{ "document": { "category": "csaf_base", "csaf_version": "2.0", "distribution": { "tlp": { "label": "WHITE" } }, "lang": "en", "notes": [ { "category": "legal_disclaimer", "text": "The Netherlands Cyber Security Center (henceforth: NCSC-NL) maintains this portal to enhance access to its information and vulnerabilities. The use of this information is subject to the following terms and conditions:\n\nThe vulnerabilities disclosed in this portal are gathered by NCSC-NL from a variety of open sources, which the user can retrieve from other platforms. NCSC-NL makes every reasonable effort to ensure that the content of this portal is kept up to date, and that it is accurate and complete. Nevertheless, NCSC-NL cannot entirely rule out the possibility of errors, and therefore cannot give any warranty in respect of its completeness, accuracy or real-time keeping up-to-date. NCSC-NL does not control nor guarantee the accuracy, relevance, timeliness or completeness of information obtained from these external sources. The vulnerabilities disclosed in this portal are intended solely for the convenience of professional parties to take appropriate measures to manage the risks posed to the cybersecurity. No rights can be derived from the information provided therein.\n\nNCSC-NL and the Kingdom of the Netherlands assume no legal liability or responsibility for any damage resulting from either the use or inability of use of the vulnerabilities disclosed in this portal. This includes damage resulting from the inaccuracy of incompleteness of the information contained in it.\nThe information on this page is subject to Dutch law. All disputes related to or arising from the use of this portal regarding the disclosure of vulnerabilities will be submitted to the competent court in The Hague. This choice of means also applies to the court in summary proceedings." } ], "publisher": { "category": "coordinator", "contact_details": "cert@ncsc.nl", "name": "National Cyber Security Centre", "namespace": "https://www.ncsc.nl/" }, "title": "CVE-2026-26961", "tracking": { "current_release_date": "2026-04-03T15:45:08.033607Z", "generator": { "date": "2026-02-17T15:00:00Z", "engine": { "name": "V.E.L.M.A", "version": "1.7" } }, "id": "CVE-2026-26961", "initial_release_date": "2026-04-02T17:28:29.395806Z", "revision_history": [ { "date": "2026-04-02T17:28:29.395806Z", "number": "1", "summary": "CVE created.| Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| References created (1).| CWES updated (1)." }, { "date": "2026-04-02T17:28:31.620665Z", "number": "2", "summary": "NCSC Score created." }, { "date": "2026-04-02T17:39:38.026382Z", "number": "3", "summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| Products connected (3).| References created (1).| CWES updated (1)." }, { "date": "2026-04-02T17:39:39.861122Z", "number": "4", "summary": "NCSC Score updated." }, { "date": "2026-04-02T20:52:58.877248Z", "number": "5", "summary": "Source created.| CVE status created. (valid)| Description created for source.| CVSS created.| References created (3).| CWES updated (1)." }, { "date": "2026-04-02T20:53:02.103467Z", "number": "6", "summary": "NCSC Score updated." }, { "date": "2026-04-03T00:44:09.575643Z", "number": "7", "summary": "Source created.| CVE status created. (valid)| Products connected (2)." }, { "date": "2026-04-03T00:44:16.705828Z", "number": "8", "summary": "NCSC Score updated." }, { "date": "2026-04-03T06:44:06.520183Z", "number": "9", "summary": "Description created for source." }, { "date": "2026-04-03T15:44:57.329536Z", "number": "10", "summary": "Source connected.| CVE status created. (valid)| EPSS created." }, { "date": "2026-04-03T15:45:05.719060Z", "number": "11", "summary": "NCSC Score updated." } ], "status": "interim", "version": "11" } }, "product_tree": { "branches": [ { "branches": [ { "branches": [ { "category": "product_version_range", "name": "vers:unknown/<2.2.23", "product": { "name": "vers:unknown/<2.2.23", "product_id": "CSAFPID-5985093" } }, { "category": "product_version_range", "name": "vers:unknown/>=3.0.0.beta1|<3.1.21", "product": { "name": "vers:unknown/>=3.0.0.beta1|<3.1.21", "product_id": "CSAFPID-5985094" } }, { "category": "product_version_range", "name": "vers:unknown/>=3.2.0|<3.2.6", "product": { "name": "vers:unknown/>=3.2.0|<3.2.6", "product_id": "CSAFPID-5985095" } } ], "category": "product_name", "name": "rack" } ], "category": "vendor", "name": "rack" }, { "branches": [ { "branches": [ { "branches": [ { "category": "product_version_range", "name": "vers:deb/unknown", "product": { "name": "vers:deb/unknown", "product_id": "CSAFPID-2065710" } } ], "category": "product_name", "name": "ruby-rack" } ], "category": "product_family", "name": "bookworm" }, { "branches": [ { "branches": [ { "category": "product_version_range", "name": "vers:deb/unknown", "product": { "name": "vers:deb/unknown", "product_id": "CSAFPID-2065711" } } ], "category": "product_name", "name": "ruby-rack" } ], "category": "product_family", "name": "bullseye" } ], "category": "vendor", "name": "Debian" } ] }, "vulnerabilities": [ { "cve": "CVE-2026-26961", "cwe": { "id": "CWE-436", "name": "Interpretation Conflict" }, "notes": [ { "category": "description", "text": "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.", "title": "nvd - https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-26961" }, { "category": "description", "text": "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.", "title": "cveprojectv5 - https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/26xxx/CVE-2026-26961.json" }, { "category": "description", "text": "## 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.", "title": "github - https://api.github.com/advisories/GHSA-vgpv-f759-9wx3" }, { "category": "description", "text": "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.", "title": "debian - https://security-tracker.debian.org/tracker/CVE-2026-26961" }, { "category": "other", "text": "0.00029", "title": "EPSS" }, { "category": "other", "text": "3.8", "title": "NCSC Score" }, { "category": "other", "text": "The value of the most recent CVSS (V3) score, Is related to (a version of) an uncommon product, There is cwe data available from source Nvd, The value of the most recent EPSS score", "title": "NCSC Score top decreasing factors" } ], "product_status": { "known_affected": [ "CSAFPID-5985093", "CSAFPID-5985094", "CSAFPID-5985095", "CSAFPID-2065710", "CSAFPID-2065711" ] }, "references": [ { "category": "external", "summary": "Source - nvd", "url": "https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-26961" }, { "category": "external", "summary": "Source - cveprojectv5", "url": "https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/26xxx/CVE-2026-26961.json" }, { "category": "external", "summary": "Source - github", "url": "https://api.github.com/advisories/GHSA-vgpv-f759-9wx3" }, { "category": "external", "summary": "Source - debian", "url": "https://security-tracker.debian.org/tracker/CVE-2026-26961" }, { "category": "external", "summary": "Source - first", "url": "https://api.first.org/data/v1/epss?limit=10000&offset=0" }, { "category": "external", "summary": "Reference - cveprojectv5; github; nvd", "url": "https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3" }, { "category": "external", "summary": "Reference - github", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-26961" }, { "category": "external", "summary": "Reference - github", "url": "https://github.com/advisories/GHSA-vgpv-f759-9wx3" } ], "scores": [ { "cvss_v3": { "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" }, "products": [ "CSAFPID-2065710", "CSAFPID-2065711", "CSAFPID-5985093", "CSAFPID-5985094", "CSAFPID-5985095" ] } ], "title": "CVE-2026-26961" } ] }