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🚨 [security] [ruby] Update rack 3.1.17 → 3.1.18 (patch)

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depfu/update/rack-3.1.18 into master

🚨 Your current dependencies have known security vulnerabilities 🚨

This dependency update fixes known security vulnerabilities. Please see the details below and assess their impact carefully. We recommend to merge and deploy this as soon as possible!

Here is everything you need to know about this update. Please take a good look at what changed and the test results before merging this pull request.

What changed?

↗️ rack (indirect, 3.1.17 → 3.1.18) · Repo · Changelog
Security Advisories 🚨

🚨 Rack has a Possible Information Disclosure Vulnerability

Summary

A possible information disclosure vulnerability existed in Rack::Sendfile when running behind a proxy that supports x-sendfile headers (such as Nginx). Specially crafted headers could cause Rack::Sendfile to miscommunicate with the proxy and trigger unintended internal requests, potentially bypassing proxy-level access restrictions.

Details

When Rack::Sendfile received untrusted x-sendfile-type or x-accel-mapping headers from a client, it would interpret them as proxy configuration directives. This could cause the middleware to send a "redirect" response to the proxy, prompting it to reissue a new internal request that was not subject to the proxy's access controls.

An attacker could exploit this by:

  1. Setting a crafted x-sendfile-type: x-accel-redirect header.
  2. Setting a crafted x-accel-mapping header.
  3. Requesting a path that qualifies for proxy-based acceleration.

Impact

Attackers could bypass proxy-enforced restrictions and access internal endpoints intended to be protected (such as administrative pages). The vulnerability did not allow arbitrary file reads but could expose sensitive application routes.

This issue only affected systems meeting all of the following conditions:

  • The application used Rack::Sendfile with a proxy that supports x-accel-redirect (e.g., Nginx).
  • The proxy did not always set or remove the x-sendfile-type and x-accel-mapping headers.
  • The application exposed an endpoint that returned a body responding to .to_path.

Mitigation

  • Upgrade to a fixed version of Rack which requires explicit configuration to enable x-accel-redirect:

    use Rack::Sendfile, "x-accel-redirect"

  • Alternatively, configure the proxy to always set or strip the headers (you should be doing this!):

    proxy_set_header x-sendfile-type x-accel-redirect;
proxy_set_header x-accel-mapping /var/www/=/files/;

  • Or in Rails applications, disable sendfile completely:

    config.action_dispatch.x_sendfile_header = nil

🚨 Rack is vulnerable to a memory-exhaustion DoS through unbounded URL-encoded body parsing

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).
Commits

See the full diff on Github. The new version differs by 5 commits:

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