Threats / Linux / CVE-2021-22555
CVE-2021-22555
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Linux Kernel vulnerability
Linux Kernel heap out-of-bounds write vulnerability in user namespace handling allows privilege escalation or denial of service through heap memory corruption.
Verdict
Today item — known-exploited.
A heap buffer overflow in the Linux Kernel's user namespace implementation enables local attackers to corrupt memory, escalate privileges to root, or crash the system. Active exploitation in the wild increases risk.
CISA KEV Yes · 2025-10-065EPSS 0.78684 (verify live)6Exploit Public PoC7CVSS 8.33CISA SSVC Exploitation: active · Automatable: no · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Public proof-of-concept exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
7 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2025-10-06).
Probability (EPSS)
EPSS 0.78684 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 8.3 (CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H) · Affected: Linux, Kernel. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-787 Out-of-bounds Write — weakness family: Memory safety.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 1552 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a functional/weaponized public exploit). Catalog entries: Metasploit module. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious user namespace configuration to trigger an out-of-bounds write in kernel heap memory.
Business
An unprivileged local user gains root-level code execution on affected systems.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I exploit the heap corruption to overwrite kernel data structures and bypass security controls.
Business
Compromised systems face complete loss of confidentiality, integrity, and availability; attackers gain persistent control.
3
Lateral reach — past segmentation narrative 3
Attacker
I trigger the vulnerability repeatedly to cause kernel panic and system crash.
Business
Critical infrastructure and services experience unplanned downtime and operational disruption.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05
Coverage & confidence
— what we know, and what we don’tEstablished (cited)
Coverage gaps — stated, not hidden
Disclosure & credit2
Catalogued by GoogleCNA
Credited with finding itAndy Nguyenunspecified