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Threats / SolarWinds / CVE-2021-35247
CVE-2021-35247 · EUVD no mirror located · GCVE no mirror located Verified 2026-08-04

SolarWinds Serv-U vulnerability

SolarWinds Serv-U versions 15.2.5 and earlier contain improper input validation allowing attackers to send unsanitized queries, enabling unauthorized access and data manipulation.

Verdict

Today item — known-exploited.

This improper input validation vulnerability in Serv-U has been exploited in the wild. Attackers can bypass security controls by crafting malicious queries, potentially leading to unauthorized system access, data exfiltration, or lateral movement within affected environments.

CISA KEV Yes · 2022-01-215EPSS 0.03453 (verify live)6CVSS 4.33CISA SSVC Exploitation: active · Automatable: no · Impact: partial4
01

Is it exploitable?

— the evidence, ranked above the score
Reported exploitation
3 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
microsoft.com ↗cisa.gov ↗mandiant.com ↗Confirmed
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2022-01-21).
CISA KEV ↗Confirmed
Probability (EPSS)
EPSS 0.03453 — 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 4.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N) · Affected: SolarWinds, Serv-U. 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-20 Improper Input Validation.CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CVE.org ↗Reported
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · 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 14 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-20 · Improper Input Validation
02

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.

03

Why it matters

— the attack path, told twice: adversary, then board
1

Front door — unauthenticated access narrative 1

Attacker
I craft and send queries without proper sanitization to bypass input validation controls.
Business
Attackers gain unauthorized access to file transfer systems and sensitive data stored within Serv-U instances.
2

Keys to the kingdom — privilege/identity takeover narrative 2

Attacker
I execute commands or access resources through the unvalidated query interface to establish persistence.
Business
Compromised Serv-U servers become pivot points for lateral movement and extended network compromise.
3

Lateral reach — past segmentation narrative 3

Attacker
I extract credentials, configuration data, or customer files accessible through the vulnerable query mechanism.
Business
Data breach exposes confidential information, triggering regulatory penalties, customer notification costs, and reputational damage.
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’t

Established (cited)

  • KEV listing (CISA)
  • EPSS probability (FIRST)
  • 3 reported-exploitation source(s)
  • CVSS 4.3 — CNA-assigned (CVE.org)
  • CISA SSVC triage disposition (CVE.org ADP)
  • CWE weakness mapping — CNA-assigned (CVE.org)
  • Catalogued by SolarWinds (CNA)
  • Named finder/reporter credit (CVE.org)

    • 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.
    • Disclosure & credit2
    Catalogued by SolarWindsCNA
    Credited with finding itSolarWinds would like to thank Jonathan Bar Or of Microsoft (@yo_yo_yo_jbo) for reporting this vulnerabilityunspecified