CVE-2000-0553 in IPFilter
Summary
by MITRE
Race condition in IPFilter firewall 3.4.3 and earlier, when configured with overlapping "return-rst" and "keep state" rules, allows remote attackers to bypass access restrictions.
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Analysis
by VulDB Data Team • 04/21/2026
The vulnerability described in CVE-2000-0553 represents a critical race condition within the IPFilter firewall implementation that affects versions 3.4.3 and earlier. This flaw manifests when the firewall is configured with specific rule combinations involving "return-rst" and "keep state" directives, creating a temporal window where the system's state management becomes inconsistent. The race condition occurs during the processing of network packets when multiple concurrent operations attempt to modify the firewall's internal state table simultaneously, leading to unpredictable behavior in packet filtering decisions.
The technical implementation of this vulnerability stems from the improper synchronization mechanisms within the IPFilter kernel module. When overlapping rules are present, the firewall's state tracking system fails to properly coordinate access to shared memory structures that maintain connection state information. The "return-rst" rule is designed to send a reset packet in response to certain connection attempts, while "keep state" rules maintain persistent connection tracking. The interaction between these conflicting rule sets during high-concurrency scenarios creates a window where packets can traverse the firewall without proper state validation, effectively bypassing the intended access controls.
From an operational perspective, this vulnerability poses significant security implications for systems relying on IPFilter for network protection. Remote attackers can exploit this race condition to establish unauthorized connections or bypass existing firewall restrictions without requiring local access or authentication credentials. The attack requires only network connectivity to the target system and can be executed from any remote location, making it particularly dangerous for perimeter defense implementations. The timing aspect of the race condition means that successful exploitation depends on precise packet timing and concurrent processing conditions, but the vulnerability remains exploitable across various network environments.
The flaw aligns with CWE-362, which specifically addresses race conditions in concurrent programming, and demonstrates how improper synchronization can lead to security vulnerabilities. From an attack framework perspective, this vulnerability maps to the privilege escalation and lateral movement tactics described in the MITRE ATT&CK framework, as it allows attackers to bypass network-level controls and potentially gain access to restricted network segments. The impact extends beyond simple access bypass to potentially enable more sophisticated attacks such as port scanning, service enumeration, or even full network infiltration when combined with other exploitation techniques.
Mitigation strategies for this vulnerability require immediate patching of IPFilter installations to versions that address the race condition in state management. System administrators should also consider implementing additional network segmentation measures and monitoring for unusual connection patterns that might indicate exploitation attempts. The recommended approach includes disabling overlapping rule configurations where possible and implementing proper state synchronization mechanisms. Organizations should conduct thorough security assessments of their firewall configurations to identify and remediate similar race conditions in other network security appliances, as this type of vulnerability often indicates broader architectural weaknesses in concurrent system design that could affect other security controls.