CVE-2026-34829 — Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length

Affected: rack, Red Hat Logging Subsystem for Red Hat OpenShift, Red Hat 3scale API Management Platform 2, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9

Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser only wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit. For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space. This results in a denial of service condition for Rack applications that accept multipart form data. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

GHSA-8vqr-qjwx-82mw — Rack's multipart parsing without Content-Length header allows unbounded chunked file uploads

Affected: RubyGems: rack

## Summary

`Rack::Multipart::Parser` only wraps the request body in a `BoundedIO` when `CONTENT_LENGTH` is present. When a `multipart/form-data` request is sent without a `Content-Length` header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit.

For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space.

This results in a denial of service condition for Rack applications that accept multipart form data.

## Details

`Rack::Multipart::Parser.parse` applies `BoundedIO` only when `content_length` is not `nil`:

```ruby io = BoundedIO.new(io, content_length) if content_length ```

When `CONTENT_LENGTH` is absent, the parser reads the multipart body until EOF without a global byte limit.

Although Rack enforces `BUFFERED_UPLOAD_BYTESIZE_LIMIT` for retained non-file parts, file uploads are handled differently. When a multipart part includes a filename, the body is streamed to a `Tempfile`, and the retained-size accounting is not applied to that file content. As a result, file parts are not subject to the same upload size bound.

An attacker can exploit this by sending a chunked `multipart/form-data` request containing a file part and continuously streaming data without declaring a `Content-Length…

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CVE-2026-34829 — Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length

GHSA-8vqr-qjwx-82mw — Rack's multipart parsing without Content-Length header allows unbounded chunked file uploads