CVE-2018-15399 in ASA
Summary
by MITRE
A vulnerability in the TCP syslog module of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to exhaust the 1550-byte buffers on an affected device, resulting in a denial of service (DoS) condition. The vulnerability is due to a missing boundary check in an internal function. An attacker could exploit this vulnerability by establishing a man-in-the-middle position between an affected device and its configured TCP syslog server and then maliciously modifying the TCP header in segments that are sent from the syslog server to the affected device. A successful exploit could allow the attacker to exhaust buffer on the affected device and cause all TCP-based features to stop functioning, resulting in a DoS condition. The affected TCP-based features include AnyConnect SSL VPN, clientless SSL VPN, and management connections such as Secure Shell (SSH), Telnet, and HTTPS.
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Analysis
by VulDB Data Team • 05/22/2023
The vulnerability described in CVE-2018-15399 represents a critical denial of service weakness within Cisco's network security infrastructure, specifically affecting Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) software implementations. This flaw resides in the TCP syslog module where insufficient input validation creates a buffer exhaustion scenario that can be exploited remotely without authentication. The vulnerability stems from a missing boundary check in an internal function that processes TCP syslog traffic, creating an exploitable condition that allows attackers to manipulate buffer allocation and consumption within the affected devices.
The technical implementation of this vulnerability involves a man-in-the-middle attack vector where an unauthenticated remote attacker can position themselves between the vulnerable Cisco device and its configured TCP syslog server. Through this intermediary position, the attacker can maliciously modify TCP headers in segments transmitted from the syslog server to the affected device. The missing boundary check in the internal processing function fails to validate the size or boundaries of incoming TCP segments, allowing oversized or malformed data to be processed without proper buffer size validation. This creates a scenario where the 1550-byte buffers become exhausted through controlled manipulation of TCP header parameters, leading to complete service disruption.
The operational impact of this vulnerability extends beyond simple service interruption to affect core network security functionality. When the buffer exhaustion occurs, all TCP-based features on the affected device cease to function properly, including critical management connections such as Secure Shell (SSH), Telnet, and HTTPS protocols. Additionally, the vulnerability compromises essential VPN services including AnyConnect SSL VPN and clientless SSL VPN capabilities, effectively cutting off legitimate administrative access and user connectivity. This comprehensive service degradation creates a cascading effect that can severely impact network operations and security posture, as administrators lose access to their security devices while the underlying threat detection and prevention capabilities become compromised.
From a cybersecurity framework perspective, this vulnerability aligns with CWE-129, which addresses insufficient boundary checking, and maps to ATT&CK technique T1499.004 for network denial of service attacks. The vulnerability demonstrates poor input validation practices that violate fundamental security principles of defensive programming and buffer management. Organizations implementing Cisco ASA and FTD solutions face significant risk exposure as this vulnerability can be exploited by attackers with minimal privileges and no authentication requirements, making it particularly dangerous in environments where network security devices are exposed to untrusted network segments. The lack of authentication requirements and the remote exploitability make this vulnerability particularly attractive to threat actors seeking to disrupt network operations and potentially create opportunities for additional attacks.
Mitigation strategies for this vulnerability should include immediate application of Cisco's security patches and updates, along with network segmentation to limit exposure of affected devices to untrusted traffic. Network administrators should implement TCP header inspection mechanisms and consider deploying intrusion prevention systems that can detect and block malformed TCP segments. Additionally, organizations should establish monitoring procedures to detect unusual buffer consumption patterns and implement network access controls that limit direct communication between syslog servers and security devices to reduce the attack surface. Regular security assessments and vulnerability scanning should be conducted to identify similar boundary checking issues in other network security components and ensure comprehensive protection against similar threats.