ruby-advisory-db/gems/rack/CVE-2026-26961.yml at 57d27b1ffd7182be75ff7a6c8834b20533c988bb · rubysec/ruby-advisory-db · GitHub

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---
gem: rack
cve: 2026-26961
ghsa: vgpv-f759-9wx3
url: https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3
title: Rack's greedy multipart boundary parsing can cause parser
  differentials and WAF bypass.
date: 2026-04-02
description: |
  ## Summary

  `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.

  ## Details

  Rack identifies the multipart boundary using logic equivalent to:

  ```ruby
  MULTIPART = %r|\Amultipart/.*boundary=\"?([^\";,]+)\"?|ni
  ```

  Because the expression is greedy, it matches the last `boundary=`
  parameter in a header such as:

  ```http
  Content-Type: multipart/form-data; boundary=safe; boundary=malicious
  ```

  As a result, Rack parses the request body using `malicious`, while
  another component may interpret the same header using `safe`.

  This creates an interpretation conflict. If an upstream WAF or proxy
  inspects multipart parts using the first boundary and Rack later
  parses the body using the last boundary, a client may be able to
  place malicious form fields or uploaded content in parts that Rack
  accepts but the upstream component did not inspect as intended.

  This issue is most relevant in layered deployments where security
  decisions are made before the request reaches Rack.

  ## Impact

  Applications that accept `multipart/form-data` uploads behind an
  inspecting proxy or WAF may be affected.

  In such deployments, an attacker may be able to bypass upstream
  filtering of uploaded files or form fields by sending a request
  with multiple `boundary` parameters and relying on the intermediary
  and Rack to parse the request differently.

  The practical impact depends on deployment architecture. If no
  upstream component relies on a different multipart interpretation,
  this behavior may not provide meaningful additional attacker capability.

  ## Mitigation

  * Update to a patched version of Rack that rejects ambiguous multipart
    `Content-Type` headers or parses duplicate `boundary` parameters
    consistently.
  * Reject requests containing multiple `boundary` parameters.
  * Normalize or regenerate multipart metadata at the trusted edge
    before forwarding requests to Rack.
  * Avoid relying on upstream inspection of malformed multipart
    requests unless duplicate parameter handling is explicitly
    consistent across components.
cvss_v3: 3.7
patched_versions:
  - "~> 2.2.23"
  - "~> 3.1.21"
  - ">= 3.2.6"
related:
  url:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-26961
    - https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3
    - https://github.com/advisories/GHSA-vgpv-f759-9wx3