CVE-2026-54904 — concurrent-ruby: `AtomicReference#update` livelocks when the stored value is `Float::NAN`

Affected: ruby-concurrency concurrent-ruby

concurrent-ruby is a modern concurrency tools for Ruby. Prior to 1.3.7, Concurrent::AtomicReference#update can enter a permanent busy retry loop when the current value is Float::NAN. The issue is caused by the interaction between AtomicReference#update, which retries until compare_and_set(old_value, new_value) succeeds; Numeric compare_and_set, which checks old == old_value before attempting the underlying atomic swap.; and Ruby NaN semantics, where Float::NAN == Float::NAN is always false. As 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. This vulnerability is fixed in 1.3.7.

GHSA-h8w8-99g7-qmvj — Concurrent Ruby : `AtomicReference#update` livelocks when the stored value is `Float::NAN`

Affected: RubyGems: concurrent-ruby

### Summary `Concurrent::AtomicReference#update` can enter a permanent busy retry loop when the current value is `Float::NAN`.

The issue is caused by the interaction between: - `AtomicReference#update`, which retries until `compare_and_set(old_value, new_value)` succeeds. - Numeric `compare_and_set`, which checks `old == old_value` before attempting the underlying atomic swap. - Ruby NaN semantics, where `Float::NAN == Float::NAN` is always `false`.

As 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.

### Version Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab ### Details

`AtomicReference#update` retries until `compare_and_set` returns true:

```ruby def update true until compare_and_set(old_value = get, new_value = yield(old_value)) new_value end ```

For numeric expected values, `compare_and_set` uses numeric equality before attempting the underlying atomic compare-and-set:

```ruby def compare_and_set(old_value, new_value) if old_value.kind_of? Numeric while true old = get

return false unless old.kind_of? Numeric return false unless old == old_value

result = _compare_and_set(old, new_value) return result if result end else _compare_and_set(old_value,…

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CVE-2026-54904 — concurrent-ruby: `AtomicReference#update` livelocks when the stored value is `Float::NAN`

GHSA-h8w8-99g7-qmvj — Concurrent Ruby : `AtomicReference#update` livelocks when the stored value is `Float::NAN`