Linux Kernel

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

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 6 Oct 2025, 1,552 days after the CVE record was published.

How likely is exploitation in the next 30 days?

78.7%

EPSS score, percentile 99.6 of all scored records

How severe?

8.3 High

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 27 Oct 2025. Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

The CVE record links no vendor advisory, patch or mitigation.

Which versions are affected?

Affected

2.6.19-rc1

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

Required action, set by CISA

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

CISA due date for this action: 27 October 2025.

Advisories and patches

The CVE record links no vendor advisory, patch or mitigation.

Linked from CISA's catalog entry

Affected products and versions

As listed in the CVE record.

n/a Linux Kernel
Status	Versions
Affected	2.6.19-rc1

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

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.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

CISA added CVE-2021-22555 to its Known Exploited Vulnerabilities (KEV) catalog on 6 Oct 2025.
CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
The due date CISA sets for the required action is 27 Oct 2025.
The CVE record was published on 7 Jul 2021, 1,552 days before it was added to KEV.
The CVE record carries a CVSS v3.1 base score of 8.3 (High), given by its CNA.
FIRST's EPSS model puts the probability of exploitation in the next 30 days at 78.7% (percentile 99.6), as of 28 Oct 2026.
CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Heap Out-Of-Bounds Write in Netfilter IP6T_SO_SET_REPLACE

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

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

CWE-787: Out-of-bounds Write

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

Date	Event
7 Jul 2021	CVE record published
6 Oct 2025	Added to CISA KEV
27 Oct 2025	CISA due date for the required action
30 Dec 2025	CVE record last updated

References

More exploited Linux vulnerabilities

CVE-2026-46300: Linux Kernel Missing Initialization of a Variable Vulnerability (added 28 Oct 2026)
CVE-2025-39682: Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability (added 18 Sep 2026)
CVE-2025-39964: Linux Kernel Race Condition Vulnerability (added 18 Sep 2026)
CVE-2026-53266: Linux Kernel Out-of-Bounds Write Vulnerability (added 18 Sep 2026)
CVE-2026-53362: Linux Kernel Unspecified Vulnerability (added 27 Aug 2026)
CVE-2022-0995: Linux Kernel Out-of-Bounds Write Vulnerability (added 26 Aug 2026)