CriticalCVE-2020-0601

    Microsoft Windows CryptoAPI Spoofing Vulnerability (CVE-2020-0601)

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

    What is affected, what to do

    A spoofing vulnerability exists in the way Windows CryptoAPI (Crypt32.dll) validates Elliptic Curve Cryptography (ECC) certificates.An attacker could exploit the vulnerability by using a spoofed code-signing certificate to sign a malicious executable, making it appear the file was from a trusted, legitimate source, aka 'Windows CryptoAPI Spoofing Vulnerability'.

    The vulnerability is being actively exploited; treat it as a priority. Update go to version 1.13.7. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows eq 10 Version 1803 for 32-bit Systems..10 Version 1803 for 32-bit Systems
    • Microsoft Windows eq 10 Version 1803 for x64-based Systems..10 Version 1803 for x64-based Systems
    • Microsoft Windows eq 10 Version 1803 for ARM64-based Systems..10 Version 1803 for ARM64-based Systems
    • Microsoft Windows eq 10 Version 1809 for 32-bit Systems..10 Version 1809 for 32-bit Systems
    • Microsoft Windows eq 10 Version 1809 for x64-based Systems..10 Version 1809 for x64-based Systems
    • Microsoft Windows eq 10 Version 1809 for ARM64-based Systems..10 Version 1809 for ARM64-based Systems
    • Microsoft Windows eq 10 Version 1709 for 32-bit Systems..10 Version 1709 for 32-bit Systems
    • Microsoft Windows eq 10 Version 1709 for x64-based Systems..10 Version 1709 for x64-based Systems
    • Microsoft Windows eq 10 Version 1709 for ARM64-based Systems..10 Version 1709 for ARM64-based Systems
    • Microsoft Windows eq 10 for 32-bit Systems..10 for 32-bit Systems
    • Microsoft Windows eq 10 for x64-based Systems..10 for x64-based Systems
    • Microsoft Windows eq 10 Version 1607 for 32-bit Systems..10 Version 1607 for 32-bit Systems
    • Microsoft Windows eq 10 Version 1607 for x64-based Systems..10 Version 1607 for x64-based Systems
    • Microsoft Windows Server eq version 1803 (Core Installation)..version 1803 (Core Installation)
    • 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)

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

    Metrics

    8.1

    Priority

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

    Source: NVD, CVSS v3

    8.1

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: Schneider Electric, CVE.org, OSV

    89.4 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.79

    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
    220.16 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
    8.1 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
    8.1
    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
    89.4 %
    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
    99.79
    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-2020-0601 (CurveBall) carries a CVSS v3 score of 8.1 and sits at the 99.8th EPSS percentile, placing it among the most likely-to-be-exploited vulnerabilities across the entire CVE corpus. The flaw in Crypt32.dll allows an attacker to craft ECC certificates that Windows accepts as legitimately chained to a trusted root CA, effectively nullifying the OS-level PKI trust model. For NIS2-scoped organisations, the risk is compounded because both code-signing trust and TLS connection integrity can be undermined simultaneously — two foundational security assumptions in KRITIS environments. CISA has not flagged known ransomware use, but the vulnerability has been documented in targeted espionage and MitM campaigns; prioritisation should reflect the organisation's exposure of internet-facing Windows systems and the criticality of its internal PKI infrastructure.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Apply the vendor patch immediately: Deploy Microsoft security update KB4528760 (Windows 10 1903/1909) or KB4534273 (Windows 10 1809), or the latest cumulative update for your specific build. All Windows 10 and Windows Server 2016+ systems are in scope — verify patch status in WSUS/SCCM and escalate any unpatched assets.
      • Update Go runtimes: Systems running Go stdlib < 1.13.0 (listed as affected) must be updated to a patched Go release. Patch available from the vendor — confirm the exact version in the upstream Go security advisory.
      • Prioritise internet-facing and PKI-adjacent systems: Web servers, reverse proxies, VPN gateways, code-signing infrastructure, and CA components are the highest-value targets for MitM exploitation and forged certificate chains — patch these first.
      • Enforce certificate revocation checking: Confirm that CRL/OCSP checking is active on all endpoints and has not been disabled via Group Policy — especially for code-signing certificates. Disable any policy that suppresses revocation errors.
      • Enable TLS inspection at the perimeter: Until the patch is fully deployed, activate SSL/TLS inspection on the perimeter proxy to detect ECC certificates with non-standard curve parameters being presented to internal clients.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Isolate systems with no legitimate internet requirement (internal CA servers, code-signing workstations) from direct internet access to reduce the MitM attack surface.
      • WAF/TLS inspection rule: Configure the perimeter proxy to reject or alert on TLS certificates using ECC curves with unrecognised OIDs or manipulated NIST curve parameters (P-256, P-384). Suricata starting point: alert tls any any -> any any (msg:"CVE-2020-0601 Suspicious ECC cert"; tls.cert_subject; content:"CN="; pcre:"/unusual_curve_oid/i"; sid:2020601; rev:1;) — tune to your environment.
      • AppLocker / WDAC policy hardening: Configure Windows Defender Application Control or AppLocker to allow execution only from explicitly trusted publishers. This directly limits the impact of forged code-signing certificates as an execution vector.
      • PKI hardening (least privilege): Restrict access to internal code-signing certificates and CA private keys to the minimum required set of accounts; mandate HSM storage for all CA private keys.
      • GPO — enforce revocation checking: Via Computer Configuration > Windows Settings > Security Settings > Public Key Policies > Certificate Path Validation Settings, set OCSP as mandatory and enable "Do not allow certificates with no revocation information."

    3. Runbook step 3

      Detection rules

      • Windows Security Event Log — Event ID 4688: Monitor for process creation events where the executable is signed by an unknown or newly appearing publisher, especially when the parent process is a browser, email client, or downloader. KQL: SecurityEvent | where EventID == 4688 | where SubjectUserName !in~ ("SYSTEM","LOCAL SERVICE") | where CommandLine contains ".exe"
      • Sysmon EID 7 (Image Load): Alert on DLLs or executables with ECC-based code-signing certificates whose issuer is not in the organisation's trusted CA list. Sigma shape: title: Suspicious ECC Code-Signing Certificate | logsource: category: image_load | detection: selection: Signed: 'true' SignatureStatus: 'Valid' | condition: selection and not filter_known_publishers
      • Network telemetry (Zeek/Suricata): Log TLS handshakes where the server certificate presents an ECC curve with an unknown OID or non-standard parameters. Filter Zeek ssl.log on validation_status != "ok" combined with curve not in {secp256r1, secp384r1, secp521r1}.
      • EDR process ancestry: Alert on signed binaries that execute immediately after download from a browser or email attachment and subsequently spawn PowerShell, cmd.exe, or WScript — a classic post-exploitation chain following abuse of a forged code-signing certificate.
      • Windows Defender / AV logs: Monitor for detections labelled "Exploit:Win32/CVE-2020-0601" or equivalent in Defender event logs (Event IDs 1116 and 1117).

    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 spoofing vulnerability exists in the way Windows CryptoAPI (Crypt32.dll) validates Elliptic Curve Cryptography (ECC) certificates.An attacker could exploit the vulnerability by using a spoofed code-signing certificate to sign a malicious executable, making it appear the file was from a trusted, legitimate source, aka 'Windows CryptoAPI Spoofing Vulnerability'.

    Source: schneider_csaf, NVD (NIST), OSV (Open Source Vulnerabilities), 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
    • 10 Version 1803 for x64-based Systems
    • 10 Version 1709 for ARM64-based Systems
    • 10 Version 1809 for ARM64-based Systems
    • 10 Version 1709 for x64-based Systems
    • 10 for 32-bit Systems
    • 10 for x64-based Systems
    • 10 Version 1803 for 32-bit Systems
    • 10 Version 1607 for 32-bit Systems
    • 10 Version 1607 for x64-based Systems
    • 10 Version 1809 for x64-based Systems
    • 10 Version 1809 for 32-bit Systems
    • 10 Version 1709 for 32-bit Systems
    • 10 Version 1803 for ARM64-based Systems
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    • —
    MicrosoftWindows Server
    • 2019
    • 2019 (Core installation)
    • 2016 (Core installation)
    • 2016
    • version 1803 (Core Installation)
    • —
    • —
    • —
    • —
    • —
    golanggo
    • 1.13 – < 1.13.7
    • 1.12 – < 1.12.16
    • 1.13.7
    • 1.12.16
    Schneider ElectricMicrosoft Windows
    • 10
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    FamilyVendorProductVersion
    Windowsmicrosoftwindows_10_1507—
    Windowsmicrosoftwindows_10_1607—
    Windowsmicrosoftwindows_10_1709—
    Windowsmicrosoftwindows_10_1803—
    Windowsmicrosoftwindows_10_1809—
    Windowsmicrosoftwindows_10_1903—
    Windowsmicrosoftwindows_10_1909—
    Windowsmicrosoftwindows_server_1803—
    Windowsmicrosoftwindows_server_1903—
    Windowsmicrosoftwindows_server_1909—
    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-004 – Vulnérabilité dans Microsoft Windows (clôturée 2020-03-24) (opens in a new tab)cert.cyber.gouv.fractively exploited
    Packet StormCurveBall Microsoft Windows CryptoAPI Spoofing Proof Of Concept (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormCurveBall Microsoft Windows CryptoAPI Spoofing Proof Of Concept (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • go.devcl/215905fix
    • go.googlesource.comgo/+/953bc8f391a63adf00bac2515dba62abe8a1e2c2fix
    • go.devissue/36834report
    • groups.google.comg/golang-announce/c/Hsw4mHYc470/m/WJeW5wguEgAJweb
    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2020-0601x_refsource_MISC, Patch, Vendor advisory
    • packetstormsecurity.comfiles/155961/CurveBall-Microsoft-Windows-CryptoAPI-Spoofing-Pro…x_refsource_MISC, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/155960/CurveBall-Microsoft-Windows-CryptoAPI-Spoofing-Pro…x_refsource_MISC, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2020-0601government-resource, US government resource
    • NVD (NIST)vuln/detail/CVE-2020-0601web
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2020-ALE-004 – Vulnérabilité dans Microsoft Windows (clôturée 2020-03-24)cert-fr
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