CVE-2017-18017 in Linux
Summary
by MITRE • 01/25/2023
The tcpmss_mangle_packet function in net/netfilter/xt_TCPMSS.c in the Linux kernel before 4.11, and 4.9.x before 4.9.36, allows remote attackers to cause a denial of service (use-after-free and memory corruption) or possibly have unspecified other impact by leveraging the presence of xt_TCPMSS in an iptables action.
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Analysis
by VulDB Data Team • 09/14/2025
The vulnerability identified as CVE-2017-18017 represents a critical use-after-free condition within the Linux kernel's netfilter subsystem, specifically affecting the tcpmss_mangle_packet function in the xt_TCPMSS module. This flaw exists in kernel versions prior to 4.11 and 4.9.x versions before 4.9.36, creating a persistent security risk across multiple kernel release lines. The vulnerability stems from improper memory management within the packet processing pipeline where the function fails to properly validate or manage memory references after certain packet processing operations, leading to potential exploitation by remote attackers who can craft malicious network traffic to trigger the vulnerable code path.
The technical implementation of this vulnerability involves the xt_TCPMSS target within iptables rules, which is designed to modify the Maximum Segment Size (MSS) field in TCP packets. When an attacker constructs packets that traverse through iptables chains containing xt_TCPMSS rules, the tcpmss_mangle_packet function processes these packets and subsequently frees memory associated with packet data structures. However, due to inadequate reference counting or improper memory deallocation logic, the function may attempt to access memory that has already been freed, resulting in memory corruption that can manifest as system crashes or potentially more severe consequences depending on the execution context and memory layout.
This vulnerability directly maps to CWE-416, which describes the use of freed memory condition, and can be categorized under the broader ATT&CK technique T1499.004 for Network Denial of Service. The operational impact extends beyond simple denial of service to potentially enabling more sophisticated attacks, as memory corruption can lead to arbitrary code execution if attackers can control the memory layout or if the corruption affects critical kernel data structures. The vulnerability is particularly concerning because it can be exploited remotely without authentication requirements, making it accessible to anyone capable of sending crafted packets to a target system.
The exploitation of CVE-2017-18017 typically involves crafting specific TCP packets that match the conditions required to trigger the vulnerable code path within the kernel's netfilter framework. Attackers can leverage this vulnerability to cause system instability through kernel panics and system crashes, effectively creating a denial of service condition that impacts network connectivity and system availability. The memory corruption aspects of this vulnerability also pose risks for information disclosure or privilege escalation if the attacker can manipulate the memory corruption to gain control over kernel execution flow, though direct exploitation for privilege escalation requires additional attack vectors or conditions.
Mitigation strategies for this vulnerability include applying the official kernel patches released by the Linux kernel security team, which address the memory management issues in the tcpmss_mangle_packet function by implementing proper reference counting and memory deallocation procedures. System administrators should prioritize updating to kernel versions 4.11 or 4.9.36 and later, as these releases contain the necessary fixes that prevent the use-after-free condition. Additionally, temporary mitigations can involve removing or disabling iptables rules that utilize the xt_TCPMSS target, though this approach may impact legitimate network functionality and should be considered a short-term solution. Network administrators should also implement monitoring to detect unusual packet patterns that might indicate exploitation attempts, as the vulnerability requires specific packet construction to trigger the memory corruption behavior.