CVE-2023-4207 in Linux
Summary
by MITRE • 09/06/2023
A use-after-free vulnerability in the Linux kernel's net/sched: cls_fw component can be exploited to achieve local privilege escalation.
When fw_change() is called on an existing filter, the whole tcf_result struct is always copied into the new instance of the filter. This causes a problem when updating a filter bound to a class, as tcf_unbind_filter() is always called on the old instance in the success path, decreasing filter_cnt of the still referenced class and allowing it to be deleted, leading to a use-after-free.
We recommend upgrading past commit 76e42ae831991c828cffa8c37736ebfb831ad5ec.
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Analysis
by VulDB Data Team • 12/04/2025
The vulnerability described in CVE-2023-4207 represents a critical use-after-free flaw within the Linux kernel's traffic control subsystem, specifically in the net/sched/cls_fw component. This issue manifests as a local privilege escalation vector that leverages improper memory management during filter updates. The vulnerability stems from a fundamental flaw in how the kernel handles filter state transitions when updating existing traffic control filters, creating a window where freed memory can be accessed by subsequent operations.
The technical root cause lies in the fw_change() function implementation where the entire tcf_result structure is copied to a new filter instance without proper reference counting or memory lifecycle management. When a filter bound to a class undergoes an update operation, the system calls tcf_unbind_filter() on the old filter instance during the successful path of execution. This premature unbinding operation reduces the filter_cnt counter of the class structure, even though the class may still be referenced elsewhere in the system. The timing of this operation creates a race condition where the class structure can be freed while references to it still exist, leading to the use-after-free scenario that can be exploited by local attackers.
This vulnerability directly maps to CWE-416, which describes the use of memory after it has been freed, and aligns with ATT&CK technique T1068, which covers local privilege escalation through kernel vulnerabilities. The exploitation potential arises from the fact that the kernel's traffic control subsystem is accessible to local users through standard network configuration interfaces, making this a particularly concerning issue for systems where untrusted local users might exist. The use-after-free condition can be leveraged to execute arbitrary code with kernel privileges, effectively allowing attackers to escalate their access level from user to root.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it represents a fundamental memory safety issue that can lead to system instability and potential denial of service conditions. Systems running affected kernel versions are susceptible to crashes or unauthorized access when traffic control filters are modified, particularly in environments where network management operations are frequently performed. The vulnerability is particularly concerning in containerized environments or systems where network filtering is extensively used, as these scenarios increase the likelihood of triggering the problematic code path.
Mitigation strategies for CVE-2023-4207 focus primarily on kernel version upgrades that contain the fix for the memory management issue in the cls_fw component. The recommended approach involves applying the patch corresponding to commit 76e42ae831991c828cffa8c37736ebfb831ad5ec, which addresses the improper reference counting and memory lifecycle management during filter updates. Organizations should prioritize patching their kernel versions to ensure that the fix is properly implemented, as manual workarounds for this specific use-after-free condition are not practical. Additionally, system administrators should monitor for any unusual network behavior or kernel crashes that might indicate exploitation attempts, and consider implementing network access controls to limit local user privileges where possible. The vulnerability demonstrates the importance of rigorous memory management practices in kernel code and highlights the need for thorough testing of state transition logic in critical subsystems.