# CVE-2025-40271: fs/proc: fix uaf in proc_readdir_de()

> A use-after-free vulnerability in the Linux kernel's proc filesystem allows local attackers to potentially execute arbitrary code or escalate privileges via race conditions.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** Yes
- **Published:** 6 Dec 2025
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2025-40271, DEBIAN-CVE-2025-40271, GHSA-r37x-wmxh-7hvh, ROOT-OS-DEBIAN-12-CVE-2025-40271, ROOT-OS-DEBIAN-13-CVE-2025-40271, ROOT-OS-UBUNTU-2204-CVE-2025-40271, ROOT-OS-UBUNTU-2404-CVE-2025-40271, UBUNTU-CVE-2025-40271

## What is CVE-2025-40271?

CVE-2025-40271 is a high-severity use after free vulnerability in Linux, published on 6 Dec 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:

fs/proc: fix uaf in proc_readdir_de()

Pde is erased from subdir rbtree through rb_erase(), but not set the node
to EMPTY, which may result in uaf access.  We should use RB_CLEAR_NODE()
set the erased node to EMPTY, then pde_subdir_next() will return NULL to
avoid uaf access.

We found an uaf issue while using stress-ng testing, need to run testcase
getdent and tun in the same time.  The steps of the issue is as follows:

1) use getdent to traverse dir /proc/pid/net/dev_snmp6/, and current
   pde is tun3;

2) in the [time windows] unregister netdevice tun3 and tun2, and erase
   them from rbtree.  erase tun3 first, and then erase tun2.  the
   pde(tun2) will be released to slab;

3) continue to getdent process, then pde_subdir_next() will return
   pde(tun2) which is released, it will case uaf access.

CPU 0                                      |    CPU 1
-------------------------------------------------------------------------
traverse dir /proc/pid/net/dev_snmp6/      |   unregister_netdevice(tun->dev)   //tun3 tun2
sys_getdents64()                           |
  iterate_dir()                            |
    proc_readdir()                         |
      proc_readdir_de()                    |     snmp6_unregister_dev()
        pde_get(de);                       |       proc_remove()
        read_unlock(&proc_subdir_lock);    |         remove_proc_subtree()
                                           |           write_lock(&proc_subdir_lock);
        [time window]                      |           rb_erase(&root->subdir_node, &parent->subdir);
                                           |           write_unlock(&proc_subdir_lock);
        read_lock(&proc_subdir_lock);      |
        next = pde_subdir_next(de);        |
        pde_put(de);                       |
        de = next;    //UAF                |

rbtree of dev_snmp6
                        |
                    pde(tun3)
                     /    \
                  NULL  pde(tun2)

## Which versions are affected by CVE-2025-40271?


## How do I fix CVE-2025-40271?

Apply the vendor's patch.

## Is CVE-2025-40271 being exploited?

There is no known exploitation of CVE-2025-40271 in the wild and no public exploit code.

Public exploit code:

- CVE-2025-40271 Modifed By MadEploits (github_poc): https://github.com/MadExploits/CVE-2025-40271
- Linux Kernel proc_readdir_de() 6.18-rc5 - Local Privilege Escalation (exploitdb)
- https://github.com/kaleth4/CVE-2025-40271 (github_poc): https://github.com/kaleth4/CVE-2025-40271

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40271 and fix it if it is affected.

What it is: CVE-2025-40271 is a high-severity use after free vulnerability in Linux, published on 6 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-40271 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-40271/ (Markdown: https://insights.sbomify.com/CVE-2025-40271/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40271
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40271
- OSV: https://osv.dev/vulnerability/CVE-2025-40271

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40271/
Summaries are generated and may contain mistakes; the official description above is the record.