Rack: Multipart boundary ambiguity guard in Rack::Multipart::Parser can be bypassed with a quoted boundary value, allowing WAF bypass
Severity and exploitation
KEV coverage does not rule out exploitation.
Assessment vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:NAssessment vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:NSeverity
low3.7CVSS measures technical severity, not the likelihood of an attack.
Known exploitation
Not listed in the checked CISA KEV catalog
Absence from CISA KEV does not rule out attacks.
Public exploit
Not established by this record
Missing evidence does not mean no public exploit exists.
Remediation
Package fixes recorded
Fix versions are recorded for 1 of 1 mapped packages. Check each release branch and any separate product guidance.
What this means
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.
Source excerpt · OSVAttack conditions
No attack-condition excerpt has been identified here. Review the full source description and references for authentication, network access, and configuration requirements.
Review supporting sources →Affected versions
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rack · GitHub_M
rack
3 ranges
rack · GitHub_M
rack
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affected
< 2.2.25
>= 3.0, < 3.1.23
>= 3.2, < 3.2.8
Other versions: unknown
Packages
Compare each installed version with the publisher’s affected ranges. Where a fix is recorded, choose the appropriate release branch and test compatibility before upgrading.
| Affected range | Recorded fix |
|---|---|
| Earlier releases before 2.2.25Ecosystem versions | 2.2.25 |
| From 3.0.0 before 3.1.23Ecosystem versions | 3.1.23 |
| From 3.2.0 before 3.2.8Ecosystem versions | 3.2.8 |
Version evidence: OSV · GHSA-p8x9-v4hv-2w4g
View original version evidence (179 explicit versions)
What to do
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Use the recorded fix for your release branch. Review the publisher’s instructions and test compatibility before upgrading.
cve.new guidance unless attributed to a publisher. This page does not inspect your installation.
Public exploit evidence
No public exploit reference is identified in the imported evidence. This is not a claim that no exploit exists.
Original source description
Read the full publisher description
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-Typecontains more than oneboundaryparameter at the reverse proxy.
References
10 sourcesInside the record
Evidence available
Describes the evidence in this view, not whether the product is safe.
Original records
CVE-2026-125108View the latest unmodified CVE JSON from the official source. It may be newer than this imported page.
cve.new JSON →Underlying weaknesses
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Contributing organizations · 2
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Timeline
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What changed
Follow changes- CVE · CVE-2026-125108
Observed 2026-11-13T18:02:48.220Z · upstream updated 2026-11-13T13:22:27.675Z
Prioritization updated
before
[]
after
[ { "source": "CISA-ADP", "type": "SSVC", "role": "CISA Coordinator", "timestamp": "2026-11-13T13:22:10.976638Z", "decisions": [ { "name": "Exploitation", "value": "none" }, { "name": "Automatable", "value": "no" }, { "name": "Technical Impact", "value": "partial" } ] } ]
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