Linux Kernel Heap Out-of-Bounds Write (CVE-2021-22555)

Historical catalog analysis: CISA added this entry on October 06, 2025. The entry reflects catalog information retrieved on August 31, 2026, not a snapshot archived on the inclusion date. Check current vendor guidance before acting.

What the vulnerability is

CVE-2021-22555 is a heap out-of-bounds write vulnerability (CWE-787) located within the Linux Kernel. This flaw involves heap memory corruption that could be leveraged by an attacker to achieve privilege escalation or trigger a Denial of Service (DoS).

Exposure and applicability

The vulnerability is accessible through the user name space. Systems utilizing the Linux Kernel are potentially affected if they allow the conditions necessary for this specific heap write to occur. Because this flaw involves core kernel memory management, it applies to any infrastructure running susceptible versions of the kernel regardless of the distribution, provided the vulnerable code paths are reachable.

Remediation priorities

Remediation should be prioritized based on the asset’s exposure and the presence of user name space functionality. This vulnerability was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on October 6, 2025, indicating active exploitation in the wild.

Our analysis suggests the following priority actions:
1. Identify Affected Kernels: Infrastructure owners should audit all Linux instances to determine if they are running versions lacking the specific fixes identified in kernel.org commits 9fa492cdc160cd27ce1046cb36f47d3b2b1efa21 and b29c457a6511435960115c0f548c4360d5f4801d.
2. Apply Vendor Patches: Deploy updates provided by the OS vendor or apply the specific kernel commits to resolve the out-of-bounds write condition.
3. Restrict User Name Spaces: In environments where immediate patching is not feasible, defenders should evaluate if restricting access to user name spaces can reduce the attack surface available to unprivileged users.

How to validate remediation

Verification must go beyond a simple version string check, as kernel versions vary across distributions and backported patches may be present.

• Commit Verification: The most reliable method of validation is confirming that the specific fixes in commits 9fa492cdc160cd27ce1046cb36f47d3b2b1efa21 and b29c457a6511435960115c0f548c4360d5f4801d have been integrated into the running kernel image.
• Vendor Advisory Alignment: Cross-reference the installed package version with official vendor advisories (such as those from NetApp or Google Security Research) to ensure the specific CVE-2021-22555 fix is included in the build.

Limits and open questions

While patching addresses the root cause of the heap corruption, residual risk remains if other undiscovered vulnerabilities exist within the same subsystem. It is currently unknown whether this vulnerability has been utilized specifically in ransomware campaigns. Furthermore, while restricting user name spaces may limit the entry path, it does not remove the underlying vulnerability from the kernel code.

Source and editorial note

CVE-2021-22555: Linux Kernel Heap Out-of-Bounds Write Vulnerability · Source date: October 06, 2025 · Retrieved August 31, 2026.

Material facts are tied to the cited primary source. Recommendations are independent defensive analysis unless attributed to the source. Confirm consequential decisions against current authoritative guidance.

Archive date: October 09, 2025. The displayed post date is assigned three days after the source date to organize this retrospective archive; it does not mean this site published the analysis then. First published by this site: October 01, 2026 at 00:36 UTC.

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