Vulnerability-Lookup

CVE-2026-26961 (GCVE-0-2026-26961)

Vulnerability from cvelistv5 – Published: 2026-04-02 16:42 – Updated: 2026-04-03 17:58

Title

Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass

Summary

Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

Severity ?

3.7 (Low)


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

CWE

CWE-436 - Interpretation Conflict

Assigner

GitHub_M

References

URL

Tags

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

x_refsource_CONFIRM

Impacted products

	Vendor	Product	Version
	rack	rack	Affected: < 2.2.23 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-2026-26961",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-04-03T17:57:50.290547Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-04-03T17:58:12.149Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "rack",
          "vendor": "rack",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 2.2.23"
            },
            {
              "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. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "HIGH",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 3.7,
            "baseSeverity": "LOW",
            "confidentialityImpact": "NONE",
            "integrityImpact": "LOW",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-436",
              "description": "CWE-436: Interpretation Conflict",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-04-02T16:42:16.766Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3"
        }
      ],
      "source": {
        "advisory": "GHSA-vgpv-f759-9wx3",
        "discovery": "UNKNOWN"
      },
      "title": "Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-26961",
    "datePublished": "2026-04-02T16:42:16.766Z",
    "dateReserved": "2026-02-16T22:20:28.611Z",
    "dateUpdated": "2026-04-03T17:58:12.149Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-26961\",\"sourceIdentifier\":\"security-advisories@github.com\",\"published\":\"2026-04-02T17:16:21.973\",\"lastModified\":\"2026-04-03T16:10:23.730\",\"vulnStatus\":\"Undergoing Analysis\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.\"}],\"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:N/I:L/A:N\",\"baseScore\":3.7,\"baseSeverity\":\"LOW\",\"attackVector\":\"NETWORK\",\"attackComplexity\":\"HIGH\",\"privilegesRequired\":\"NONE\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"NONE\",\"integrityImpact\":\"LOW\",\"availabilityImpact\":\"NONE\"},\"exploitabilityScore\":2.2,\"impactScore\":1.4}]},\"weaknesses\":[{\"source\":\"security-advisories@github.com\",\"type\":\"Primary\",\"description\":[{\"lang\":\"en\",\"value\":\"CWE-436\"}]}],\"references\":[{\"url\":\"https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3\",\"source\":\"security-advisories@github.com\"}]}}",
    "vulnrichment": {
      "containers": "{\"cna\": {\"title\": \"Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass\", \"problemTypes\": [{\"descriptions\": [{\"cweId\": \"CWE-436\", \"lang\": \"en\", \"description\": \"CWE-436: Interpretation Conflict\", \"type\": \"CWE\"}]}], \"metrics\": [{\"cvssV3_1\": {\"attackComplexity\": \"HIGH\", \"attackVector\": \"NETWORK\", \"availabilityImpact\": \"NONE\", \"baseScore\": 3.7, \"baseSeverity\": \"LOW\", \"confidentialityImpact\": \"NONE\", \"integrityImpact\": \"LOW\", \"privilegesRequired\": \"NONE\", \"scope\": \"UNCHANGED\", \"userInteraction\": \"NONE\", \"vectorString\": \"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N\", \"version\": \"3.1\"}}], \"references\": [{\"name\": \"https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3\", \"tags\": [\"x_refsource_CONFIRM\"], \"url\": \"https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3\"}], \"affected\": [{\"vendor\": \"rack\", \"product\": \"rack\", \"versions\": [{\"version\": \"\u003c 2.2.23\", \"status\": \"affected\"}, {\"version\": \"\u003e= 3.0.0.beta1, \u003c 3.1.21\", \"status\": \"affected\"}, {\"version\": \"\u003e= 3.2.0, \u003c 3.2.6\", \"status\": \"affected\"}]}], \"providerMetadata\": {\"orgId\": \"a0819718-46f1-4df5-94e2-005712e83aaa\", \"shortName\": \"GitHub_M\", \"dateUpdated\": \"2026-04-02T16:42:16.766Z\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.\"}], \"source\": {\"advisory\": \"GHSA-vgpv-f759-9wx3\", \"discovery\": \"UNKNOWN\"}}, \"adp\": [{\"title\": \"CISA ADP Vulnrichment\", \"metrics\": [{\"other\": {\"type\": \"ssvc\", \"content\": {\"id\": \"CVE-2026-26961\", \"role\": \"CISA Coordinator\", \"options\": [{\"Exploitation\": \"none\"}, {\"Automatable\": \"no\"}, {\"Technical Impact\": \"partial\"}], \"version\": \"2.0.3\", \"timestamp\": \"2026-04-03T17:57:50.290547Z\"}}}], \"providerMetadata\": {\"orgId\": \"134c704f-9b21-4f2e-91b3-4a467353bcc0\", \"shortName\": \"CISA-ADP\", \"dateUpdated\": \"2026-04-03T17:57:59.363Z\"}}]}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-26961\", \"assignerOrgId\": \"a0819718-46f1-4df5-94e2-005712e83aaa\", \"state\": \"PUBLISHED\", \"assignerShortName\": \"GitHub_M\", \"dateReserved\": \"2026-02-16T22:20:28.611Z\", \"datePublished\": \"2026-04-02T16:42:16.766Z\", \"dateUpdated\": \"2026-04-03T17:58:12.149Z\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

GHSA-VGPV-F759-9WX3

Vulnerability from github – Published: 2026-04-02 20:30 – Updated: 2026-04-02 20:30

Summary

Rack's greedy multipart boundary parsing can cause parser differentials and WAF bypass.

Details

Summary

Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first.

In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated.

Details

Rack identifies the multipart boundary using logic equivalent to:

MULTIPART = %r|\Amultipart/.*boundary=\"?([^\";,]+)\"?|ni

Because the expression is greedy, it matches the last boundary= parameter in a header such as:

Content-Type: multipart/form-data; boundary=safe; boundary=malicious

As a result, Rack parses the request body using malicious, while another component may interpret the same header using safe.

This creates an interpretation conflict. If an upstream WAF or proxy inspects multipart parts using the first boundary and Rack later parses the body using the last boundary, a client may be able to place malicious form fields or uploaded content in parts that Rack accepts but the upstream component did not inspect as intended.

This issue is most relevant in layered deployments where security decisions are made before the request reaches Rack.

Impact

Applications that accept multipart/form-data uploads behind an inspecting proxy or WAF may be affected.

In such deployments, an attacker may be able to bypass upstream filtering of uploaded files or form fields by sending a request with multiple boundary parameters and relying on the intermediary and Rack to parse the request differently.

The practical impact depends on deployment architecture. If no upstream component relies on a different multipart interpretation, this behavior may not provide meaningful additional attacker capability.

Mitigation

Update to a patched version of Rack that rejects ambiguous multipart Content-Type headers or parses duplicate boundary parameters consistently.
Reject requests containing multiple boundary parameters.
Normalize or regenerate multipart metadata at the trusted edge before forwarding requests to Rack.
Avoid relying on upstream inspection of malformed multipart requests unless duplicate parameter handling is explicitly consistent across components.

Severity ?

3.7 (Low)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.2.23"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "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-2026-26961"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-436"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-02T20:30:40Z",
    "nvd_published_at": "2026-04-02T17:16:21Z",
    "severity": "LOW"
  },
  "details": "## Summary\n\n`Rack::Multipart::Parser` extracts the `boundary` parameter from `multipart/form-data` using a greedy regular expression. When a `Content-Type` header contains multiple `boundary` parameters, Rack selects the last one rather than the first.\n\nIn deployments where an upstream proxy, WAF, or intermediary interprets the first `boundary` parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated.\n\n## Details\n\nRack identifies the multipart boundary using logic equivalent to:\n\n```ruby\nMULTIPART = %r|\\Amultipart/.*boundary=\\\"?([^\\\";,]+)\\\"?|ni\n```\n\nBecause the expression is greedy, it matches the last `boundary=` parameter in a header such as:\n\n```http\nContent-Type: multipart/form-data; boundary=safe; boundary=malicious\n```\n\nAs a result, Rack parses the request body using `malicious`, while another component may interpret the same header using `safe`.\n\nThis creates an interpretation conflict. If an upstream WAF or proxy inspects multipart parts using the first boundary and Rack later parses the body using the last boundary, a client may be able to place malicious form fields or uploaded content in parts that Rack accepts but the upstream component did not inspect as intended.\n\nThis issue is most relevant in layered deployments where security decisions are made before the request reaches Rack.\n\n## Impact\n\nApplications that accept `multipart/form-data` uploads behind an inspecting proxy or WAF may be affected.\n\nIn such deployments, an attacker may be able to bypass upstream filtering of uploaded files or form fields by sending a request with multiple `boundary` parameters and relying on the intermediary and Rack to parse the request differently.\n\nThe practical impact depends on deployment architecture. If no upstream component relies on a different multipart interpretation, this behavior may not provide meaningful additional attacker capability.\n\n## Mitigation\n\n* Update to a patched version of Rack that rejects ambiguous multipart `Content-Type` headers or parses duplicate `boundary` parameters consistently.\n* Reject requests containing multiple `boundary` parameters.\n* Normalize or regenerate multipart metadata at the trusted edge before forwarding requests to Rack.\n* Avoid relying on upstream inspection of malformed multipart requests unless duplicate parameter handling is explicitly consistent across components.",
  "id": "GHSA-vgpv-f759-9wx3",
  "modified": "2026-04-02T20:30:40Z",
  "published": "2026-04-02T20:30:40Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-26961"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/rack/rack"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Rack\u0027s greedy multipart boundary parsing can cause parser differentials and WAF bypass."
}

FKIE_CVE-2026-26961

Vulnerability from fkie_nvd - Published: 2026-04-02 17:16 - Updated: 2026-04-03 16:10

Severity ?

3.7 (Low) - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6."
    }
  ],
  "id": "CVE-2026-26961",
  "lastModified": "2026-04-03T16:10:23.730",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "HIGH",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 3.7,
          "baseSeverity": "LOW",
          "confidentialityImpact": "NONE",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.2,
        "impactScore": 1.4,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-04-02T17:16:21.973",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Undergoing Analysis",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-436"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

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-2026-26961 (GCVE-0-2026-26961)

GHSA-VGPV-F759-9WX3

Summary

Details

Impact

Mitigation

FKIE_CVE-2026-26961

Tags

Sightings

Nomenclature