Vulnerability · Published Apr 2, 2026 · Assigned by GitHub_M
CVE-2026-26961Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass
Low 3.7
In plain English
A low-severity security flaw in rack from rack. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 18% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | ruby-rack | 2.2.22-0+deb12u2 | DEBIAN-CVE-2026-26961 |
| Debian:13 | ruby-rack | 3.1.20-0+deb13u2 | DEBIAN-CVE-2026-26961 |
| Debian:14 | ruby-rack | 3.2.6-2 | DEBIAN-CVE-2026-26961 |
| RubyGems | rack | 2.2.23 | GHSA-vgpv-f759-9wx3 |
| RubyGems | rack | 3.1.21 | GHSA-vgpv-f759-9wx3 |
| RubyGems | rack | 3.2.6 | GHSA-vgpv-f759-9wx3 |
| Ubuntu:24.04:LTS | ruby-rack | 2.2.7-1ubuntu0.7 | UBUNTU-CVE-2026-26961 |
| Ubuntu:25.10 | ruby-rack | 3.1.16-0.1ubuntu0.3 | UBUNTU-CVE-2026-26961 |
| Ubuntu:26.04:LTS | ruby-rack | 3.2.4-1ubuntu1.1 | UBUNTU-CVE-2026-26961 |
| Ubuntu:26.04:LTS | ruby-rack | 3.2.4-1ubuntu1.1 | USN-8989-1 |
Affected products (as reported)
Weakness: CWE-436 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
References
- https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3 x_refsource_CONFIRM