Threats / Cisco / CVE-2026-20131
CVE-2026-20131
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-09-02
Cisco Secure Firewall Management Center (FMC) vulnerability
Cisco Secure Firewall Management Center contains a deserialization vulnerability in its web management interface allowing unauthenticated remote code execution as root.
Verdict
Today item, not a backlog item.
An unauthenticated remote attacker can exploit unsafe deserialization of untrusted data to execute arbitrary Java code with root privileges on affected FMC instances, enabling full device compromise and lateral network movement.
CISA KEV Yes · 2026-03-195Ransomware use Flagged5EPSS 0.31231 (verify live)6CVSS 103CISA SSVC Exploitation: active · Automatable: yes · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
15 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 2026-03-19), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.31231 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 10 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H) · Affected: Cisco, Secure Firewall Management Center (FMC). Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-502 Deserialization of Untrusted Data — weakness family: Injection.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: yes · 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 15 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.
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 serialized Java object and send it to the FMC web management interface without authentication.
Business
The organization's firewall management infrastructure is directly accessible to remote attackers without credential barriers.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
The vulnerable deserialization handler processes my object and instantiates arbitrary classes, executing my embedded payload.
Business
Attackers gain root-level code execution on the central firewall management platform, the highest-trust security appliance.
3
Lateral reach — past segmentation narrative 3
Attacker
I establish persistence and move laterally through the network using the compromised FMC as a pivot point.
Business
The breach extends from the management layer to protected internal networks, enabling ransomware deployment and data exfiltration at scale.
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 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
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