CVE-2002-0515 in IPFilter
Summary
by MITRE
IPFilter 3.4.25 and earlier sets a different TTL when a port is being filtered than when it is not being filtered, which allows remote attackers to identify filtered ports by comparing TTLs.
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Analysis
by VulDB Data Team • 06/20/2024
The vulnerability described in CVE-2002-0515 represents a significant information disclosure flaw in IPFilter versions 3.4.25 and earlier. This issue stems from the network filtering mechanism's inconsistent handling of Time To Live (TTL) values in IP packets. When a port is being filtered by IPFilter, the system assigns a different TTL value compared to when a port is not filtered, creating a distinguishable pattern that can be exploited by remote attackers to determine which ports are actively being monitored or blocked by the firewall.
The technical implementation of this vulnerability lies in the fundamental network protocol handling within the IPFilter software. The TTL field in IP packets serves as a hop counter that decrements with each router hop, and normally should remain consistent for identical network paths. However, IPFilter's flawed implementation causes the system to set varying TTL values based on whether a port is in a filtered state or not. This inconsistency creates a timing and packet analysis opportunity for attackers who can observe the TTL values of packets sent to different ports and infer which ports are filtered based on the TTL differences.
From an operational impact perspective, this vulnerability enables attackers to perform port scanning with enhanced accuracy and efficiency. Traditional port scanning techniques rely on observing response types such as open, closed, or filtered ports through connection attempts. However, this vulnerability allows attackers to identify filtered ports without attempting connections, simply by analyzing TTL values. This capability significantly reduces the time and resources required for network reconnaissance, as attackers can quickly determine which ports are protected by IPFilter rules. The vulnerability essentially provides a passive reconnaissance method that bypasses typical active scanning limitations, making network discovery attacks more effective.
The implications of this vulnerability extend beyond simple port identification and represent a violation of network security principles. According to CWE-200, this represents an information disclosure vulnerability where system behavior reveals sensitive information about network configuration and security controls. The flaw undermines the fundamental security assumption that filtered network traffic should not reveal information about the underlying filtering mechanisms. From an ATT&CK framework perspective, this vulnerability maps to technique T1046 Network Service Scanning and T1592 Reconnaissance, as it provides attackers with enhanced capabilities for mapping network services and identifying protected ports without direct interaction with the target systems.
Mitigation strategies for this vulnerability require immediate attention and system updates to newer versions of IPFilter where the TTL handling has been corrected. System administrators should upgrade to IPFilter versions beyond 3.4.25 where the inconsistent TTL behavior has been resolved. Additionally, network administrators should implement additional monitoring and logging of network traffic patterns to detect potential exploitation attempts. The fix typically involves ensuring that IPFilter maintains consistent TTL values regardless of whether ports are filtered or not, eliminating the distinguishable pattern that attackers can exploit. Organizations should also consider implementing more robust network segmentation and access control measures to limit the potential impact of such information disclosure vulnerabilities.