CVE-2026-45409 is a denial-of-service vulnerability in the python-idna library, which handles Internationalized Domain Names in Python applications. The vulnerability arises from incomplete validation in the idna.encode() function, allowing attackers to send overly long inputs that cause excessive CPU resource consumption. This flaw is a regression or incomplete fix of CVE-2024-3651. The fix in python-idna versions 3.14 and later rejects long inputs early to prevent resource exhaustion. Red Hat released updated RPM packages (python3-idna-3.18-1.hum1 and python-idna-3.18-1.hum1.src) as part of a Hardened Images update to remediate this issue.

Exploitation of this vulnerability can cause denial of service by exhausting CPU resources in affected applications using the python-idna library without proper input validation. There is no impact on confidentiality or integrity. The vulnerability affects availability by making the application unavailable to legitimate users. No known exploits are reported in the wild.

Red Hat has released updated python-idna packages (python3-idna-3.18-1.hum1 and python-idna-3.18-1.hum1.src) that address this vulnerability. Users should apply these updates to affected Red Hat Hardened Images and related products. Additionally, applications should implement input validation to ensure domain names do not exceed the standard 253-character length before passing them to idna.encode(). This operational control prevents processing of maliciously long inputs. No further action is required once the update is applied.