CVE-2021-22555
Linux Kernel Heap Out-of-Bounds Write Vulnerability
Linux Kernel
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
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 6 October 2025. US federal agencies were required to remediate it by 27 October 2025.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 78.7%, which is higher than 99.6% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Frequently asked questions
Is CVE-2021-22555 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2021-22555.
What does CVE-2021-22555 affect?
CVE-2021-22555 affects Linux Kernel. 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
How severe is CVE-2021-22555?
CVE-2021-22555 is rated HIGH with a CVSS base score of 8.3. EPSS puts the probability of exploitation in the next 30 days at 78.7%, higher than 99.6% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 27 October 2025.
How should organisations respond to CVE-2021-22555?
Apply the vendor patch for Linux Kernel as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.