CVE-2025-21857 net/sched: cls_api: fix error handling causing NULL dereference
A NULL pointer dereference in the Linux kernel's traffic control subsystem allows local attackers to cause a system crash.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21857?
CVE-2025-21857 is a medium-severity vulnerability in Linux kernel, published on 12 Mar 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
net/sched: cls_api: fix error handling causing NULL dereference
tcf_exts_miss_cookie_base_alloc() calls xa_alloc_cyclic() which can return 1 if the allocation succeeded after wrapping. This was treated as an error, with value 1 returned to caller tcf_exts_init_ex() which sets exts->actions to NULL and returns 1 to caller fl_change().
fl_change() treats err == 1 as success, calling tcf_exts_validate_ex() which calls tcf_action_init() with exts->actions as argument, where it is dereferenced.
Example trace:
BUG: kernel NULL pointer dereference, address: 0000000000000000 CPU: 114 PID: 16151 Comm: handler114 Kdump: loaded Not tainted 5.14.0-503.16.1.el9_5.x86_64 #1 RIP: 0010:tcf_action_init+0x1f8/0x2c0 Call Trace: tcf_action_init+0x1f8/0x2c0 tcf_exts_validate_ex+0x175/0x190 fl_change+0x537/0x1120 [cls_flower]
Weakness: CWE-476
Which versions are affected by CVE-2025-21857?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21857?
Apply the vendor's patch.
Is CVE-2025-21857 being exploited?
There is no known exploitation of CVE-2025-21857 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-58511, BELL-CVE-2025-21857, DEBIAN-CVE-2025-21857, GHSA-6rpq-7wmg-7qjv, ROOT-OS-UBUNTU-2404-CVE-2025-21857, UBUNTU-CVE-2025-21857.