CVE-2026-26961Network attack vector

WAF bypass via greedy multipart boundary parsing

Published Apr 2, 2026 · Updated Apr 2, 2026

Interpretation conflict in Rack allows remote attackers to bypass upstream multipart inspection with duplicate boundary parameters. Rack::Multipart::Parser uses a greedy regular expression for Content-Type and consequently selects the last boundary instead of rejecting duplicate parameters. Exploitation requires a proxy or WAF that uses the first boundary, allowing uninspected form fields or uploads to reach the application.

CVSS severity3.7
Low
EPSS probability0.25%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Interpretation conflict in Rack allows remote attackers to bypass upstream multipart inspection with duplicate boundary parameters. Rack::Multipart::Parser uses a greedy regular expression for Content-Type and consequently selects the last boundary instead of rejecting duplicate parameters. Exploitation requires a proxy or WAF that uses the first boundary, allowing uninspected form fields or uploads to reach the application.

The record

CVE
CVE-2026-26961
Published
Apr 2, 2026
Updated
Apr 2, 2026
Vendor
Rack
Product
Rack
Classifications
CWE-436, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 2, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 2, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.2.23
  2. Affected versionversion=>= 3.0.0.beta1, < 3.1.21
  3. Affected versionversion=>= 3.2.0, < 3.2.6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Rack · Rackversion=< 2.2.23; version=>= 3.0.0.beta1, < 3.1.21; version=>= 3.2.0, < 3.2.6

Published CVSS scores

3.7GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-436Interpretation Conflict
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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