CVE-2025-21700: 
Linux Kernel vulnerability analysis and mitigation

A use-after-free (UAF) vulnerability was discovered in the Linux kernel's network scheduler component, identified as CVE-2025-21700. The vulnerability was discovered by Lion Ackermann and affects the traffic control (tc) subsystem, specifically when replacing child qdisc from one parent to another. This vulnerability was reported in February 2025 and affects the Linux kernel's network scheduling functionality (NVD).

The vulnerability exists in the network scheduler's handling of qdisc (queueing discipline) replacement operations. The issue occurs when attempting to replace a child qdisc by moving it from one parent to another, which can lead to a use-after-free condition. The bug is triggered when using the 'tc qdisc replace' command with specific parameters that attempt to move a qdisc between different parents. This results in incorrect reference counting where the refcount increases by two and both the original and new parent point to the same qdisc, leading to potential memory corruption (Kernel Commit).

The vulnerability can be exploited for privilege escalation, allowing a local attacker to elevate their privileges to root. This is particularly severe as it affects the kernel's network traffic control system, a core component of Linux networking. The CVSS v3.1 base score of 7.8 (HIGH) indicates significant severity, with the vector string CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (NVD).

The vulnerability has been patched in the Linux kernel by adding validation that prevents moving a qdisc to a different parent during replace operations. The fix implements a preventive approach by explicitly disallowing such configurations rather than attempting a more complex solution. The patch adds a check to verify that the parent handle matches the existing parent before allowing the replace operation (Kernel Commit).