TI-2026-21446 Aliases CVE-2026-26961 GHSA-vgpv-f759-9wx3

Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass

LOW 24CWE-436Published 2026-04-02 · Updated 2026-04-03

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Impact type: —
Confidentiality · integrity · availability: None · Low · None
Fix status: Fixed

Attack profile from CVSS 3.1 · NVD. Fix status from CNA and GitHub.

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00262 18th percentileScored 2026-11-07

Fix Fixed rack (rack): 2.2.23, 3.1.21, 3.2.6rack (RubyGems): 2.2.23, 3.1.21, 3.2.6

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
rack	rack	before 2.2.23; 3.0.0.beta1 to before 3.1.21; 3.2.0 to before 3.2.6	2.2.23, 3.1.21, 3.2.6	Patch available	CNA
—	rack (RubyGems)	before 2.2.23	2.2.23	Patch available	GHSA-vgpv-f759-9wx3
—	rack (RubyGems)	3.0.0.beta1 to before 3.1.21	3.1.21	Patch available	GHSA-vgpv-f759-9wx3
—	rack (RubyGems)	3.2.0 to before 3.2.6	3.2.6	Patch available	GHSA-vgpv-f759-9wx3

rack Vendorrack Affected before 2.2.23; 3.0.0.beta1 to before 3.1.21; 3.2.0 to before 3.2.6 Fixed in 2.2.23, 3.1.21, 3.2.6 Patch available CNA
rack (RubyGems) Vendor— Affected before 2.2.23 Fixed in 2.2.23 Patch available GHSA-vgpv-f759-9wx3
rack (RubyGems) Vendor— Affected 3.0.0.beta1 to before 3.1.21 Fixed in 3.1.21 Patch available GHSA-vgpv-f759-9wx3
rack (RubyGems) Vendor— Affected 3.2.0 to before 3.2.6 Fixed in 3.2.6 Patch available GHSA-vgpv-f759-9wx3

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:rack:rack::::::ruby::*	before 2.2.23; 3.0.0 to before 3.1.21; 3.2.0 to before 3.2.6	Vulnerable	rack

cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:* Versions before 2.2.23; 3.0.0 to before 3.1.21; 3.2.0 to before 3.2.6 Vulnerable ·rack

Remediation

Fix · CNA rack (rack) Fixed in 2.2.23, 3.1.21, 3.2.6
Fix · GHSA-vgpv-f759-9wx3 rack (RubyGems) Fixed in 2.2.23 Advisory →
Fix · GHSA-vgpv-f759-9wx3 rack (RubyGems) Fixed in 3.1.21 Advisory →
Fix · GHSA-vgpv-f759-9wx3 rack (RubyGems) Fixed in 3.2.6 Advisory →

Vendor assessments

Red Hat

Red Hat rating: Low. Red Hat's page for this CVE →

Not affected: 1 product

Red Hat Satellite 6

Timeline

2026-10-02 Threatist record created from CVE-2026-26961
2026-10-03 Alias added: GHSA-vgpv-f759-9wx3

References

Advisory 2

Other 2

TI-2024-16580 Rack ReDos in content type parsing (2nd degree polynomial) CVSS 3.1HIGH 7.5
TI-2024-17236 Possible Denial of Service Vulnerability in Rack Header Parsing CVSS 3.1HIGH 7.5
TI-2024-17231 Possible DoS Vulnerability with Range Header in Rack CVSS 3.1HIGH 7.5
TI-2023-12254 Uncontrolled resource consumption in https://github.com/rack/rack CVSS 3.1HIGH 7.5
TI-2025-38165 Rack's multipart parser buffers unbounded per-part headers, enabling DoS (memory exhaustion) CVSS 3.1HIGH 7.5
TI-2025-38163 Rack's unbounded multipart preamble buffering enables DoS (memory exhaustion) CVSS 3.1HIGH 7.5

See all 36 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences