CriticalCVE-2023-29357

    Microsoft SharePoint Server Elevation of Privilege Vulnerability (CVE-2023-29357)

    Published In CISA KEV since Sources: SingCERT, pwn.watch, CVE.org

    What is affected, what to do

    Microsoft SharePoint Server Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft SharePoint Server 2019 to version 16.0.10399.20005. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft SharePoint Server 2019 lt 16.0.0..16.0.10399.20005
    Fixed in
    • Microsoft SharePoint Server 2019 16.0.10399.20005

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: SingCERT, pwn.watch, CVE.org

    99.98 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.98

    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
    256.84 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: CSA/SingCERT 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.

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    9.8 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
    9.8
    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“.

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    99.98 %
    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.

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    99.98
    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
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-303

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE earns its KEV spot: unauthenticated, network-reachable, and results in full administrator rights on a platform that typically holds high-value documents, cached credentials and lateral-movement pivots. Chained with CVE-2023-24955 it forms one of the most reliable pre-auth RCE paths against SharePoint and is confirmed to be used in ransomware operations, placing exploitation likelihood at the top of the scale. Treat any internet-exposed or cross-tenant reachable SharePoint 2019 farm as P1 with a 24-48 h patch deadline; purely internal farms stay P2 with a seven-day deadline, since the chain can equally be triggered from any compromised workstation on the LAN.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy Microsoft's June 2023 security update (KB5002402 for SharePoint Server 2019); the fixed build is 16.0.10399.20005 or higher. Bring Subscription Edition and 2016 farms to the current CU in the same change window.
      • Audit internet exposure of SharePoint web front-ends on TCP 80/443. If exposed, immediately front them with a pre-authenticating reverse proxy (Entra Application Proxy, WAF) or take them offline until the patch is verified.
      • After patching, rotate farm machine keys and validation keys (Update-SPMachineKey), plus farm account and app-pool credentials — JWT signing material may already be in attacker hands.
      • Compromise assessment: review every Site Collection Administrator, _spfarm account and newly created admin from the last 180 days; disable suspicious accounts and kill active sessions.
      • Preserve IIS logs and SQL-tier snapshots before the patch reboot — service restarts will overwrite part of the forensic trail.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: isolate SharePoint farms in a dedicated VLAN; permit only reverse-proxy/WAF ingress and block direct user-LAN access to intra-farm ports TCP 32843/32844/32845, 808 and 1433.
      • WAF/reverse-proxy rule: block or alert on HTTP requests carrying Authorization: Bearer towards /_api/, /_vti_bin/client.svc and /_layouts/15/ that are not preceded by a valid Entra/ADFS auth flow. Watch POSTs to /_api/web/siteusers and /_api/web/currentuser closely.
      • IIS hardening: cut anonymous access to /_api/ and /_vti_bin/ to the strict minimum; enable Request Filtering for oversized Authorization headers (> 8 KB) since spoofed JWTs are typically unusually long.
      • Least privilege: shrink the Farm Administrators group to under five named accounts; remove service accounts from local Administrators on WFE servers wherever SharePoint docs permit.
      • Compensating control for farms that cannot be patched immediately: expose SharePoint only through VPN or a zero-trust broker (Entra Private Access, ZScaler ZPA) with MFA pre-auth; end direct internet exposure now.
      • Egress filtering from WFE servers: restrict outbound traffic to defined Microsoft update, Entra and SQL endpoints — post-exploitation typically pulls a second-stage loader.

    3. Runbook step 3

      Detection rules

      • IIS access logs: HTTP-200 responses on /_api/web/currentuser without a prior /_login/ or /_trust/ request from the same client IP — the signature left by the public STAR Labs PoC. KQL sketch: W3CIISLog | where csUriStem has "/_api/web/currentuser" and scStatus == 200 | join kind=leftanti (W3CIISLog | where csUriStem has_any ("/_login","/_trust")) on cIP.
      • Windows Security EID 4624 logon type 3 on WFE servers followed by EID 4672 (Special Privileges) for accounts that normally only log on interactively, in particular token swaps to SHAREPOINT\system.
      • Sysmon EID 1 with w3wp.exe as parent of cmd.exe, powershell.exe, csc.exe, certutil.exe or rundll32.exe — strong indicator of webshell deployment after auth bypass. Sigma shape: parent_image endswith '\\w3wp.exe' and image endswith_any ('\\powershell.exe','\\cmd.exe','\\csc.exe').
      • File-system monitoring on %ProgramFiles%\Common Files\Microsoft Shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\ and _catalogs\masterpage for new .aspx, .ashx, .asmx files — SharePoint-specific webshell drop paths.
      • Network/Zeek/Suricata: TLS sessions to SharePoint carrying Authorization: Bearer headers of unusually large size. Suricata idea: alert http any any -> $SHAREPOINT_SERVERS any (msg:"Possible CVE-2023-29357 spoofed JWT"; flow:to_server,established; http.header; content:"Authorization|3a 20|Bearer"; dsize:>4000;).

    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

    Microsoft SharePoint Server Elevation of Privilege Vulnerability

    Source: CSA/SingCERT, pwn.watch, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

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

    Affected products
    Affected versionsFixed in
    MicrosoftMicrosoft SharePoint Server 2019
    • 16.0.0 – < 16.0.10399.20005
    • 16.0.10399.20005
    microsoftsharepoint_server
    • 2019
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-303Incorrect Implementation of Authentication Algorithm

      Base

      The requirements for the product dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect.

      Entry at MITRE CWE-303

    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
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    CSA/SingCERTActive Exploitation of Critical Vulnerability in Microsoft SharePoint Server (opens in a new tab)CSA/SingCERTactively exploited

    References & sources

    • Microsoft Security Update GuideMicrosoft SharePoint Server Elevation of Privilege VulnerabilityVendor advisory, Patch, Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-29357government-resource, US government resource
    • pwn.watchpwn.watch: Microsoft SharePoint Server Privilege Escalationpwn-watch
    • NVD (NIST)NVD: CVE-2023-29357pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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