CVE-2006-1242 in Linux
Summary
by MITRE
The ip_push_pending_frames function in Linux 2.4.x and 2.6.x before 2.6.16 increments the IP ID field when sending a RST after receiving unsolicited TCP SYN-ACK packets, which allows remote attackers to conduct an Idle Scan (nmap -sI) attack, which bypasses intended protections against such attacks.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 06/18/2019
The vulnerability described in CVE-2006-1242 represents a significant weakness in the Linux kernel's TCP/IP implementation that directly impacts network scanning security measures. This flaw exists in kernel versions 2.4.x and 2.6.x prior to 2.6.16, specifically within the ip_push_pending_frames function that handles IP packet transmission. The issue stems from improper handling of TCP connection state transitions when the kernel receives unsolicited TCP SYN-ACK packets from external sources. When the kernel processes these unexpected packets and subsequently sends a RST (reset) packet in response, it increments the IP ID field even though no actual data transmission is occurring. This behavior creates a predictable pattern that malicious actors can exploit to bypass standard network scanning protections.
The technical exploitation of this vulnerability enables what is known as an idle scan attack, commonly performed using the nmap -sI flag. In traditional TCP scanning, attackers send packets to target systems to determine which ports are open or closed, but these scans can be detected and blocked by firewalls. The idle scan technique leverages the IP ID field increment behavior to create a covert scanning mechanism that appears to originate from a different host, effectively allowing attackers to perform reconnaissance without direct connection to the target. The flaw allows attackers to determine the next IP ID value that will be used by the target system, enabling them to construct packets that appear to come from the target itself. This creates a false positive detection scenario where the network monitoring systems believe the target is scanning itself, rather than an external attacker performing the scan.
The operational impact of this vulnerability extends beyond simple network reconnaissance, as it fundamentally undermines the security assumptions of network firewalls and intrusion detection systems. Network administrators who rely on IP ID field monitoring as part of their security posture may be misled into believing that their systems are not under active scanning, when in fact sophisticated attackers are using this vulnerability to conduct stealthy reconnaissance. The vulnerability affects systems running older kernel versions where the IP ID increment behavior was not properly controlled during RST packet generation. This creates a window of opportunity for attackers to map network topologies, identify open ports, and potentially discover vulnerable services without triggering standard security alerts. The flaw particularly impacts systems that depend on IP ID-based detection mechanisms for identifying scanning activities, as the predictable increments eliminate the effectiveness of this security feature.
Mitigation strategies for this vulnerability require immediate kernel updates to versions 2.6.16 or later where the ip_push_pending_frames function properly handles IP ID field increments during RST packet generation. System administrators should prioritize patching affected systems, particularly those running kernel versions 2.4.x and 2.6.x prior to 2.6.16. Network security measures should include implementing more robust detection mechanisms that do not rely solely on IP ID field patterns, as well as monitoring for anomalous TCP packet behavior that might indicate exploitation attempts. Organizations should also consider implementing additional layers of protection such as TCP SYN cookies and enhanced firewall rules that can detect and block suspicious packet sequences. This vulnerability aligns with CWE-191, which addresses integer underflow and overflow conditions, and represents a specific case where improper integer handling during network packet processing creates security implications. From an ATT&CK perspective, this vulnerability maps to technique T1046 for network service scanning and T1592 for reconnaissance using network scanning tools, as it enables attackers to perform covert reconnaissance without detection by traditional security measures. The fix implemented in kernel 2.6.16 specifically addresses the improper handling of IP ID increments during RST packet generation, ensuring that these fields are only incremented when legitimate packet transmission occurs rather than during error response handling.