Rack is a modular Ruby web server interface. From version 3.2.0 to before version 3.2.6, Rack::Multipart::Parser unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as filename or name instead of removing the folded line break during unfolding. As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting. This issue has been patched in version 3.2.6.

Affected: RubyGems: rack

## Summary

`Rack::Multipart::Parser` unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as `filename` or `name` instead of removing the folded line break during unfolding.

As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting.

## Details

`Rack::Multipart::Parser` accepts folded multipart header values and unfolds them during parsing. However, the unfolding behavior does not fully remove the embedded line break sequence from the parsed value.

This means a multipart part header such as:

```http Content-Disposition: form-data; name="file"; filename="test\r\n foo.txt" ```

can result in a parsed parameter value that still contains CRLF characters.

The issue is not that Rack creates a second multipart header field. Rather, the problem is that CRLF remains embedded in the parsed metadata value after unfolding. If an application later uses that value in a security-sensitive context, such as constructing an HTTP response header, the preserved CRLF may alter downstream header parsing.

Affected values may include multipart parameters such as `filename`, `name`, or similar parsed header attributes.

## Impact

Applications that accept multipart form uploads may be affected if they later reuse parsed multipart metadata in HTTP headers or other header-sensiti…

Security update for rmt-server

Security update for rmt-server

Affected: Ubuntu:Pro:14.04:LTS: ruby-rack, Ubuntu:Pro:16.04:LTS: ruby-rack, Ubuntu:Pro:18.04:LTS: ruby-rack, Ubuntu:Pro:20.04:LTS: ruby-rack, Ubuntu:22.04:LTS: ruby-rack, Ubuntu:24.04:LTS: ruby-rack, Ubuntu:25.10: ruby-rack, Ubuntu:26.04:LTS: ruby-rack

Rack is a modular Ruby web server interface. From version 3.2.0 to before version 3.2.6, Rack::Multipart::Parser unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as filename or name instead of removing the folded line break during unfolding. As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting. This issue has been patched in version 3.2.6.