CVE-2026-59099 affects Apereo CAS 7.3.0 prior to 8.0.0-RC6 and involves a cryptographic vulnerability where the AES-GCM encryption uses a fixed all-zero initialization vector (IV) reused across the server lifetime with the same key. This nonce reuse breaks the security guarantees of AES-GCM, enabling remote unauthenticated attackers to collect multiple client-side webflow execution tokens from the login page and perform known-plaintext attacks to decrypt the webflow conversation state. The vulnerability allows recovery of plaintext sensitive data without authentication.

The vulnerability allows remote unauthenticated attackers to decrypt sensitive webflow conversation state data by exploiting nonce reuse in AES-GCM encryption. This compromises confidentiality of the conversation state, potentially exposing sensitive session or authentication-related information. There is no indication of impact on integrity or availability. No known exploits in the wild have been reported.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Since the vulnerability is due to nonce reuse in AES-GCM, upgrading to Apereo CAS version 8.0.0-RC6 or later (where the issue is fixed) is recommended once available. Until then, consider restricting access to the login page or monitoring for suspicious activity as temporary measures.