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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-06-24 · updated 2026-10-25
Weakness: CWE-835

Description

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.

Affected products

ruby-concurrency concurrent-ruby: before 1.3.7 — fixed in 1.3.7
Rubygems Concurrent-Ruby: before 1.3.7; 0.0.1, 0.1.0, 0.1.1, 0.1.1.pre.1, 0.1.1.pre.2, 0.1.1.pre.3 (+71) — fixed in 1.3.7

How to fix

Upgrade to a fixed version: ruby-concurrency concurrent-ruby 1.3.7; Rubygems Concurrent-Ruby 1.3.7. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-54904 or the next vulnerability affects your stack