CVE-2019-15705 in FortiOS
Summary
by MITRE
An Improper Input Validation vulnerability in the SSL VPN portal of FortiOS versions 6.2.1 and below, and 6.0.6 and below may allow an unauthenticated remote attacker to crash the SSL VPN service by sending a crafted POST request.
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Analysis
by VulDB Data Team • 11/28/2019
The vulnerability identified as CVE-2019-15705 represents a critical improper input validation flaw within the SSL VPN portal component of Fortinet FortiOS operating systems. This weakness exists in versions 6.2.1 and earlier, as well as 6.0.6 and earlier, creating a significant attack surface for unauthenticated remote adversaries seeking to disrupt critical network infrastructure services. The vulnerability specifically targets the SSL VPN service functionality, which serves as a primary gateway for remote access to corporate networks, making it a particularly attractive target for attackers seeking to establish persistent access or cause operational disruption.
The technical nature of this vulnerability stems from insufficient validation of input parameters within the SSL VPN portal's POST request handling mechanism. When an attacker crafts and submits a malicious POST request to the affected system, the lack of proper input sanitization allows the malformed data to propagate through the system's processing pipeline. This failure in input validation creates a condition where the system cannot properly distinguish between legitimate and malicious requests, leading to a state where the SSL VPN service becomes unstable and eventually crashes. The vulnerability operates at the application layer and specifically exploits the web interface component that handles authentication and session management for SSL VPN connections.
The operational impact of CVE-2019-15705 extends beyond simple service disruption, as it can severely compromise network security operations and business continuity. Organizations relying on FortiOS SSL VPN services for remote workforce access face potential exposure to extended downtime, particularly during critical business hours when remote access is most needed. The unauthenticated nature of the attack means that adversaries can exploit this vulnerability without requiring valid credentials, making it especially dangerous for organizations that may not have robust network segmentation or monitoring in place. This vulnerability directly impacts the availability aspect of the CIA triad, potentially allowing attackers to create denial of service conditions that can persist until the system is manually restarted or the underlying software is patched.
Security professionals should recognize this vulnerability as a variant of CWE-20, Improper Input Validation, which is a fundamental weakness that consistently appears across multiple software systems and applications. The attack pattern aligns with techniques described in the MITRE ATT&CK framework under the T1499.004 sub-technique for Network Denial of Service, where adversaries specifically target network infrastructure components to create service disruptions. Organizations should implement immediate mitigations including applying the latest Fortinet patches, implementing network segmentation to limit exposure of the SSL VPN service, and establishing enhanced monitoring for unusual POST request patterns. Additionally, network administrators should consider deploying intrusion detection systems capable of identifying and blocking malformed POST requests that match the vulnerability's exploitation patterns, as this represents a common attack vector that can be effectively mitigated through proper network security controls and timely software updates.