Threats / Cisco / CVE-2020-3566

CVE-2020-3566 · EUVD no mirror located · GCVE no mirror located Verified 2026-09-19

Cisco IOS XR vulnerability

Cisco IOS XR improperly handles IGMP packets in DVMRP, allowing unauthenticated remote attackers to crash the IGMP process or exhaust memory, causing denial of service.

Verdict

Today item — known-exploited.

An unauthenticated remote attacker can send crafted IGMP packets to trigger resource exhaustion or immediate process termination in Cisco IOS XR systems, disrupting multicast routing availability without authentication requirements.

CISA KEV Yes · 2021-11-03⁵EPSS 0.03702 (verify live)⁶CVSS 8.6³CISA SSVC Exploitation: active · Automatable: yes · Impact: partial⁴

Is it exploitable?

— the evidence, ranked above the score

Reported exploitation

2 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.

sec.cloudapps.cisco.com ↗cisa.gov ↗Confirmed

Exploited in the wild

Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03).

CISA KEV ↗Confirmed

Probability (EPSS)

EPSS 0.03702 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.

FIRST EPSS ↗High

Severity / affected

CVSS 8.6 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H) · Affected: Cisco, IOS XR. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.

CVE.org ↗Reported

Weakness (CWE)

Mapped to CWE-400 Uncontrolled Resource Consumption — weakness family: Resource / availability.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.

CVE.org ↗Reported

CISA SSVC (triage disposition)

Exploitation: active · Automatable: yes · Technical Impact: partialCISA'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.

CISA-ADP ↗Reported

Currency of the record

CISA added this to its KEV catalog 431 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.

CISA KEV ↗CVE.org ↗Reported

WeaknessCWE-400 · Uncontrolled Resource ConsumptionResource / availability

Who’s exploiting it?

— attribution turns risk into urgency

Attribution 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.

Why it matters

— the attack path, told twice: adversary, then board

Front door — unauthenticated access narrative 1

Attacker

I send malformed IGMP packets to the target IOS XR device over the network.

Business

Network multicast services become unavailable, disrupting video distribution, IPTV, or other group communication applications.

Keys to the kingdom — privilege/identity takeover narrative 2

Attacker

I exploit improper packet handling to exhaust memory or crash the IGMP process on the routing device.

Business

Router instability forces emergency maintenance windows and potential failover to backup infrastructure, increasing operational costs.

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.

Coverage & confidence

— what we know, and what we don’t

Established (cited)

KEV listing (CISA)

EPSS probability (FIRST)

2 reported-exploitation source(s)

CVSS 8.6 — CNA-assigned (CVE.org)

CISA SSVC triage disposition (CVE.org ADP)

CWE weakness mapping — CNA-assigned (CVE.org)

Catalogued by cisco (CNA)

Coverage gaps — stated, not hidden

No EUVD / GCVE mirror in feed — single-authority dependency for the identifier.

EPSS & exposure are time-varying; verify live at the source.

Threat-actor attribution not established from feed data — absence of a name is not absence of compromise.

No finder is named in the public CVE record or the scanned vendor advisory — attribution scoped to the sources checked.

Disclosure & credit²

Catalogued by ciscoCNA

Credited with finding itNo finder is named in the public CVE record or the scanned vendor advisory — attribution scoped to the sources checked.

Found an error? Propose a correction.

A correction is a cited counter-claim — it must cite a source, gets reviewed by a second human, and is never auto-applied. See the correction convention and our identity npub1j7gt9ky7sfcrjn8jjee6693dkzvk4rahs0fk2suu7968dfns62zs3mqm3y.

With JavaScript off: publish a signed kind 1111 Nostr event tagging ["r", "https://basicsecurity.net/threat/cve-2020-3566.html"], ["v", "sha256:030e8007865a2a1437764939d64034926e1401e3d819f4472671d3d924d469b1"] (the record version), plus ["src", "<your source URL>"] and ["t", "correction"], per the correction convention. Our identity: npub1j7gt9ky7sfcrjn8jjee6693dkzvk4rahs0fk2suu7968dfns62zs3mqm3y. Cite-or-die: an event with no src is a coverage finding, not a proposal.