CVE-2026-59099 describes a cryptographic vulnerability in Apereo CAS 7.3.0 where the AES-GCM encryption uses a fixed all-zero initialization vector (IV) paired with the same encryption key across the server lifetime. This nonce reuse violates cryptographic best practices and results in keystream reuse, enabling remote unauthenticated attackers to recover plaintext conversation state. Attackers can gather multiple client-side webflow execution tokens from the unauthenticated login page and leverage known-plaintext attacks to decrypt the webflow conversation state. The vulnerability is present in version 7.3.0 before 8.0.0-RC6. No official patch or remediation level is provided in the available data.

The vulnerability allows remote unauthenticated attackers to decrypt sensitive conversation state data by exploiting AES-GCM nonce reuse. This compromises the confidentiality and integrity of the webflow conversation state, potentially exposing sensitive authentication or session information. The CVSS score of 9.1 indicates a critical severity with high impact on confidentiality and integrity, but no impact on availability. There are no known exploits in the wild as of the published date.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Since no official fix or remediation level is provided, users should monitor Apereo's advisories for updates. Until a fix is available, consider restricting access to the affected version or applying compensating controls if feasible. Avoid using version 7.3.0 in production environments where this vulnerability could be exploited.