GHSA-p8x9-v4hv-2w4g

Suggest an improvement
Source
https://github.com/advisories/GHSA-p8x9-v4hv-2w4g
Import Source
https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/11/GHSA-p8x9-v4hv-2w4g/GHSA-p8x9-v4hv-2w4g.json
JSON Data
https://api.osv.dev/v1/vulns/GHSA-p8x9-v4hv-2w4g
Aliases
Published
2026-11-10T19:26:28Z
Modified
2026-11-10T19:45:09Z
Severity
  • 3.7 (Low) CVSS_V3 - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N CVSS Calculator
Summary
Rack: Multipart boundary ambiguity guard in Rack::Multipart::Parser can be bypassed with a quoted boundary value, allowing WAF bypass
Details

Summary

The guard added for CVE-2026-26961 (GHSA-vgpv-f759-9wx3), which rejects multipart requests with more than one boundary parameter, only inspects the part of the Content-Type header after the boundary it extracts. A Content-Type whose first boundary parameter is empty or begins with a quote, semicolon, or comma is skipped by the extraction pattern, so a second boundary parameter is accepted silently and Rack parses the body with a boundary that a conforming parser would not derive from the header.

Details

Rack::Multipart::Parser.parse_boundary extracts the boundary with a regular expression that uses a lazy prefix and a value class excluding ", ;, and ,. For a header such as multipart/form-data; boundary=";boundary=AaB03x" or multipart/form-data; boundary=; boundary=AaB03x, the pattern skips the first boundary= and anchors on the second, and the subsequent check for another boundary= in the remaining text finds nothing. Rack then uses AaB03x.

RFC 9110 parses the first header as a single parameter whose value is ;boundary=AaB03x, which is not a legal boundary under RFC 2046, so the only conformant outcome is rejection. Inspecting intermediaries that implement the standard grammar, such as ModSecurity, see a different boundary or no multipart body at all.

The fix counts boundary= occurrences across the entire header value and rejects the request when there is more than one.

Impact

As with CVE-2026-26961, an attacker can craft a multipart request that a web application firewall or other inspecting proxy in front of a Rack application interprets differently from Rack, so form fields and file uploads reach the application without inspection. Rack itself grants no capability beyond that of a well-formed request with the same body. All supported versions, including 2.2, are affected.

Mitigation

  • Update to a patched version of Rack.
  • Reject requests whose Content-Type contains more than one boundary parameter at the reverse proxy.
Database specific 
{
    "cwe_ids": [
        "CWE-436"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-11-10T19:26:28Z",
    "nvd_published_at": "2026-11-09T22:18:41Z",
    "severity": "LOW"
}
References

Affected packages

RubyGems / rack

Package

Name
rack
Purl
pkg:gem/rack

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected
Fixed
2.2.25

Affected versions

0.*
0.1.0
0.2.0
0.3.0
0.4.0
0.9.0
0.9.1
1.*
1.0.0
1.0.1
1.1.0
1.1.1.pre
1.1.1
1.1.2
1.1.3
1.1.4
1.1.5
1.1.6
1.2.0
1.2.1
1.2.2
1.2.3
1.2.4
1.2.5
1.2.6
1.2.7
1.2.8
1.3.0.beta
1.3.0.beta2
1.3.0
1.3.1
1.3.2
1.3.3
1.3.4
1.3.5
1.3.6
1.3.7
1.3.8
1.3.9
1.3.10
1.4.0
1.4.1
1.4.2
1.4.3
1.4.4
1.4.5
1.4.6
1.4.7
1.5.0.beta.1
1.5.0.beta.2
1.5.0
1.5.1
1.5.2
1.5.3
1.5.4
1.5.5
1.6.0.beta
1.6.0.beta2
1.6.0
1.6.1
1.6.2
1.6.3
1.6.4
1.6.5
1.6.6
1.6.7
1.6.8
1.6.9
1.6.10
1.6.11
1.6.12
1.6.13
2.*
2.0.0.alpha
2.0.0.rc1
2.0.1
2.0.2
2.0.3
2.0.4
2.0.5
2.0.6
2.0.7
2.0.8
2.0.9
2.0.9.1
2.0.9.2
2.0.9.3
2.0.9.4
2.1.0
2.1.1
2.1.2
2.1.3
2.1.4
2.1.4.1
2.1.4.2
2.1.4.3
2.1.4.4
2.2.0
2.2.1
2.2.2
2.2.3
2.2.3.1
2.2.4
2.2.5
2.2.6
2.2.6.1
2.2.6.2
2.2.6.3
2.2.6.4
2.2.7
2.2.8
2.2.8.1
2.2.9
2.2.10
2.2.11
2.2.12
2.2.13
2.2.14
2.2.15
2.2.16
2.2.17
2.2.18
2.2.19
2.2.20
2.2.21
2.2.22
2.2.23
2.2.24

Database specific

source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/11/GHSA-p8x9-v4hv-2w4g/GHSA-p8x9-v4hv-2w4g.json"

RubyGems / rack

Package

Name
rack
Purl
pkg:gem/rack

Affected ranges

Type
ECOSYSTEM
Events
Introduced
3.0.0
Fixed
3.1.23

Affected versions

3.*
3.0.0
3.0.1
3.0.2
3.0.3
3.0.4
3.0.4.1
3.0.4.2
3.0.5
3.0.6
3.0.6.1
3.0.7
3.0.8
3.0.9
3.0.9.1
3.0.10
3.0.11
3.0.12
3.0.13
3.0.14
3.0.15
3.0.16
3.0.17
3.0.18
3.1.0
3.1.1
3.1.2
3.1.3
3.1.4
3.1.5
3.1.6
3.1.7
3.1.8
3.1.9
3.1.10
3.1.11
3.1.12
3.1.13
3.1.14
3.1.15
3.1.16
3.1.17
3.1.18
3.1.19
3.1.20
3.1.21
3.1.22

Database specific

source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/11/GHSA-p8x9-v4hv-2w4g/GHSA-p8x9-v4hv-2w4g.json"

RubyGems / rack

Package

Name
rack
Purl
pkg:gem/rack

Affected ranges

Type
ECOSYSTEM
Events
Introduced
3.2.0
Fixed
3.2.8

Affected versions

3.*
3.2.0
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
3.2.6
3.2.7

Database specific

source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/11/GHSA-p8x9-v4hv-2w4g/GHSA-p8x9-v4hv-2w4g.json"