Summary
Rack::Request#POST reads the entire request body into memory for Content-Type: application/x-www-form-urlencoded, calling rack.input.read(nil) without enforcing a length or cap. Large request bodies can therefore be buffered completely into process memory before parsing, leading to denial of service (DoS) through memory exhaustion.
Details
When handling non-multipart form submissions, Rack’s request parser performs:
form_vars = get_header(RACK_INPUT).read
Since read is called with no argument, the entire request body is loaded into a Ruby String. This occurs before query parameter parsing or enforcement of any params_limit. As a result, Rack applications without an upstream body-size limit can experience unbounded memory allocation proportional to request size.
Impact
Attackers can send large application/x-www-form-urlencoded bodies to consume process memory, causing slowdowns or termination by the operating system (OOM). The effect scales linearly with request size and concurrency. Even with parsing limits configured, the issue occurs before those limits are enforced.
Mitigation
- Update to a patched version of Rack that enforces form parameter limits using
query_parser.bytesize_limit, preventing unbounded reads of application/x-www-form-urlencoded bodies.
- Enforce strict maximum body size at the proxy or web server layer (e.g., Nginx
client_max_body_size, Apache LimitRequestBody).
References
Pirikara Reporter
jeremyevans Remediation developer
ioquatix Remediation reviewer
Summary
Rack::Request#POSTreads the entire request body into memory forContent-Type: application/x-www-form-urlencoded, callingrack.input.read(nil)without enforcing a length or cap. Large request bodies can therefore be buffered completely into process memory before parsing, leading to denial of service (DoS) through memory exhaustion.Details
When handling non-multipart form submissions, Rack’s request parser performs:
Since
readis called with no argument, the entire request body is loaded into a RubyString. This occurs before query parameter parsing or enforcement of anyparams_limit. As a result, Rack applications without an upstream body-size limit can experience unbounded memory allocation proportional to request size.Impact
Attackers can send large
application/x-www-form-urlencodedbodies to consume process memory, causing slowdowns or termination by the operating system (OOM). The effect scales linearly with request size and concurrency. Even with parsing limits configured, the issue occurs before those limits are enforced.Mitigation
query_parser.bytesize_limit, preventing unbounded reads ofapplication/x-www-form-urlencodedbodies.client_max_body_size, ApacheLimitRequestBody).References