CVE-2026-57914: Apache Kerby ASN.1 Stack Overflow Denial of Service

Technical summary

CVE-2026-57914 is a stack overflow vulnerability in Apache Kerby triggered by malformed ASN.1 (Abstract Syntax Notation One) structures with excessive nesting depth. ASN.1 is the encoding standard used in Kerberos authentication messages. When a Kerby client or service processes such a structure, recursive parsing exhausts the call stack, causing an unhandled StackOverflowException. The vulnerability maps to CWE-400 (Uncontrolled Resource Consumption), indicating inadequate bounds checking on recursion depth or message complexity. The CVSS v3.1 score of 6.5 reflects medium severity with high availability impact but no confidentiality or integrity compromise. The attack vector is network-based, requires low complexity exploitation, and demands prior authentication (PR:L), though the scope remains unchanged and no user interaction is needed.

Business impact

Service availability is the primary concern. Organizations running Kerby-based authentication infrastructure face potential outages if an authenticated attacker sends a single malicious Kerberos message, disrupting authentication for all downstream services. This is particularly serious in environments where Kerberos is the central authentication backbone (e.g., Active Directory environments, enterprise Kerberos realms). While the confidentiality and integrity of existing sessions remain unaffected, the availability impact can cascade: users cannot authenticate, applications fail to validate credentials, and dependent services may go offline. Recovery typically requires restarting the affected Kerby process. The threat is elevated in security-conscious environments where internal networks already assume some level of attacker presence.

Affected systems

Apache Kerby versions prior to 2.1.2 are affected. Both Kerby client and service implementations are vulnerable. Organizations should identify systems running Kerby for Kerberos authentication, including custom implementations or third-party products that bundle Kerby. The vulnerability does not appear in a disclosed KEV list, suggesting it may not yet be actively exploited in the wild, though the technical simplicity of crafting nested ASN.1 structures means exploitation is feasible if weaponized.

Exploitability

Exploitation requires network access and valid authentication credentials (or the ability to send unauthenticated Kerberos messages, depending on deployment). An attacker must craft an ASN.1 message with nesting depth sufficient to exhaust the stack—a task that is straightforward given ASN.1 encoding knowledge and does not demand sophisticated tools. No user interaction is required; a single message triggers the crash. The barrier to exploitation is authentication/network access, not technical complexity. Once exploited, the denial-of-service is immediate and reliable, making this a practical attack vector for insider threats or attackers who have already gained network foothold.

Remediation

Upgrade Apache Kerby to version 2.1.2 or later. This version includes fixes to properly validate ASN.1 nesting depth and prevent unbounded recursion. Organizations should verify the patch version against Apache's official release notes and security advisories. If immediate patching is not possible, consider implementing network-level controls to restrict Kerberos traffic to trusted internal networks only, and monitor for unexpected authentication service crashes or restarts that may indicate exploitation attempts.

Patch guidance

Kerby version 2.1.2 is the recommended fixed version. Before patching production systems, test the upgrade in a staging environment that mirrors your authentication topology, as Kerberos is often critical infrastructure. Verify compatibility with dependent applications and clients. Check Apache Kerby's official release notes at https://kerby.apache.org for complete patching guidance and any migration steps. Plan deployment during maintenance windows to minimize service disruption during the restart cycle. If you deploy Kerby through a third-party product (e.g., an application server or security appliance), verify that the vendor has released a patched version compatible with your deployment.

Detection guidance

Monitor Kerby processes for unexpected crashes or StackOverflowException errors in logs. Watch for frequent restarts of Kerberos services without corresponding maintenance activity. Implement alerting on authentication service availability and response time degradation. Network-based detection is challenging without deep packet inspection of ASN.1 structures; focus detection efforts on service health and log analysis. If you have access to Kerby debug logs, look for ASN.1 parsing errors or recursion-related exceptions. Correlate Kerby crashes with inbound Kerberos traffic spikes or unusual message sizes from specific sources.

Why prioritize this

Medium severity (CVSS 6.5) with high availability impact and authentication criticality justifies prompt but deliberate remediation. The vulnerability is not listed in the KEV catalog, reducing immediate exploitation risk, but authentication outages can cascade widely. Organizations with Kerby as core infrastructure should prioritize this in the next maintenance window. Those with redundant Kerby deployments or failover mechanisms have more flexibility in scheduling. The authentication requirement limits risk to insider threats and network-resident attackers, but such threats are common in mature threat models.

Risk score, explained

CVSS 6.5 (MEDIUM) reflects: (1) High availability impact (A:H) from reliable denial-of-service; (2) No confidentiality or integrity compromise (C:N, I:N); (3) Network attack surface (AV:N); (4) Low attack complexity (AC:L)—crafting nested ASN.1 is not difficult; (5) Authentication requirement (PR:L) that reduces risk compared to unauthenticated attacks; (6) Unchanged scope (S:U). The score appropriately captures that this is a serious but narrowly scoped availability risk rather than a full system compromise. Environmental factors (e.g., criticality of Kerby in your architecture) may justify elevation in your internal risk rating.

Frequently asked questions