CVE-2008-1804 in Snort
Summary
by MITRE
preprocessors/spp_frag3.c in Sourcefire Snort before 2.8.1 does not properly identify packet fragments that have dissimilar TTL values, which allows remote attackers to bypass detection rules by using a different TTL for each fragment.
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Analysis
by VulDB Data Team • 08/11/2019
The vulnerability described in CVE-2008-1804 resides within the fragment reassembly functionality of Sourcefire Snort intrusion detection system version 2.8.0 and earlier. This issue specifically affects the spp_frag3.c module responsible for processing fragmented IP packets. The flaw represents a significant weakness in the network traffic analysis capabilities of the system, as it fails to properly validate the consistency of Time To Live (TTL) values across packet fragments that should logically belong together. According to CWE-200, this vulnerability stems from insufficient input validation and improper handling of network protocol elements that should maintain consistency within a single logical packet.
The technical implementation of this vulnerability allows malicious actors to exploit the lack of TTL consistency checking during fragment reassembly. When network packets are fragmented, the original packet is broken into smaller pieces that must be reassembled to form the complete message. In normal operation, all fragments of a given packet should maintain the same TTL value to ensure proper reassembly and prevent network anomalies. However, Snort's fragment processing module fails to enforce this consistency check, enabling attackers to send fragmented packets with varying TTL values for each fragment. This behavior bypasses the system's detection capabilities because the IDS cannot properly reconstruct the original packet for signature analysis.
From an operational perspective, this vulnerability creates a significant security gap in network monitoring and intrusion detection. Attackers can leverage this flaw to evade signature-based detection rules that would normally identify malicious traffic patterns. The technique allows for the delivery of malicious payloads that would otherwise be detected by Snort's rule engine, as the fragmented traffic appears benign when analyzed at the fragment level. This vulnerability directly impacts the integrity of network security monitoring and can lead to false negatives where actual threats go undetected. The attack vector is remote and requires no authentication, making it particularly dangerous in network environments where Snort is deployed as a primary security monitoring tool.
The mitigation strategy for this vulnerability involves upgrading to Snort version 2.8.1 or later, where the fragment reassembly logic has been corrected to properly validate TTL consistency across packet fragments. Organizations should also consider implementing additional network monitoring techniques beyond signature-based detection to compensate for potential gaps in the IDS functionality. Network administrators should review and test their existing Snort configurations to ensure proper handling of fragmented traffic. According to ATT&CK framework concept T1071.004, this vulnerability aligns with techniques involving protocol manipulation to evade detection systems. The fix implemented in the updated Snort version addresses the core issue by enforcing proper fragment validation logic that ensures TTL consistency, thereby restoring the system's ability to properly reconstruct and analyze fragmented network traffic for security purposes.