External risk intelligence

Apereo CAS AES-GCM Initialization Vector Reuse Information Disclosure

CVE advisorySeverity: CRITICAL (CVSS 9.3)

CVE-2026-59099

Apereo CAS is an enterprise single sign-on product designed as an identity portal. It is typically deployed as a public-facing authentication gateway to handle login requests for web applications, making its login endpoints and associated services accessible via the internet by design.

Halo Surface Signal: 5 out of 5 — more likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

Apereo CAS, a widely used identity and access management system, has a cryptographic vulnerability that could allow unauthenticated attackers to decrypt sensitive conversation data. This occurs due to a weakness in how the system reuses encryption components, potentially exposing user session information.

Issue: Encryption weakness can expose conversation data.
Leadership concern: Potential for unauthorized access to user sessions.
Executive takeaway: Assess relevance and confirm exposure.

Attack Path

How an attacker could exploit the issue

An attacker could potentially gain access to sensitive conversation details by exploiting a flaw in how the system encrypts session data. By observing multiple unauthenticated login attempts, an attacker could collect enough encrypted information to decrypt past conversations, revealing plaintext data. This attack is possible because the encryption method reuses a predictable initialization vector, which is a critical component for secure encryption, allowing for a known-plaintext attack.

Exposed login page.
Reused encryption initialization vector.
Disclosure of plaintext conversation state.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, remote unauthenticated attackers could recover plaintext conversation state by exploiting AES-GCM initialization vector reuse across the server lifetime. This could allow attackers to collect multiple client-side webflow execution tokens from the unauthenticated login page and perform known-plaintext analysis to decrypt the webflow conversation state due to keystream reuse.

Conversation state data at risk.
Decryption via known-plaintext analysis.
Disclosure of sensitive session information.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability affects Apereo CAS, likely deployed as a public-facing authentication gateway. The primary responsibility for addressing this issue lies with the platform or application owners responsible for the CAS deployment, in coordination with the security and network teams to assess and mitigate exposure. The first practical step is to identify all CAS instances, determine their reachability and criticality, and locate the accountable owner to plan remediation.

Platform and application owners should address this.
Verify external accessibility and business criticality.
Plan remediation based on risk assessment.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is Apereo CAS?

Apereo CAS is an enterprise-grade identity and access management system. It functions as a central authentication gateway, providing single sign-on capabilities that allow users to access multiple web applications through one login portal. It is widely used by organizations to manage authentication flows, verify user credentials, and handle secure session tokens for connected services.

What is the cryptographic weakness in CVE-2026-59099?

The vulnerability involves improper reuse of cryptographic keys, specifically classified as CWE-323: Misuse of Initialization Vector (IV). In this instance, Apereo CAS uses a static, all-zero IV with the same encryption key for AES-GCM, which is a process that requires unique values to stay secure. This mistake leads to keystream reuse, allowing an attacker to mathematically derive the original, unencrypted information from the encrypted conversation data.

How does an attacker trigger this vulnerability?

An attacker triggers this by interacting with the unauthenticated login page of a vulnerable Apereo CAS instance. They collect multiple client-side webflow execution tokens generated by the server. Because the server uses a predictable IV, the attacker can perform known-plaintext analysis on these tokens to decrypt the conversation state. Simply navigating to a protected resource or authenticating as a user is not required; the flaw is exposed via the login process itself.

Why is this CVE considered relevant for my environment?

Halo Surface Signal indicates that Apereo CAS is typically deployed as a public-facing authentication gateway by design to manage external login requests. Because the vulnerability is remotely exploitable without authentication, any instance reachable via the internet is highly accessible to attackers. You should consider this relevant if your deployment handles public-facing traffic, as the flaw targets the very interface meant to secure your applications.

What is the first step to address CVE-2026-59099?

Your priority is to identify all Apereo CAS instances within your infrastructure and confirm their current versions. Once you have an inventory, verify which instances are exposed to the internet. Coordinate with your application owners to plan an upgrade to a patched version that resolves the AES-GCM initialization vector reuse. Focus on isolating any critical, externally-facing gateways while you finalize the patching schedule.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.