# CVE-2021-22555: Linux Kernel heap out-of-bounds write in Netfilter x_tables

Source: https://junglewise.ai/threats/cve-2021-22555-linux-kernel-heap-out-of-bounds-write-in-netfilter-x-tables  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-10-06. Updated: 2026-05-15  
License: CC BY 4.0 with a link back

CVE-2021-22555 · Severity: critical · CVSS 7.8 · Exploited in the wild · Published 2025-10-06

A vulnerability in the Linux Kernel's networking subsystem allows a local user to gain full administrative control over a system. By exploiting a memory error in the firewall component, an attacker can bypass security restrictions and potentially crash the system or steal sensitive data. This issue is particularly significant because it has been observed being used in active attacks.

Technologies: [Linux Kernel](https://junglewise.ai/threats/technologies/kernel). Vendors: [Linux](https://junglewise.ai/threats/vendors/linux).

## Quick answers

**Is CVE-2021-22555 exploited in the wild?** Yes. CVE-2021-22555 is known to be exploited in the wild, first recorded on 6 October 2025. Treat it as urgent.

**How severe is CVE-2021-22555?** Critical severity, with a CVSS score of 7.8, published 6 October 2025.

**Which products and versions are affected?** Linux Linux Kernel v2.6.19-rc1 and later.

**Is there a fix for CVE-2021-22555?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2021-22555 attacked?** It is attacked with local access, in an operating system.

**Am I affected by CVE-2021-22555?** Check whether you run Linux Kernel. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A heap out-of-bounds write vulnerability exists in net/netfilter/x_tables.c in the Linux Kernel. The flaw is triggered when handling certain setsockopt calls, where an integer underflow or incorrect size calculation leads to memory corruption. An attacker with local access and the ability to create user namespaces can exploit this to overwrite kernel memory, leading to local privilege escalation (LPE) to root or a system crash (DoS). The vulnerability has been present since version 2.6.19-rc1 and is confirmed to be exploited in the wild. Patches are available in the upstream kernel.

## Affected products

- Linux Linux Kernel v2.6.19-rc1 and later

## Timeline

- 2021-04-20: disclosed: Initial discovery and disclosure by Google security researchers.
- 2025-10-06: kev added: Added to CISA Known Exploited Vulnerabilities catalog.

## Related threats

- [Linux kernel xen-netback double unmap memory corruption](https://junglewise.ai/threats/cve-2026-99287-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel PMC uninitialized clock rate in 32-bit Tegra](https://junglewise.ai/threats/cve-2026-99259-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel x86 Hyper-V memory access violation](https://junglewise.ai/threats/cve-2026-99258-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel io_uring signal loss in worker exit](https://junglewise.ai/threats/cve-2026-99228-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel mv643xx_eth NULL pointer dereference in PHY probe](https://junglewise.ai/threats/cve-2026-99197-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)