CriticalCVE-2020-1350

    Microsoft Windows DNS Server Remote Code Execution Vulnerability (CVE-2020-1350)

    Published In CISA KEV since Sources: Schneider Electric, CVE.org

    What is affected, what to do

    A remote code execution vulnerability exists in Windows Domain Name System servers when they fail to properly handle requests, aka 'Windows DNS Server Remote Code Execution Vulnerability'.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows Server eq 2019..2019
    • Microsoft Windows Server eq 2019 (Core installation)..2019 (Core installation)
    • Microsoft Windows Server eq 2016..2016
    • Microsoft Windows Server eq 2016 (Core installation)..2016 (Core installation)
    • Microsoft Windows Server eq 2008 for 32-bit Systems Service Pack 2..2008 for 32-bit Systems Service Pack 2
    • Microsoft Windows Server eq 2008 for 32-bit Systems Service Pack 2 (Core installation)..2008 for 32-bit Systems Service Pack 2 (Core installation)
    • Microsoft Windows Server eq 2008 for x64-based Systems Service Pack 2..2008 for x64-based Systems Service Pack 2
    • Microsoft Windows Server eq 2008 for x64-based Systems Service Pack 2 (Core installation)..2008 for x64-based Systems Service Pack 2 (Core installation)
    • Microsoft Windows Server eq 2008 R2 for x64-based Systems Service Pack 1..2008 R2 for x64-based Systems Service Pack 1
    • Microsoft Windows Server eq 2008 R2 for x64-based Systems Service Pack 1 (Core installation)..2008 R2 for x64-based Systems Service Pack 1 (Core installation)
    • Microsoft Windows Server eq 2012..2012
    • Microsoft Windows Server eq 2012 (Core installation)..2012 (Core installation)
    • Microsoft Windows Server eq 2012 R2..2012 R2
    • Microsoft Windows Server eq 2012 R2 (Core installation)..2012 R2 (Core installation)

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    10.0

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    10.0

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: Schneider Electric, CVE.org

    96.7 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.89

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    229.82 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: ANSSI / CERT-FR How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    10.0 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    10.0
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    96.7 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.89
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary

    Response and mitigation

    Why act now?

    Prioritisation rationale

    CVE-2020-1350 (SIGRed) carries a perfect CVSS 10.0 score with a fully network-based, unauthenticated attack vector and a scope change — a compromised DNS server directly threatens the entire Active Directory ecosystem, since DNS and AD authentication are architecturally inseparable. The EPSS score of 91.4 % (99.8th percentile) reflects the wide availability of working proof-of-concept code. For NIS2-scoped KRITIS organisations, this is a maximum-priority finding: a single internet-exposed Windows DNS server is sufficient as an entry point for full domain compromise. Even without the CISA ransomware-campaign flag, the combination of SYSTEM-level code execution and DNS's central role in every Windows environment justifies treating this as a Severity 1 incident until patched and verified.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Apply the patch immediately: Install Microsoft security update KB4569509 (July 2020 Patch Tuesday) on all affected Windows DNS servers (Server 2008 R2 SP1, 2012, 2012 R2, 2016, 2019). The patch is available — no deferral is acceptable for a CVSS 10.0 vulnerability.
      • Activate the registry workaround if patch deployment takes more than one hour: Set HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters\TcpReceivePacketSize to 0xFF00 and restart the DNS service. This caps the maximum TCP DNS packet size and breaks the exploit path through oversized SIG records.
      • Block inbound TCP port 53 from untrusted networks: Apply perimeter firewall and NSG/ACL rules to drop TCP/53 from the internet and DMZ segments to Windows DNS servers. Internal resolvers should never be directly internet-facing — audit and close any exceptions immediately.
      • Inventory all exposed DNS servers: Identify every Windows DNS instance acting as an authoritative resolver or internet-facing forwarder. Prioritise AD-integrated DNS zones in KRITIS/NIS2-scoped environments.
      • Audit privileged sessions on DNS servers: Because exploitation runs in the Local System Account context, immediately review all active sessions and running processes on affected hosts (netstat -ano, tasklist /v, whoami /all).

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Place DNS servers in a dedicated management VLAN; restrict inbound TCP/53 to known internal resolver IPs only. UDP/53 remains open for normal operation but is not the primary exploit vector for SIGRed.
      • IPS signature: Enable Snort/Suricata rules targeting oversized DNS TCP responses (> 65 535 bytes in SIG/RRSIG records). Starting point: alert dns any any -> $DNS_SERVERS any (msg:"CVE-2020-1350 SIGRed oversized SIG record"; dns.query; content:"|00 18|"; depth:2; dsize:>65000; sid:2020135001;) — tune and test before production deployment.
      • Harden DNS forwarder configuration: Configure internal Windows DNS servers to forward exclusively to known, hardened resolvers (e.g. an internal Unbound or BIND instance) — disable direct recursive lookups to the internet.
      • Least-privilege for the DNS service: Where AD integration does not mandate Local System, run the DNS service under a dedicated low-privilege service account. Where Local System is required, network isolation becomes the primary compensating control.
      • DNS firewall / Response Rate Limiting: Enable Windows DNS Response Rate Limiting (RRL) and, where available, a DNS RPZ feed to filter anomalous record types (SIG/RRSIG with unusual lengths) before they reach the resolver logic.

    3. Runbook step 3

      Detection rules

      • Windows Application Log — dns.exe crash: Event ID 1000 (Application Error) with dns.exe as the faulting application indicates exploit attempts destabilising the service. KQL: Event | where EventID == 1000 and RenderedDescription contains "dns.exe".
      • Sysmon EID 1 — Unexpected child processes of dns.exe: ParentImage ending in \dns.exe spawning anything other than dns.exe itself is a strong post-exploitation indicator. Sigma shape: detection: selection: ParentImage|endswith: '\dns.exe' filter: Image|endswith: '\dns.exe'.
      • Network telemetry — Oversized DNS TCP packets: In Zeek or Suricata, alert on DNS transactions over TCP with payload exceeding 65 000 bytes destined for Windows DNS server IPs. Zeek filter: dns.log | where proto == "tcp" and query_len > 60000.
      • Windows Security Log — SYSTEM process creation following DNS TCP activity: Event ID 4688 (New Process Created) with Creator Subject: SYSTEM and an unusual New Process Name (e.g. cmd.exe, powershell.exe, wscript.exe) on DNS server hosts, correlated with recent inbound DNS TCP connections.
      • Lateral movement from DNS server: Monitor for outbound SMB (TCP 445) or WinRM (TCP 5985/5986) connections originating from DNS server IPs — these are typical post-exploitation pivoting patterns after a successful SYSTEM-level compromise.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A remote code execution vulnerability exists in Windows Domain Name System servers when they fail to properly handle requests, aka 'Windows DNS Server Remote Code Execution Vulnerability'.

    Source: schneider_csaf, NVD (NIST), cvelistv5, CISA KEV Catalog

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    VendorProductAffected versionsFixed in
    MicrosoftWindows Server
    • 2012 R2
    • 2019
    • 2008 for x64-based Systems Service Pack 2 (Core installation)
    • 2008 R2 for x64-based Systems Service Pack 1
    • 2012 (Core installation)
    • 2008 for 32-bit Systems Service Pack 2 (Core installation)
    • 2008 for 32-bit Systems Service Pack 2
    • 2012
    • 2012 R2 (Core installation)
    • 2019 (Core installation)
    • 2008 R2 for x64-based Systems Service Pack 1 (Core installation)
    • 2016 (Core installation)
    • 2016
    • 2008 for x64-based Systems Service Pack 2
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
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    Schneider ElectricMicrosoft Windows
    • 10
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    FamilyVendorProductVersion
    Windowsmicrosoftwindows_server_2008r2
    Windowsmicrosoftwindows_server_2008—
    Windowsmicrosoftwindows_server_2012r2
    Windowsmicrosoftwindows_server_2012—
    Windowsmicrosoftwindows_server_2016—
    Windowsmicrosoftwindows_server_2019—

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    SourceReferenceMaturityPublished
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2020-ALE-016 – Vulnérabilité dans Microsoft Domain Name System (DNS) Server (clôturée 2020-10-12) (opens in a new tab)cert.cyber.gouv.fractively exploited
    Packet StormSIGRed Windows DNS Denial Of Service (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2020-1350x_refsource_MISC, Patch, Vendor advisory
    • packetstormsecurity.comfiles/158484/SIGRed-Windows-DNS-Denial-Of-Service.htmlx_refsource_MISC, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2020-1350government-resource, US government resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2020-ALE-016 – Vulnérabilité dans Microsoft Domain Name System (DNS) Server (clôturée 2020-10-12)cert-fr
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    <a href="https://intel.neosec.eu/en/cve/CVE-2020-1350"><img src="https://intel.neosec.eu/badge/cve/CVE-2020-1350.svg?lang=en" alt="CVE-2020-1350 – NEOSEC Intel" height="20"></a>