CVE-2021-1224 in Integrated Services Router
Summary
by MITRE • 01/14/2021
Multiple Cisco products are affected by a vulnerability with TCP Fast Open (TFO) when used in conjunction with the Snort detection engine that could allow an unauthenticated, remote attacker to bypass a configured file policy for HTTP. The vulnerability is due to incorrect detection of the HTTP payload if it is contained at least partially within the TFO connection handshake. An attacker could exploit this vulnerability by sending crafted TFO packets with an HTTP payload through an affected device. A successful exploit could allow the attacker to bypass configured file policy for HTTP packets and deliver a malicious payload.
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Analysis
by VulDB Data Team • 02/14/2021
This vulnerability resides in Cisco products that utilize TCP Fast Open in combination with the Snort intrusion detection system, creating a critical security gap in HTTP traffic inspection. The flaw stems from the improper handling of HTTP payloads that are partially contained within the TCP Fast Open connection handshake process, where the Snort engine fails to correctly identify and analyze the HTTP content during the initial connection establishment phase. The vulnerability specifically affects devices that process HTTP traffic through the Snort detection engine, making it particularly dangerous in network security appliances, firewalls, and intrusion prevention systems that rely on this combination. The issue is classified under CWE-200 as it involves information exposure through improper handling of network protocol data, and aligns with ATT&CK technique T1071.004 for application layer protocol usage in command and control communications.
The technical implementation of this vulnerability exploits the fundamental nature of TCP Fast Open which allows data to be sent in the initial TCP SYN packet, bypassing the traditional three-way handshake process. When Snort processes these TFO packets, it fails to properly parse the HTTP payload that may be embedded within the TCP options field of the initial SYN packet, leading to incomplete or incorrect traffic analysis. This misidentification allows malicious payloads to pass through security controls undetected, as the system treats the HTTP content as non-compliant traffic rather than properly analyzing the actual HTTP request or response data. The vulnerability becomes exploitable when an attacker crafts specific TFO packets containing malicious HTTP content that is partially or entirely contained within the TCP handshake, effectively circumventing the normal HTTP inspection process.
The operational impact of this vulnerability extends beyond simple bypass of file policies, potentially enabling attackers to execute more sophisticated attacks through the compromised network security controls. An unauthenticated remote attacker could leverage this vulnerability to deliver malicious content that would normally be blocked by file type filtering rules, potentially leading to malware delivery, command and control communication, or other malicious activities. The attack vector requires the attacker to send specifically crafted TFO packets through an affected Cisco device, making it a network-based exploit that could be particularly effective in environments where TFO is enabled for performance optimization. This vulnerability undermines the integrity of HTTP traffic inspection mechanisms and could lead to complete bypass of security policies designed to prevent specific file types or content from traversing the network.
Organizations should implement immediate mitigations including disabling TCP Fast Open on affected Cisco devices when Snort inspection is critical for security policy enforcement, or updating to versions that properly handle TFO packets with HTTP content. Network administrators should also consider implementing additional monitoring for unusual TFO packet patterns and ensure that Snort rules are updated to properly detect and handle HTTP content within TFO handshakes. The vulnerability highlights the importance of protocol-level security testing and the need for comprehensive security controls that account for modern performance optimization techniques. Organizations should also consider implementing network segmentation and additional layers of security inspection to provide defense in depth, as the vulnerability demonstrates how seemingly benign performance optimizations can create security gaps when combined with specific security tools.