CVE-2026-126080: Active Support: Possible DoS vulnerability in Active Support safe_constantize

Impact When constant lookup raises LoadError, ActiveSupport::Inflector#safeconstantize (and String#safeconstantize) builds a regular expression from the requested constant name without escaping it, so a caller that passes untrusted input lets the attacker choose the source of a pattern that the application compiles. Crafted patterns can trigger defects in Ruby's regular expression compiler, possibly leading to a crash of the server process, and repeated requests could keep the application unavailable.

Action Cable passes the channel value of each subscribe command to safeconstantize before any channel lookup, so in the default configuration an unauthenticated client of the Action Cable endpoint can reach this code.

Details The check that decides whether safeconstantize returns nil or re-raises the LoadError now matches the first segment of the constant name against the error message as a literal string, and no longer compiles a regular expression. It returns the same result as before for constant names that contain no regular expression syntax.

Affected applications An application is affected if it meets all of these requirements: - Mounts Action Cable, which a new Rails application does by default at /cable (config.actioncable.mountpath), or otherwise passes untrusted input to safeconstantize. - Accepts Action Cable connections from untrusted clients. ApplicationCable::Connection#connect authenticates nobody unless the application adds a check.

Workarounds Applications that do not use Action Cable can set config.actioncable.mountpath = nil to remove the endpoint. Applications that use Action Cable can call rejectunauthorizedconnection in ApplicationCable::Connection#connect for unauthenticated clients, which limits exposure to authenticated users but does not remove it.

Releases The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once: - 8.1: https://github.com/rails/rails/commit/95aa9e416016524956d4904fed96eaed079580bb.patch - 8.0: https://github.com/rails/rails/commit/210ac22d446c8e2625b091e79256806b836eee20.patch

Versions affected

- activesupport < 8.0.5.2 - activesupport >= 8.1, < 8.1.4.1

Credit This issue was responsibly reported by Felix Tscheulin (flixt) through HackerOne researcher scheja, and independently by Bucky Wolfe (Igneous), who provided a proof of concept that crashes the server. It was also found by the Anthropic OSS Scanner (ANT-2026-ZUN5KU3Q).