Summary
Rack::Multipart::Parser stores non-file form fields (parts without a filename) entirely in memory as Ruby String objects. A single large text field in a multipart/form-data request (hundreds of megabytes or more) can consume equivalent process memory, potentially leading to out-of-memory (OOM) conditions and denial of service (DoS).
Details
During multipart parsing, file parts are streamed to temporary files, but non-file parts are buffered into memory:
body = String.new # non-file → in-RAM buffer
@mime_parts[mime_index].body << content
There is no size limit on these in-memory buffers. As a result, any large text field—while technically valid—will be loaded fully into process memory before being added to params.
Impact
Attackers can send large non-file fields to trigger excessive memory usage. Impact scales with request size and concurrency, potentially leading to worker crashes or severe garbage-collection overhead. All Rack applications processing multipart form submissions are affected.
Mitigation
- Upgrade: Use a patched version of Rack that enforces a reasonable size cap for non-file fields (e.g., 2 MiB).
- Workarounds:
- Restrict maximum request body size at the web-server or proxy layer (e.g., Nginx
client_max_body_size).
- Validate and reject unusually large form fields at the application level.
References
kwkr Reporter
jeremyevans Remediation developer
ioquatix Remediation reviewer
Summary
Rack::Multipart::Parserstores non-file form fields (parts without afilename) entirely in memory as RubyStringobjects. A single large text field in a multipart/form-data request (hundreds of megabytes or more) can consume equivalent process memory, potentially leading to out-of-memory (OOM) conditions and denial of service (DoS).Details
During multipart parsing, file parts are streamed to temporary files, but non-file parts are buffered into memory:
There is no size limit on these in-memory buffers. As a result, any large text field—while technically valid—will be loaded fully into process memory before being added to
params.Impact
Attackers can send large non-file fields to trigger excessive memory usage. Impact scales with request size and concurrency, potentially leading to worker crashes or severe garbage-collection overhead. All Rack applications processing multipart form submissions are affected.
Mitigation
client_max_body_size).References