CVE-2019-17656 in FortiOS
Summary
by MITRE • 04/12/2021
A Stack-based Buffer Overflow vulnerability in the HTTPD daemon of FortiOS 6.0.10 and below, 6.2.2 and below and FortiProxy 1.0.x, 1.1.x, 1.2.9 and below, 2.0.0 and below may allow an authenticated remote attacker to crash the service by sending a malformed PUT request to the server. Fortinet is not aware of any successful exploitation of this vulnerability that would lead to code execution.
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Analysis
by VulDB Data Team • 04/15/2021
This vulnerability represents a critical stack-based buffer overflow in the HTTPD daemon component of Fortinet's security appliances, specifically affecting FortiOS versions 6.0.10 and earlier, 6.2.2 and earlier, along with various FortiProxy versions up to 2.0.0. The flaw resides in how the system processes incoming PUT requests through the HTTP daemon, creating an exploitable condition where an authenticated remote attacker can manipulate memory allocation patterns. The vulnerability is classified as CWE-121 Stack-based Buffer Overflow, which occurs when a program writes data beyond the bounds of a fixed-length stack buffer, potentially overwriting adjacent memory locations including return addresses and function pointers. This type of vulnerability falls under the ATT&CK technique T1203 Exploitation for Client Execution and T1059 Command and Scripting Interpreter, as it enables attackers to disrupt service availability and potentially achieve further system compromise.
The technical implementation of this vulnerability involves the HTTPD daemon's insufficient validation of PUT request parameters, particularly when handling large or malformed data payloads. When an authenticated user submits a specially crafted PUT request containing excessive data, the daemon fails to properly bounds-check the input before copying it into a stack-allocated buffer. This allows the attacker to overwrite adjacent stack memory, potentially causing a segmentation fault or more insidiously, manipulating the program's execution flow. The buffer overflow manifests as a service crash, effectively creating a denial of service condition that can be exploited repeatedly to maintain persistent availability disruption. The authentication requirement means that attackers must first establish valid credentials, but this does not prevent the exploitation from being highly impactful given that many security devices operate with elevated privileges.
The operational impact of this vulnerability extends beyond simple service disruption, as it represents a significant threat to network security infrastructure reliability. In production environments, the crash of HTTPD daemons on FortiOS appliances can compromise critical security functions including web-based management interfaces, API endpoints, and potentially the underlying security policies that the appliance enforces. The vulnerability's presence in multiple product lines including FortiOS and FortiProxy creates widespread exposure across enterprise networks, as these components typically serve as primary interfaces for administrative access and policy management. Organizations may experience cascading failures when the HTTPD service crashes, potentially affecting other dependent services that rely on the same daemon for communication. The lack of known code execution exploits does not diminish the severity, as the denial of service impact can effectively disable security controls for the affected devices.
Mitigation strategies for this vulnerability should prioritize immediate patch deployment across all affected FortiOS and FortiProxy versions, as Fortinet has released security updates addressing the buffer overflow condition. Network administrators should implement strict access controls limiting PUT request handling to only authorized administrative users and consider disabling unnecessary HTTP services when not required for operational functions. The implementation of web application firewalls and intrusion prevention systems can help detect and block malformed PUT requests before they reach the vulnerable HTTPD daemon. Additionally, monitoring should be enhanced to detect service crashes or abnormal behavior patterns that may indicate exploitation attempts, while regular security assessments should verify that only essential HTTP functionality remains enabled. Organizations should also consider implementing network segmentation to isolate affected appliances and reduce the potential impact of service disruption on broader network operations, ensuring that the vulnerability cannot be leveraged to create lateral movement opportunities within the network infrastructure.