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Actively exploited vulnerability

Linux Kernel: memory corruption flaw under attack

CVE-2021-22555 · Linux Kernel Heap Out-of-Bounds Write Vulnerability

Concerns everyone who uses it7.8 high

What is affected

Linux Kernel is found on ordinary personal computers and phones, so this is not only a problem for companies. If you use it and have not updated since the fix was released, you are exposed.

Linux Kernel contains a heap out-of-bounds write vulnerability that could allow an attacker to gain privileges or cause a DoS (via heap memory corruption) through user name space.

CISA's description

What an attacker can do

This is a memory corruption flaw. It lets an attacker corrupt the program's memory by feeding it specially crafted content: a web page, a file or a network message. The result ranges from a crash to the attacker running their own code.

How it is reached
Needs access to the device or a file opened on it
Access the attacker needs
A basic user account
Does the victim have to do something?
No, works without the victim doing anything

What to do

  1. Update Linux Kernel now. Open the update or "About" screen, install whatever is offered and restart the app or device, because the fix is not active until you do.
  2. Turn on automatic updates so the next fix arrives without you having to look for it.
  3. If your device is too old to receive this update, stop using the affected app for anything sensitive and plan a replacement.

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Action required by CISA for US federal agencies

Vendor advisories and fixes

  • git.kernel.orggit.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/net/netfilter/x_tables.c
  • git.kernel.orggit.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/net/netfilter/x_tables.c
  • security.netapp.comsecurity.netapp.com/advisory/ntap-20210805-0010/
  • github.comgithub.com/google/security-research/security/advisories/GHSA-xxx5-8mvq-3528

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on October 6, 2025, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by October 27, 2025, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 79% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99.6% of all scored vulnerabilities.

Its CVSS severity score is 7.8 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

A heap out-of-bounds write affecting Linux since v2.6.19-rc1 was discovered in net/netfilter/x_tables.c. This allows an attacker to gain privileges or cause a DoS (via heap memory corruption) through user name space

NIST National Vulnerability Database
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