# CVE-2023-53635: netfilter: conntrack: fix wrong ct->timeout value

> A logic error in the Linux kernel netfilter subsystem allows attackers to read incorrect connection tracking timeout values, potentially leaking internal state information.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 Oct 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-76410, BELL-CVE-2023-53635, DEBIAN-CVE-2023-53635, GHSA-v8v9-wq25-mffx, ROOT-OS-DEBIAN-11-CVE-2023-53635, ROOT-OS-UBUNTU-2204-CVE-2023-53635, UBUNTU-CVE-2023-53635

## What is CVE-2023-53635?

CVE-2023-53635 is a high-severity vulnerability in Linux kernel, published on 7 Oct 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

netfilter: conntrack: fix wrong ct->timeout value

(struct nf_conn)->timeout is an interval before the conntrack
confirmed.  After confirmed, it becomes a timestamp.

It is observed that timeout of an unconfirmed conntrack:
- Set by calling ctnetlink_change_timeout(). As a result,
  `nfct_time_stamp` was wrongly added to `ct->timeout` twice.
- Get by calling ctnetlink_dump_timeout(). As a result,
  `nfct_time_stamp` was wrongly subtracted.

Call Trace:
 <TASK>
 dump_stack_lvl
 ctnetlink_dump_timeout
 __ctnetlink_glue_build
 ctnetlink_glue_build
 __nfqnl_enqueue_packet
 nf_queue
 nf_hook_slow
 ip_mc_output
 ? __pfx_ip_finish_output
 ip_send_skb
 ? __pfx_dst_output
 udp_send_skb
 udp_sendmsg
 ? __pfx_ip_generic_getfrag
 sock_sendmsg

Separate the 2 cases in:
- Setting `ct->timeout` in __nf_ct_set_timeout().
- Getting `ct->timeout` in ctnetlink_dump_timeout().

Pablo appends:

Update ctnetlink to set up the timeout _after_ the IPS_CONFIRMED flag is
set on, otherwise conntrack creation via ctnetlink breaks.

Note that the problem described in this patch occurs since the
introduction of the nfnetlink_queue conntrack support, select a
sufficiently old Fixes: tag for -stable kernel to pick up this fix.

## Which versions are affected by CVE-2023-53635?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2023-53635?

Apply the vendor's patch.

## Is CVE-2023-53635 being exploited?

There is no known exploitation of CVE-2023-53635 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-53635 and fix it if it is affected.

What it is: CVE-2023-53635 is a high-severity vulnerability in Linux kernel, published on 7 Oct 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-53635 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2023-53635/ (Markdown: https://insights.sbomify.com/CVE-2023-53635/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-53635
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-53635
- OSV: https://osv.dev/vulnerability/CVE-2023-53635

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-53635/
Summaries are generated and may contain mistakes; the official description above is the record.