CriticalCVE-2019-9875

    Sitecore CMS and Experience Platform (XP) Deserialization Vulnerability (CVE-2019-9875)

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

    What is affected, what to do

    This vulnerability is a deserialization issue in the Sitecore Security.AntiCSRF module, allowing an authenticated attacker to execute arbitrary code by sending a serialized .NET object in the HTTP POST parameter __CSRFTOKEN. It affects Sitecore CMS version 9.1 and is actively exploited in the wild. Organizations using Sitecore CMS version 9.1 should immediately update to the recommended patches provided by Sitecore. The vulnerability is listed in the CISA KEV catalog, indicating its critical nature and the urgency for a response.

    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
    • Sitecore CMS and Experience Platform (XP) lte *..9.1

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

    Metrics

    8.8

    Priority

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

    Source: NVD, CVSS v3

    8.8

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: pwn.watch, CVE.org

    25.4 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 98.0

    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
    220.75 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.

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    actively exploited
    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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    8.8 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
    8.8
    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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    25.4 %
    Elevated — model estimates 10-50% exploitation likelihood.
    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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    98.0
    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
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 26 March 2025
    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-502

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable with low attack complexity and yields full code execution as the IIS worker process. The only barrier is a valid backend account, which attackers can readily obtain through default passwords, credential stuffing or phishing of content editors. Sitecore is widely deployed for corporate, public-sector and customer-portal sites. Off-the-shelf .NET deserialization tooling such as ysoserial.net makes weaponisation trivial, and exploitation likelihood should be treated as high. CISA does not flag known ransomware use, but compromised CMS servers are a common foothold for lateral movement into internal networks. Treat any instance with an internet-reachable backend as critical and lock it down within 24 hours. Internal-only CM servers behind MFA can follow in an expedited regular patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Identify every Sitecore CMS and Sitecore XP instance up to and including 9.1, covering CM, CD and staging servers. You can check the version via /sitecore/shell/sitecore.version.xml or the control panel. Handle internet-facing Content Management (CM) servers first.
      • Patch: A patch is available from the vendor. Look up the fixed version or hotfix in the Sitecore Security Bulletin for CVE-2019-9875. Prefer upgrading to a currently supported Sitecore release, since parts of the 9.x line are past end of support.
      • Lock down the backend now: Use IIS IP restrictions or an upstream reverse proxy so that /sitecore/, /sitecore/shell/, /sitecore/admin/ and /sitecore/login are reachable only from admin networks or over VPN. Disable the backend entirely on CD servers, as recommended in Sitecore's hardening guide.
      • Rotate credentials: Exploitation requires an authenticated account. Check the default sitecore\admin account for the well-known default password and change it. Disable dormant editor accounts, and force password resets for all backend users if there are signs of credential stuffing.
      • Compromise assessment: If you suspect successful exploitation, isolate the host and preserve memory and IIS logs. Hunt the webroot for new .aspx or .ashx files. Then rotate the machineKey, connection strings and service-account secrets in web.config and ConnectionStrings.config.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Separate CM from CD and do not publish CM directly to the internet. Use the host firewall to restrict outbound traffic from w3wp.exe on Sitecore hosts to required destinations only (SQL, Solr, xConnect). This blocks reverse shells and payload staging.
      • WAF/IPS rule: Inspect POST requests whose __CSRFTOKEN parameter carries serialized .NET objects. Common Base64 prefixes are AAEAAAD///// (BinaryFormatter) and /wE (ObjectStateFormatter/LosFormatter). Block, or at minimum alert on, abnormally long __CSRFTOKEN values, for example values several kilobytes in size.
      • IAM/least privilege: Enforce MFA on the Sitecore login, either directly or through SSO with MFA (Identity Server or an authenticating reverse proxy). Reduce the number of accounts with backend access to the minimum needed. Enable account lockout to slow brute-force attempts.
      • Endpoint hardening: Run the IIS app pool under a dedicated, low-privilege identity. Use WDAC/AppLocker or an EDR prevention rule to stop w3wp.exe from launching cmd.exe, powershell.exe, certutil.exe or rundll32.exe.
      • Configuration: Check whether the vendor advisory documents a configuration workaround for the Sitecore.Security.AntiCSRF module. Apply it only as described there, because ad-hoc edits under App_Config can break functionality.

    3. Runbook step 3

      Detection rules

      • EDR/Sysmon EID 1 (process ancestry): Alert when w3wp.exe spawns a shell or a LOLBin. Sigma shape: ParentImage|endswith: '\w3wp.exe' AND Image|endswith: ['\cmd.exe','\powershell.exe','\certutil.exe','\whoami.exe']. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "w3wp.exe" and FileName in~ ("cmd.exe","powershell.exe","whoami.exe").
      • WAF or reverse-proxy logs: Search POST bodies for __CSRFTOKEN= followed by AAEAAAD or /wE. IIS logs do not record POST bodies, so you need WAF or proxy data for this. SPL: index=waf http_method=POST request_body="*__CSRFTOKEN=AAEAAAD*".
      • IIS access logs: Look for successful logins followed by POST requests to /sitecore/shell/* from external or unusual source IPs. Also watch for unusually large cs-bytes values and bursts of HTTP 500 responses after POSTs to backend paths.
      • Windows Application log: Review ASP.NET event 1309 and .NET runtime errors that reference BinaryFormatter, ObjectStateFormatter, SerializationException or Sitecore.Security.AntiCSRF. These entries often indicate failed gadget-chain attempts.
      • Sysmon EID 11 / file integrity: Alert when w3wp.exe writes new .aspx, .ashx or .asmx files, or DLLs, into the webroot or \bin\. This is a classic webshell drop indicator.

    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

    Deserialization of Untrusted Data in the anti CSRF module in Sitecore through 9.1 allows an authenticated attacker to execute arbitrary code by sending a serialized .NET object in an HTTP POST parameter.

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

    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
    sitecorecms
    • ≤ 9.1
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-502Deserialization of Untrusted Data

      Base

      The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.

      Entry at MITRE CWE-502

    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

    References & sources

    • dev.sitecore.netDownloads.aspxx_refsource_MISC, Product, Vendor advisory
    • synacktiv.comblog.htmlx_refsource_MISC, Third-party advisory
    • synacktiv.comressources/advisories/Sitecore_CSRF_deserialize_RCE.pdfx_refsource_MISC, Exploit, Patch, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2019-9875government-resource, US government resource
    • pwn.watchpwn.watch: Sitecore CMS and Experience Platform (XP): 2 exploited vulnerabilities (CVE-2019-9874, CVE-2019-9875)pwn-watch
    • support.sitecore.comSitecore advisory: Sitecore CMS and Experience Platform (XP)pwn-watch
    • support.sitecore.comSitecore advisory: Sitecore CMS and Experience Platform (XP)pwn-watch
    • NVD (NIST)NVD: CVE-2019-9874pwn-watch
    • NVD (NIST)NVD: CVE-2019-9875pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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