CVE-2006-1966 in Fortinet28
Summary
by MITRE
An unspecified Fortinet product, possibly Fortinet28, allows remote attackers to cause a denial of service via a "small synflood" to the SMTP port (TCP port 25), as demonstrated by a 10-microsecond wait between sending packets. NOTE: this issue has been disputed in followup posts that suggest that a protection feature is triggering a RST.
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Analysis
by VulDB Data Team • 08/15/2018
The vulnerability identified as CVE-2006-1966 represents a significant denial of service weakness in certain Fortinet network security appliances, specifically potentially affecting the Fortinet28 model. This flaw manifests through a sophisticated attack vector that exploits the Simple Mail Transfer Protocol implementation within the affected devices. The vulnerability operates by leveraging a specialized form of SYN flood attack that demonstrates how network security appliances can be overwhelmed through seemingly innocuous packet timing characteristics, creating a scenario where legitimate network services become unavailable to authorized users.
The technical mechanism behind this vulnerability involves the manipulation of TCP connection establishment patterns through what researchers termed a "small synflood" technique. The attack specifically targets TCP port 25, which serves as the standard SMTP port for email services, making this a particularly concerning weakness for organizations that rely heavily on email infrastructure. The demonstration of this attack using a 10-microsecond interval between packet transmissions reveals a critical design flaw in how the affected Fortinet appliances process incoming connection requests. This timing characteristic suggests that the device's TCP stack implementation lacks proper rate limiting or connection state management mechanisms that would normally prevent such attacks from succeeding.
The operational impact of this vulnerability extends beyond simple service disruption, as it represents a fundamental weakness in the device's ability to maintain network availability during legitimate traffic patterns. When exploited, this vulnerability can cause complete denial of email services, potentially affecting business operations and communication channels across organizations that depend on these network security appliances. The attack's effectiveness against the SMTP port indicates that even basic email infrastructure becomes compromised, which can have cascading effects throughout enterprise networks where email serves as a critical communication backbone.
The vulnerability's classification aligns with CWE-400, which addresses "Uncontrolled Resource Consumption" and specifically relates to denial of service conditions where system resources become exhausted through malicious input patterns. Additionally, this weakness demonstrates characteristics that map to ATT&CK technique T1498, "Network Denial of Service," where adversaries exploit weaknesses in network infrastructure to disrupt service availability. The disputed nature of this vulnerability, where follow-up analysis suggests that a protection feature is triggering a RST packet, indicates that the device may be experiencing a complex interaction between its defensive mechanisms and the attack pattern, potentially creating a scenario where the very protection features designed to prevent such attacks inadvertently contribute to the denial of service condition.
Organizations should implement immediate mitigations including network segmentation to isolate critical email services, deployment of additional rate limiting controls, and monitoring for unusual traffic patterns on SMTP ports. The vulnerability highlights the importance of comprehensive testing of network security appliances under various attack conditions and the need for robust defense-in-depth strategies that account for both traditional attack vectors and the potential for protection mechanisms to be subverted by sophisticated attacks. Regular firmware updates and security assessments remain critical for maintaining protection against such vulnerabilities that can exploit fundamental network protocol implementations.