# CVE-2025-39866: fs: writeback: fix use-after-free in __mark_inode_dirty() > A use-after-free in the Linux kernel writeback subsystem allows local attackers to potentially execute 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:** 19 Sep 2025 - **Updated:** 15 Oct 2026 - **Also known as:** AZL-67578, AZL-75119, BELL-CVE-2025-39866, DEBIAN-CVE-2025-39866, GHSA-65ww-5pr7-752v, ROOT-OS-DEBIAN-11-CVE-2025-39866, ROOT-OS-DEBIAN-12-CVE-2025-39866, ROOT-OS-DEBIAN-13-CVE-2025-39866, ROOT-OS-UBUNTU-2204-CVE-2025-39866, ROOT-OS-UBUNTU-2404-CVE-2025-39866, UBUNTU-CVE-2025-39866 ## What is CVE-2025-39866? CVE-2025-39866 is a high-severity use after free vulnerability in Debian Debian Linux, published on 19 Sep 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: writeback: fix use-after-free in __mark_inode_dirty() An use-after-free issue occurred when __mark_inode_dirty() get the bdi_writeback that was in the progress of switching. CPU: 1 PID: 562 Comm: systemd-random- Not tainted 6.6.56-gb4403bd46a8e #1 ...... pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : __mark_inode_dirty+0x124/0x418 lr : __mark_inode_dirty+0x118/0x418 sp : ffffffc08c9dbbc0 ........ Call trace: __mark_inode_dirty+0x124/0x418 generic_update_time+0x4c/0x60 file_modified+0xcc/0xd0 ext4_buffered_write_iter+0x58/0x124 ext4_file_write_iter+0x54/0x704 vfs_write+0x1c0/0x308 ksys_write+0x74/0x10c __arm64_sys_write+0x1c/0x28 invoke_syscall+0x48/0x114 el0_svc_common.constprop.0+0xc0/0xe0 do_el0_svc+0x1c/0x28 el0_svc+0x40/0xe4 el0t_64_sync_handler+0x120/0x12c el0t_64_sync+0x194/0x198 Root cause is: systemd-random-seed kworker ---------------------------------------------------------------------- ___mark_inode_dirty inode_switch_wbs_work_fn spin_lock(&inode->i_lock); inode_attach_wb locked_inode_to_wb_and_lock_list get inode->i_wb spin_unlock(&inode->i_lock); spin_lock(&wb->list_lock) spin_lock(&inode->i_lock) inode_io_list_move_locked spin_unlock(&wb->list_lock) spin_unlock(&inode->i_lock) spin_lock(&old_wb->list_lock) inode_do_switch_wbs spin_lock(&inode->i_lock) inode->i_wb = new_wb spin_unlock(&inode->i_lock) spin_unlock(&old_wb->list_lock) wb_put_many(old_wb, nr_switched) cgwb_release old wb released wb_wakeup_delayed() accesses wb, then trigger the use-after-free issue Fix this race condition by holding inode spinlock until wb_wakeup_delayed() finished. Weakness: CWE-416 (Use After Free) ## Which versions are affected by CVE-2025-39866? It affects 2 products. Products: Debian Debian Linux, Linux kernel. ## How do I fix CVE-2025-39866? Apply the vendor's patch. ## Is CVE-2025-39866 being exploited? There is no known exploitation of CVE-2025-39866 in the wild and no public exploit code. Public exploit code: - Proof of concept for CVE-2025-39866 (UAF and race condition) (github_poc): https://github.com/byteReaper77/CVE-2025-39866 ## Other vulnerabilities in Debian Debian Linux - [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild) - [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild) - [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild) - [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild) - [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2025-39866 and fix it if it is affected. What it is: CVE-2025-39866 is a high-severity use after free vulnerability in Debian Debian Linux, published on 19 Sep 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-2025-39866 in the wild and no public exploit code. Affected product: Debian Debian 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-2025-39866/ (Markdown: https://insights.sbomify.com/CVE-2025-39866/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39866 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39866 - OSV: https://osv.dev/vulnerability/CVE-2025-39866 --- From sbomify Insights: https://insights.sbomify.com/CVE-2025-39866/ Summaries are generated and may contain mistakes; the official description above is the record.