Impact

When constant lookup raises LoadError, ActiveSupport::Inflector#safe_constantize (and String#safe_constantize) 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 safe_constantize 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 safe_constantize 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.action_cable.mount_path), or otherwise passes untrusted input to safe_constantize.
• 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.action_cable.mount_path = nil to remove the endpoint. Applications that use Action Cable can call reject_unauthorized_connection 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).