CVE-2023-4208 in Linux
Summary
by MITRE • 09/06/2023
A use-after-free vulnerability in the Linux kernel's net/sched: cls_u32 component can be exploited to achieve local privilege escalation.
When u32_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.
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Analysis
by VulDB Data Team • 12/04/2025
The vulnerability described in CVE-2023-4208 represents a critical use-after-free condition within the Linux kernel's traffic control subsystem, specifically in the cls_u32 component that handles packet classification. This flaw exists in the net/sched directory of the kernel source code and presents a significant security risk due to its potential for local privilege escalation. The issue stems from improper memory management during filter updates, creating a scenario where freed memory can be accessed by subsequent operations, potentially allowing malicious code to execute with elevated privileges.
The technical implementation of this vulnerability occurs during the execution of the u32_change() function when updating existing network filters. The core problem manifests when the entire tcf_result struct is copied into a new filter instance without proper handling of references to the old instance. This copying operation creates a race condition where the tcf_unbind_filter() function is invoked on the old filter instance during the success path of the update operation. This premature unbinding causes the filter_cnt counter of the associated class to be decremented, even though the class may still be referenced elsewhere in the system. When the class reaches zero references, it gets deleted from memory, but the original filter instance continues to reference the now-freed memory location.
The operational impact of this vulnerability extends beyond simple memory corruption, as it enables local privilege escalation attacks that can be leveraged by unprivileged users to gain root-level access to affected systems. Attackers can exploit this condition by carefully crafting network traffic that triggers the vulnerable code path, ultimately leading to arbitrary code execution with kernel-level privileges. The vulnerability affects systems running Linux kernels that include the problematic code changes, making it particularly concerning for server environments and systems where local user access might be possible.
This vulnerability maps directly to CWE-416, which describes the use of freed memory condition, and aligns with ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation." The attack surface is primarily limited to local users with access to the system, as exploitation requires the ability to trigger the specific network filter update mechanism. The recommended mitigation involves upgrading to a kernel version that includes the fix committed at 3044b16e7c6fe5d24b1cdbcf1bd0a9d92d1ebd81, which addresses the improper memory handling by ensuring that filter references are properly managed during update operations.
The fix for this vulnerability implements proper reference counting mechanisms and ensures that tcf_unbind_filter() operations occur at appropriate times to prevent premature class deletion. This approach aligns with established kernel security practices for memory management and prevents the race condition that leads to the use-after-free scenario. Organizations should prioritize patching systems running affected kernel versions, particularly those in critical infrastructure environments where local privilege escalation could lead to significant security breaches. The vulnerability demonstrates the importance of careful memory management in kernel space operations and highlights the need for thorough code review processes when implementing network filtering components that handle dynamic updates.