CVE-2020-17469 in FNET
Summary
by MITRE • 12/12/2020
An issue was discovered in FNET through 4.6.4. The code for IPv6 fragment reassembly tries to access a previous fragment starting from a network incoming fragment that still doesn't have a reference to the previous one (which supposedly resides in the reassembly list). When faced with an incoming fragment that belongs to a non-empty fragment list, IPv6 reassembly must check that there are no empty holes between the fragments: this leads to an uninitialized pointer dereference in _fnet_ip6_reassembly in fnet_ip6.c, and causes Denial-of-Service.
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Analysis
by VulDB Data Team • 12/16/2020
The vulnerability identified as CVE-2020-17469 affects the FNET network stack version 4.6.4 and earlier, representing a critical flaw in the IPv6 fragment reassembly mechanism. This issue manifests when the network stack processes incoming IPv6 fragments that belong to existing fragment lists, creating a scenario where the reassembly logic attempts to access memory locations without proper initialization. The flaw occurs specifically within the _fnet_ip6_reassembly function located in the fnet_ip6.c source file, where the code fails to properly validate the existence and integrity of previous fragment references before attempting to dereference them.
The technical root cause of this vulnerability stems from an uninitialized pointer dereference that arises during IPv6 fragment reassembly operations. When an IPv6 fragment arrives that belongs to a non-empty fragment list, the system must verify that there are no gaps or holes between the fragments to ensure proper reassembly. However, the implementation does not adequately check for the presence of valid references to previous fragments before attempting to access them, leading to a scenario where an uninitialized pointer is dereferenced. This condition violates fundamental memory safety principles and creates a predictable crash condition that can be exploited by attackers.
From an operational perspective, this vulnerability presents a significant denial-of-service risk to systems running affected FNET versions. The uninitialized pointer dereference results in immediate system crashes or hangs during network traffic processing, effectively rendering the affected network stack non-functional. Attackers can exploit this vulnerability by sending specially crafted IPv6 fragments that trigger the problematic code path, causing the target system to become unresponsive and unable to process legitimate network traffic. The impact extends beyond simple service interruption as it can affect network availability for critical applications and infrastructure components relying on FNET for IPv6 connectivity.
This vulnerability aligns with CWE-476, which addresses the use of uninitialized pointers in software implementations, and demonstrates a classic memory safety issue that can lead to system instability and availability problems. The flaw also maps to ATT&CK technique T1499.004, which covers network denial-of-service attacks through the exploitation of software vulnerabilities. Organizations utilizing FNET for IPv6 network operations should prioritize immediate remediation by upgrading to version 4.6.5 or later, which includes proper validation of fragment references before dereferencing. Additionally, network administrators should consider implementing network segmentation and monitoring solutions to detect and mitigate potential exploitation attempts, while also establishing robust incident response procedures to address any successful exploitation attempts that may occur.