# CVE-2025-40206: netfilter: nft_objref: validate objref and objrefmap expressions

> A local attacker can crash the Linux kernel by referencing a synproxy object in the OUTPUT hook, causing infinite recursion and a stack overflow.

- **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:** 12 Nov 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-70100, BELL-CVE-2025-40206, DEBIAN-CVE-2025-40206, GHSA-3xw5-26jc-pqjh, ROOT-OS-DEBIAN-11-CVE-2025-40206, ROOT-OS-DEBIAN-12-CVE-2025-40206, ROOT-OS-DEBIAN-13-CVE-2025-40206, ROOT-OS-UBUNTU-2204-CVE-2025-40206, ROOT-OS-UBUNTU-2404-CVE-2025-40206, UBUNTU-CVE-2025-40206

## What is CVE-2025-40206?

CVE-2025-40206 is a high-severity denial of service vulnerability in Linux, published on 12 Nov 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:

netfilter: nft_objref: validate objref and objrefmap expressions

Referencing a synproxy stateful object from OUTPUT hook causes kernel
crash due to infinite recursive calls:

BUG: TASK stack guard page was hit at 000000008bda5b8c (stack is 000000003ab1c4a5..00000000494d8b12)
[...]
Call Trace:
 __find_rr_leaf+0x99/0x230
 fib6_table_lookup+0x13b/0x2d0
 ip6_pol_route+0xa4/0x400
 fib6_rule_lookup+0x156/0x240
 ip6_route_output_flags+0xc6/0x150
 __nf_ip6_route+0x23/0x50
 synproxy_send_tcp_ipv6+0x106/0x200
 synproxy_send_client_synack_ipv6+0x1aa/0x1f0
 nft_synproxy_do_eval+0x263/0x310
 nft_do_chain+0x5a8/0x5f0 [nf_tables
 nft_do_chain_inet+0x98/0x110
 nf_hook_slow+0x43/0xc0
 __ip6_local_out+0xf0/0x170
 ip6_local_out+0x17/0x70
 synproxy_send_tcp_ipv6+0x1a2/0x200
 synproxy_send_client_synack_ipv6+0x1aa/0x1f0
[...]

Implement objref and objrefmap expression validate functions.

Currently, only NFT_OBJECT_SYNPROXY object type requires validation.
This will also handle a jump to a chain using a synproxy object from the
OUTPUT hook.

Now when trying to reference a synproxy object in the OUTPUT hook, nft
will produce the following error:

synproxy_crash.nft: Error: Could not process rule: Operation not supported
  synproxy name mysynproxy
  ^^^^^^^^^^^^^^^^^^^^^^^^

## Which versions are affected by CVE-2025-40206?


## How do I fix CVE-2025-40206?

Apply the vendor's patch.

## Is CVE-2025-40206 being exploited?

There is no known exploitation of CVE-2025-40206 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40206 and fix it if it is affected.

What it is: CVE-2025-40206 is a high-severity denial of service vulnerability in Linux, published on 12 Nov 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-2025-40206 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-2025-40206/ (Markdown: https://insights.sbomify.com/CVE-2025-40206/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40206
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40206
- OSV: https://osv.dev/vulnerability/CVE-2025-40206

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40206/
Summaries are generated and may contain mistakes; the official description above is the record.