CVE-2026-84732 in OpenVPN
Summary
by MITRE • 09/07/2026
Retransmissions of ACK packet ID in OpenVPN through 2.6.22 and 2.7.6 allow remote unauthenticated attackers to cause a denial of service via crafted inputs that trigger a timeout integer overflow
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Analysis
by VulDB Data Team • 09/07/2026
The vulnerability identified involves a critical flaw within the OpenVPN software, specifically affecting versions up to 2.6.22 and 2.7.6. This issue centers on the handling of acknowledgment packets, commonly referred to as ACKs, which are essential for maintaining reliable communication channels in virtual private network connections. The core technical deficiency lies in how the application processes retransmissions of these ACK packet identifiers. When an attacker sends crafted inputs that trigger a timeout integer overflow, they can disrupt the normal state machine logic governing connection persistence and data integrity checks. This specific flaw allows remote unauthenticated attackers to exploit the system without needing prior access credentials or established sessions, significantly lowering the barrier for exploitation compared to vulnerabilities requiring authentication.
From a technical perspective, the vulnerability manifests as an integer overflow condition during the calculation of timeout values associated with retransmitted ACK packets. In network protocols like OpenVPN, timeouts are calculated based on elapsed time and packet counts to determine when a connection should be considered dead or in need of resynchronization. If the logic handling these calculations fails to properly validate input ranges or handle wrap-around conditions correctly, an attacker can supply values that cause the integer variable to overflow. This results in incorrect timeout computations, potentially leading to immediate disconnection loops or resource exhaustion as the system attempts to recover from a perceived catastrophic failure state. The lack of proper bounds checking on these packet identifiers is the primary vector for this denial of service attack.
The operational impact of this vulnerability is severe, primarily resulting in a denial of service against OpenVPN servers and clients. By triggering the integer overflow through crafted ACK retransmissions, an attacker can cause the affected system to become unresponsive or crash entirely. This disrupts network connectivity for all users relying on that specific VPN instance, leading to significant downtime and potential data loss if active sessions are abruptly terminated without proper cleanup procedures. Since the attack requires no authentication, it poses a substantial risk to public-facing OpenVPN deployments where attackers can probe from any location on the internet. The ability to remotely induce this state means that availability, one of the three pillars of information security confidentiality, integrity, and availability is directly compromised.
This vulnerability aligns with Common Weakness Enumeration (CWE) standards, specifically CWE-190: Integer Overflow or Wraparound. This classification highlights the failure to perform adequate bounds checking on arithmetic operations involving integer data types. Furthermore, in terms of cyber kill chain modeling such as MITRE ATT&CK, this behavior can be associated with techniques that disrupt availability, often categorized under resource exhaustion or impact tactics. The unauthenticated nature of the exploit places it within the initial access phase for denial-of-service attacks, although its primary effect is on service continuity rather than data theft. Understanding these classifications helps in mapping the vulnerability to broader security frameworks and prioritizing remediation efforts based on industry-standard risk assessments.
Mitigation strategies must focus on immediate patching and architectural hardening. The most effective solution is to upgrade OpenVPN to a version that includes patches for this integer overflow issue, ensuring that all instances are updated to at least 2.6.23 or later where applicable fixes have been released. Administrators should also implement network-level controls such as rate limiting on incoming UDP traffic directed at the VPN port to mitigate the volume of crafted packets an attacker can send in a given timeframe. Additionally, deploying intrusion detection systems configured with signatures for anomalous ACK patterns can help identify and block exploitation attempts before they trigger the overflow condition. Regular security audits and penetration testing should be conducted to verify that these mitigations are effective against evolving attack vectors targeting network protocol implementations.