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Security Digest · Vulnerability record · Known exploited

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

Exploited in the wild (CISA KEV). A heap out-of-bounds write affecting Linux since v2.6.19-rc1 was discovered in net/netfilter/xtables.c.

CISA KEV · added 2025-10-06 CVSS 8.3 High EPSS 78.7% · p99.6

Updated By erroneo.us Security Desk12 references Seen in the digest for Oct 6, 2025

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

  • Yes, if you run Linux Kernel. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2025-10-06, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2025-10-27; treat that as the outer limit, not the target.
  • EPSS 78.7%, percentile 99.6: FIRST’s model estimates a 78.7% chance of exploitation activity in the next 30 days, higher than 99.6% of all scored CVEs (very high). EPSS is a probability, not a verdict.

What it is

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

CISA’s summary

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.

Weakness type: CWE-787.

Product: Linux Kernel · Assigned by: Google · Published: 2021-07-07

Affected and fixed versions

VendorProductAffected versions (per CVE record)
n/aLinux Kernel2.6.19-rc1 < unspecified

Severity scores

Scored byCVSSScoreVector
Google3.18.3 HighCVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
  • Attack vector: adjacent network only
  • Attack complexity: high (needs special conditions)
  • Privileges required: none (no login)
  • User interaction: none
  • Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

  • CISA KEV: added 2025-10-06; federal due date 2025-10-27. Known ransomware use: Unknown.
  • CISA required action: 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 SSVC assessment: exploitation active, automatable no, technical impact total.
  • FIRST EPSS (model of 2026-10-21): 78.68% probability of exploitation activity in the next 30 days; percentile 99.6.

Timeline

  • — CVE record published by Google
  • — Added to CISA’s Known Exploited Vulnerabilities catalog
  • — CISA remediation deadline for US federal agencies
  • — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

  1. Listed in CISA KEV notes (git.kernel.org) git.kernel.org
  2. Listed in CISA KEV notes (git.kernel.org) git.kernel.org
  3. Listed in CISA KEV notes (security.netapp.com) security.netapp.com
  4. Listed in CISA KEV notes (github.com) github.com

Vulnerability records

  1. CVE.org record CVE-2021-22555 cve.org
  2. NVD entry CVE-2021-22555 nvd.nist.gov

Government

  1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov

Other references from the CVE record and advisory

  1. packetstormsecurity.com packetstormsecurity.com
  2. packetstormsecurity.com packetstormsecurity.com
  3. packetstormsecurity.com packetstormsecurity.com
  4. packetstormsecurity.com packetstormsecurity.com
  5. packetstormsecurity.com packetstormsecurity.com

All exploited Linux vulnerabilities · Today’s Security Digest