Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass
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.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-26961
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 4 Nov 2026, 03:05 UTC · Revision 1
Source evidence
Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass
core-osv · Original sourcePublished 2 Apr 2026, 16:42 UTC
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.
osv:CVE-2026-26961
Complete source record
{
"id": "4fa85979f90250c8a118c674e64ea3ac3c36237f8b52690cc57b54a3cddc34aa",
"url": "https://osv.dev/vulnerability/CVE-2026-26961",
"method": "structured",
"payload": {
"kind": "advisory",
"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.",
"evidence": "osv:CVE-2026-26961",
"products": [],
"severity": "low",
"structured": {
"id": "CVE-2026-26961",
"aliases": [
"CVE-2026-26961",
"GHSA-vgpv-f759-9wx3"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/rack/rack",
"type": "GIT",
"events": [
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{
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{
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"source": "CPE_RANGE"
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"v2.2.5",
"2.2.4",
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"2.2.0",
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"2.0.0.rc1",
"2.0.0.alpha",
"1.6.0.beta",
"1.5.1",
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"1.4.1",
"1.4.0",
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"1.3.0.beta2",
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],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-26961.json"
}
}
],
"modified": "2026-10-28T02:30:33.033247875Z",
"provider": "osv",
"references": [
"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/26xxx/CVE-2026-26961.json",
"https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3",
"https://nvd.nist.gov/vuln/detail/CVE-2026-26961"
],
"provider_metadata": {
"cwe_ids": [
"CWE-436"
],
"cna_assigner": "GitHub_M",
"osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/26xxx/CVE-2026-26961.json"
}
},
"related_cves": [
{
"value": "CVE-2026-26961",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 499867,
"published_at": "2026-04-02T16:42:16.766+00:00"
}