# CVE-2021-47245: netfilter: synproxy: Fix out of bounds when parsing TCP options

> An out-of-bounds read in the Linux kernel's netfilter synproxy TCP option parser can lead to denial of service or information disclosure.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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:** 21 May 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47245, GHSA-ch44-784c-7wgq, UBUNTU-CVE-2021-47245

## What is CVE-2021-47245?

CVE-2021-47245 is a high-severity memory corruption vulnerability in Linux kernel, published on 21 May 2024. 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: synproxy: Fix out of bounds when parsing TCP options

The TCP option parser in synproxy (synproxy_parse_options) could read
one byte out of bounds. When the length is 1, the execution flow gets
into the loop, reads one byte of the opcode, and if the opcode is
neither TCPOPT_EOL nor TCPOPT_NOP, it reads one more byte, which exceeds
the length of 1.

This fix is inspired by commit 9609dad263f8 ("ipv4: tcp_input: fix stack
out of bounds when parsing TCP options.").

v2 changes:

Added an early return when length < 0 to avoid calling
skb_header_pointer with negative length.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2021-47245?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47245?

Apply the vendor's patch.

## Is CVE-2021-47245 being exploited?

There is no known exploitation of CVE-2021-47245 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-2021-47245 and fix it if it is affected.

What it is: CVE-2021-47245 is a high-severity memory corruption vulnerability in Linux kernel, published on 21 May 2024. 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-2021-47245 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-2021-47245/ (Markdown: https://insights.sbomify.com/CVE-2021-47245/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47245
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47245
- OSV: https://osv.dev/vulnerability/CVE-2021-47245

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47245/
Summaries are generated and may contain mistakes; the official description above is the record.