":[{\"vendor\":\"rack\",\"product\":\"rack\",\"versions\":[{\"version\":\"\u003e= 3.0.0.beta1, \u003c 3.1.21\",\"status\":\"affected\"},{\"version\":\"\u003e= 3.2.0, \u003c 3.2.6\",\"status\":\"affected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"security-advisories@github.com\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N\",\"baseScore\":4.8,\"baseSeverity\":\"MEDIUM\",\"attackVector\":\"NETWORK\",\"attackComplexity\":\"HIGH\",\"privilegesRequired\":\"NONE\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"LOW\",\"integrityImpact\":\"LOW\",\"availabilityImpact\":\"NONE\"},\"exploitabilityScore\":2.2,\"impactScore\":2.5},{\"source\":\"nvd@nist.gov\",\"type\":\"Primary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N\",\"baseScore\":6.5,\"baseSeverity\":\"MEDIUM\",\"attackVector\":\"NETWORK\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"NONE\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"LOW\",\"integrityImpact\":\"LOW\",\"availabilityImpact\":\"NONE\"},\"exploitabilityScore\":3.9,\"impactScore\":2.5}],\"ssvcV203\":[{\"source\":\"134c704f-9b21-4f2e-91b3-4a467353bcc0\",\"ssvcData\":{\"timestamp\":\"2026-04-02T17:43:54.517566Z\",\"id\":\"CVE-2026-34835\",\"options\":[{\"exploitation\":\"none\"},{\"automatable\":\"no\"},{\"technicalImpact\":\"partial\"}],\"role\":\"CISA Coordinator\",\"version\":\"2.0.3\"}}]},\"weaknesses\":[{\"source\":\"security-advisories@github.com\",\"type\":\"Secondary\",\"description\":[{\"lang\":\"en\",\"value\":\"CWE-1286\"}]}],\"configurations\":[{\"nodes\":[{\"operator\":\"OR\",\"negate\":false,\"cpeMatch\":[{\"vulnerable\":true,\"criteria\":\"cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*\",\"versionStartIncluding\":\"3.0.0\",\"versionEndExcluding\":\"3.1.21\",\"matchCriteriaId\":\"6948AAA6-873D-46BA-AA22-4C81138128E1\"},{\"vulnerable\":true,\"criteria\":\"cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*\",\"versionStartIncluding\":\"3.2.0\",\"versionEndExcluding\":\"3.2.6\",\"matchCriteriaId\":\"3FB592AD-E826-49BE-AC6D-E5F55FDCC96E\"}]}]}],\"references\":[{\"url\":\"https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5\",\"source\":\"security-advisories@github.com\",\"tags\":[\"Exploit\",\"Mitigation\",\"Vendor Advisory\"]}]}}",
    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-06-30T11:43:18+00:00",
      "cve": "CVE-2026-34835",
      "id": "CVE-2026-34835",
      "initial_release_date": "2026-04-02T17:09:07.047000+00:00",
      "product_status:known_affected": "50",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "rack: Rack: Host header poisoning due to malformed Host header bypasses validation",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34835.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "moderate",
      "current_release_date": "2026-07-03T03:17:43Z",
      "cve": "CVE-2026-34835",
      "id": "CVE-2026-34835",
      "initial_release_date": "2026-04-03T23:24:47Z",
      "product_status:known_not_affected": "89",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-34835",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-34835.json",
      "version": "3"
    },
    "vulnrichment": {
      "containers": "{\"adp\": [{\"title\": \"CISA ADP Vulnrichment\", \"metrics\": [{\"other\": {\"type\": \"ssvc\", \"content\": {\"id\": \"CVE-2026-34835\", \"role\": \"CISA Coordinator\", \"options\": [{\"Exploitation\": \"none\"}, {\"Automatable\": \"no\"}, {\"Technical Impact\": \"partial\"}], \"version\": \"2.0.3\", \"timestamp\": \"2026-04-02T17:43:54.517566Z\"}}}], \"providerMetadata\": {\"orgId\": \"134c704f-9b21-4f2e-91b3-4a467353bcc0\", \"shortName\": \"CISA-ADP\", \"dateUpdated\": \"2026-04-02T17:43:58.046Z\"}}], \"cna\": {\"title\": \"Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.\", \"source\": {\"advisory\": \"GHSA-g2pf-xv49-m2h5\", \"discovery\": \"UNKNOWN\"}, \"metrics\": [{\"cvssV3_1\": {\"scope\": \"UNCHANGED\", \"version\": \"3.1\", \"baseScore\": 4.8, \"attackVector\": \"NETWORK\", \"baseSeverity\": \"MEDIUM\", \"vectorString\": \"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N\", \"integrityImpact\": \"LOW\", \"userInteraction\": \"NONE\", \"attackComplexity\": \"HIGH\", \"availabilityImpact\": \"NONE\", \"privilegesRequired\": \"NONE\", \"confidentialityImpact\": \"LOW\"}}], \"affected\": [{\"vendor\": \"rack\", \"product\": \"rack\", \"versions\": [{\"status\": \"affected\", \"version\": \"\u003e= 3.0.0.beta1, \u003c 3.1.21\"}, {\"status\": \"affected\", \"version\": \"\u003e= 3.2.0, \u003c 3.2.6\"}]}], \"references\": [{\"url\": \"https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5\", \"name\": \"https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5\", \"tags\": [\"x_refsource_CONFIRM\"]}], \"descriptions\": [{\"lang\": \"en\", \"value\": \"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.\"}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-1286\", \"description\": \"CWE-1286: Improper Validation of Syntactic Correctness of Input\"}]}], \"providerMetadata\": {\"orgId\": \"a0819718-46f1-4df5-94e2-005712e83aaa\", \"shortName\": \"GitHub_M\", \"dateUpdated\": \"2026-04-02T17:09:07.047Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-34835\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-04-02T17:44:03.453Z\", \"dateReserved\": \"2026-03-30T20:52:53.284Z\", \"assignerOrgId\": \"a0819718-46f1-4df5-94e2-005712e83aaa\", \"datePublished\": \"2026-04-02T17:09:07.047Z\", \"assignerShortName\": \"GitHub_M\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

FKIE_CVE-2026-34835

Vulnerability from fkie_nvd - Published: 2026-04-02 18:16 - Updated: 2026-06-17 10:39

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	Exploit, Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	rack	rack	*
	rack	rack	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "rack",
          "vendor": "rack",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 3.0.0.beta1, \u003c 3.1.21"
            },
            {
              "status": "affected",
              "version": "\u003e= 3.2.0, \u003c 3.2.6"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*",
              "matchCriteriaId": "6948AAA6-873D-46BA-AA22-4C81138128E1",
              "versionEndExcluding": "3.1.21",
              "versionStartIncluding": "3.0.0",
              "vulnerable": true
            },
            {
              "criteria": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*",
              "matchCriteriaId": "3FB592AD-E826-49BE-AC6D-E5F55FDCC96E",
              "versionEndExcluding": "3.2.6",
              "versionStartIncluding": "3.2.0",
              "vulnerable": true
            }
          ],
          "negate": false,
          "operator": "OR"
        }
      ]
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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."
    }
  ],
  "id": "CVE-2026-34835",
  "lastModified": "2026-06-17T10:39:43.007",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "HIGH",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 4.8,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.2,
        "impactScore": 2.5,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      },
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 6.5,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 2.5,
        "source": "nvd@nist.gov",
        "type": "Primary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-34835",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-04-02T17:43:54.517566Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-04-02T18:16:33.967",
  "references": [
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Exploit",
        "Mitigation",
        "Vendor Advisory"
      ],
      "url": "https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-1286"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-G2PF-XV49-M2H5

Vulnerability from github – Published: 2026-04-02 20:36 – Updated: 2026-05-13 16:19

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"
            },
            {
              "fixed": "3.1.21"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.2.0"
            },
            {
              "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": "2026-05-13T16:19:21Z",
  "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"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-34835.yml"
    }
  ],
  "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"
}

ubuntu-cve-2026-34835

Vulnerability from osv_ubuntu

Published

2026-04-02 18:16

Modified

2026-05-20 15:27

Summary

Details

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

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-34835	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-34835	REPORT
	https://github.com/rack/rack/security/advisories/…	REPORT
	https://ubuntu.com/security/notices/USN-8182-1	ADVISORY

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "availability": "No subscription required",
        "binaries": [
          {
            "binary_name": "ruby-rack",
            "binary_version": "3.1.16-0.1ubuntu0.3"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:25.10",
        "name": "ruby-rack",
        "purl": "pkg:deb/ubuntu/ruby-rack@3.1.16-0.1ubuntu0.3?arch=source\u0026distro=questing"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.1.16-0.1ubuntu0.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.2.7-1.1",
        "3.1.16-0.1",
        "3.1.16-0.1ubuntu0.1",
        "3.1.16-0.1ubuntu0.2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "ruby-rack",
            "binary_version": "3.2.4-1ubuntu1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:26.04:LTS",
        "name": "ruby-rack",
        "purl": "pkg:deb/ubuntu/ruby-rack@3.2.4-1ubuntu1?arch=source\u0026distro=resolute"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.1.16-0.1",
        "3.1.18-1",
        "3.1.18-1build1",
        "3.2.4-1ubuntu1"
      ]
    }
  ],
  "aliases": [],
  "details": "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.",
  "id": "UBUNTU-CVE-2026-34835",
  "modified": "2026-05-20T15:27:24Z",
  "published": "2026-04-02T18:16:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-34835"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-34835"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5"
    },
    {
      "type": "ADVISORY",
      "url": "https://ubuntu.com/security/notices/USN-8182-1"
    }
  ],
  "related": [
    "USN-8182-1"
  ],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-34835"
  ]
}

Sightings

Author	Source	Type	Date	Other

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.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

Action not permitted

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

FKIE_CVE-2026-34835

GHSA-G2PF-XV49-M2H5

Summary

Details

Impact

Mitigation

ubuntu-cve-2026-34835

Vulnerability from osv_ubuntu

Tags

Sightings

Nomenclature