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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 27 Oct 2025.
Linux KernelOperating system · Linux

CVE-2021-22555

HIGH · 7.8 CVSS v3.1 Published 2021-07-07
Fix available
A fix is available. Upgrade to 4.4.267 / 4.9.267 or later.
See remediation →
96/100
Remediation priority · Urgent
In the wild High EPSS Public exploit Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Heap out-of-bounds write vulnerability in Linux kernel's netfilter x_tables module (net/netfilter/x_tables.c) present since v2.6.19-rc1 (approximately 2006). Exploitable through user namespaces to achieve local privilege escalation or cause denial of service via heap memory corruption.

MitigationApply kernel security patches; consider disabling user namespaces if not required to reduce attack surface.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 2.6.19, < 4.4.267>= 4.5, < 4.9.267>= 4.10, < 4.14.231>= 4.15, < 4.19.188>= 4.20, < 5.4.113>= 5.5, < 5.10.31>= 5.11, < 5.12
C400 FirmwareOperating system
Affected:all versions
C250 FirmwareOperating system
Affected:all versions
H410c FirmwareOperating system
Affected:all versions
H300s FirmwareOperating system
Affected:all versions
H500s FirmwareOperating system
Affected:all versions
H700s FirmwareOperating system
Affected:all versions
H410s FirmwareOperating system
Affected:all versions

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check your Linux kernel version
    Run 'uname -r' or 'cat /proc/version' to retrieve the running kernel version
    Affected if The kernel version falls within any of these ranges: >= 2.6.19 and < 4.4.267; >= 4.5 and < 4.9.267; >= 4.10 and < 4.14.231; >= 4.15 and < 4.19.188; >= 4.20 and < 5.4.113; >= 5.5 and < 5.10.31; >= 5.11 and < 5.12
  2. Determine if NetApp C400 firmware is in use
    Check the system or NetApp documentation for the C400 appliance firmware version
    Affected if The firmware version is any version (all versions are affected)
  3. Determine if NetApp C250 firmware is in use
    Check the system or NetApp documentation for the C250 appliance firmware version
    Affected if The firmware version is any version (all versions are affected)
  4. Determine if NetApp H-series firmware is in use
    Check the system or NetApp documentation for H410c, H300s, H500s, H700s, or H410s appliance firmware versions
    Affected if Any of these H-series firmware versions are in use (all versions are affected)
  5. Verify if user namespaces are accessible
    Test by running 'unshare --user whoami' as a non-root user; check /proc/sys/kernel/unprivileged_userns_clone (if present)
    Affected if User namespaces can be created by unprivileged users, which is required for the exploit to be triggered

You are affected if your Linux kernel version is within any of the listed vulnerable ranges, or if you are running any version of the listed NetApp firmware, AND user namespaces are accessible to local users.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.4.267 / 4.9.267 / 4.14.231 or later
Fixed in 4.4.2674.9.2674.14.231
Vendor patch git.kernel.org →
Interim mitigation

Apply kernel security patches; consider disabling user namespaces if not required to reduce attack surface.

Recommended fix High confidence

Upgrade to Linux kernel >= 4.4.267, >= 4.9.267, >= 4.14.231, >= 4.19.188, or a modern stable release (5.x/6.x) that includes the x_tables.c fix

  1. 1. Identify the currently running kernel version using 'uname -r'
  2. 2. Check if the system is using a supported kernel version within the affected ranges (< 4.4.267, < 4.9.267, < 4.14.231, or < 4.19.188)
  3. 3. For enterprise Linux distributions (RHEL, CentOS, Ubuntu, Debian), check for and install available kernel updates via the package manager (e.g., 'yum update' or 'apt-get update && apt-get upgrade')
  4. 4. For custom or upstream kernels, compile and install a kernel version >= 4.4.267 (for 4.4.x branch), >= 4.9.267 (for 4.9.x branch), >= 4.14.231 (for 4.14.x branch), or >= 4.19.188 (for 4.19.x branch)
  5. 5. For newer kernel branches (5.x and 6.x), ensure the version includes the x_tables.c fix (commit available in git.kernel.org)
  6. 6. Reboot the system to load the new kernel
  7. 7. Verify the fix is applied by checking 'uname -r' shows a patched version and confirming the kernel config includes x_tables/iptable_filter support
Caveat Kernel upgrades may require reboot and could introduce compatibility issues with custom modules or older drivers; ensure application compatibility before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Linux Kernel Exploited in the wild — priority engagement
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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Scan for this in your stack

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dbcve dependency scanner

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-22555 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data