Linux: Use After Free
CVE-2025-21703 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 e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c, 7f31d74fcc556a9166b1bb20515542de7bb939d1, 98a2c685293aae122f688cde11d9334dddc5d207 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 0.3% 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 e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c or 7f31d74fcc556a9166b1bb20515542de7bb939d1 or 98a2c685293aae122f688cde11d9334dddc5d207 or 7b79ca9a1de6a428d486ff52fb3d602321c08f55, 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 | 83c6ab12f08dcc09d4c5ac86fdb89736b28f1d31 up to, but not including, e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c; 216509dda290f6db92c816dd54b83c1df9da9e76 up to, but not including, 7f31d74fcc556a9166b1bb20515542de7bb939d1; c2047b0e216c8edce227d7c42f99ac2877dad0e4 up to, but not including, 98a2c685293aae122f688cde11d9334dddc5d207; 10df49cfca73dfbbdb6c4150d859f7e8926ae427 up to, but not including, 7b79ca9a1de6a428d486ff52fb3d602321c08f55; 3824c5fad18eeb7abe0c4fc966f29959552dca3e up to, but not including, 1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5; 356078a5c55ec8d2061fcc009fb8599f5b0527f9 up to, but not including, 6312555249082d6d8cc5321ff725df05482d8b83; f8d4bc455047cf3903cd6f85f49978987dbb3027 up to, but not including, 839ecc583fa00fab785fde1c85a326743657fd32; f8d4bc455047cf3903cd6f85f49978987dbb3027 up to, but not including, 638ba5089324796c2ee49af10427459c2de35f71; and 7 more ranges | e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c, 7f31d74fcc556a9166b1bb20515542de7bb939d1, 98a2c685293aae122f688cde11d9334dddc5d207, 7b79ca9a1de6a428d486ff52fb3d602321c08f55, 1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5, 6312555249082d6d8cc5321ff725df05482d8b83 |
| Siemens | SIMATIC S7-1500 TM MFP - BIOS | all versions | Not stated |
| Siemens | SIMATIC S7-1500 TM MFP - GNU/Linux subsystem | all versions | Not stated |
| Linux | Linux Kernel | 5.4.288 up to, but not including, 5.4.291; 5.10.232 up to, but not including, 5.10.235; 5.15.175 up to, but not including, 5.15.179; 6.1.121 up to, but not including, 6.1.129; 6.6.67 up to, but not including, 6.6.78; 6.12.6 up to, but not including, 6.12.14; 6.13 up to, but not including, 6.13.3; 6.14 | 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.78, 6.12.14 |
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: netem: Update sch->q.qlen before qdisc_tree_reduce_backlog() qdisc_tree_reduce_backlog() notifies parent qdisc only if child qdisc becomes empty, therefore we need to reduce the backlog of the child qdisc before calling it. Otherwise it would miss the opportunity to call cops->qlen_notify(), in the case of DRR, it resulted in UAF since DRR uses ->qlen_notify() to maintain its active list.
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/1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5
- git.kernel.org/stable/c/6312555249082d6d8cc5321ff725df05482d8b83
- git.kernel.org/stable/c/638ba5089324796c2ee49af10427459c2de35f71
- git.kernel.org/stable/c/7b79ca9a1de6a428d486ff52fb3d602321c08f55
- git.kernel.org/stable/c/7f31d74fcc556a9166b1bb20515542de7bb939d1
- git.kernel.org/stable/c/839ecc583fa00fab785fde1c85a326743657fd32
- git.kernel.org/stable/c/98a2c685293aae122f688cde11d9334dddc5d207
- git.kernel.org/stable/c/e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c
Other references
Recent Use After Free vulnerabilities
- 9.8 Critical severity
- 9.8 Critical severity
- 8.1 High severity
- 8.8 High severity
- 7.8 High severity
- 7.4 High severity
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