Citrix ShareFile Improper Access Control Vulnerability (CVE-2021-22941)
Published In CISA KEV since Sources: CVE.org
What is affected, what to do
Improper Access Control in Citrix ShareFile storage zones controller before 5.11.20 may allow an unauthenticated attacker to remotely compromise the storage zones controller.
The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update sharefile_storagezones_controller to version 5.11.20. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Citrix ShareFile storage zones controller
eq 5.11.20..5.11.20
- Citrix ShareFile storage zones controller
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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: CVE.org
53.6 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.0
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- 252.39 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 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.
Read in glossary- 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“.
Read in glossary- 53.6 %
- 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- 99.0
- 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 · automatable · Technical impact: total
- Source: CISA Vulnrichment, as of 4 February 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-284
Response and mitigation
Why act now?
Prioritisation rationale
This flaw enables unauthenticated network takeover of the Storage Zones Controller, giving the attacker direct access to all files stored in the zone and an ideal pivot into internal networks, since the controller typically bridges the DMZ and internal SMB file shares. CISA added it to KEV on the basis of active exploitation including documented ransomware use; exploitation likelihood ranks among the highest in the KEV catalogue. Treat as P1 for any deployment with internet exposure or holding sensitive / personal data (NIS2 and GDPR relevance), and P2 only for strictly internal installations without external reachability — in both cases patch within 24-72 hours.
Runbook step 1
Immediate response (0-24 h)
- Upgrade the Citrix ShareFile Storage Zones Controller to version 5.11.20 or later immediately (Citrix hotfix packages available; all versions prior to 5.11.20 are vulnerable). Restart IIS after patching and verify via Citrix health check.
- If patching cannot happen within hours: take the Storage Zones Controller off the public internet or front it with a reverse proxy / WAF enforcing strict URL filtering, particularly on upload endpoints (
/upload-threaded-*,/cifs/,/sp/). - Rotate all ShareFile tenant API keys, storage account credentials and service accounts — a successful exploit may have exfiltrated them.
- Retroactively review IIS logs, ShareFile ConfigService logs and filesystem timestamps on the storage path for anomalies between deployment and patch date (compromise assessment before putting the host back in service).
- Incident readiness: consult the ShareFile data classification and determine whether personal data may be affected — if there is reasonable suspicion, trigger NIS2 and GDPR notification clocks (72 h).
Runbook step 2
Mitigation layers
- Network segmentation: place the Storage Zones Controller in a dedicated DMZ VLAN, inbound only TCP/443 from Citrix ShareFile SaaS backends (whitelist the IP ranges published by Citrix), outbound internet restricted to the Citrix FQDNs only.
- WAF / reverse-proxy rules: block requests to ShareFile upload endpoints without a valid signed upload token; filter abnormal Content-Length and Range headers; restrict HTTP methods to GET/POST/HEAD.
- IIS hardening: disable the WebDAV module unless required, tighten Request Filtering limits for URL and query-string length, deny anonymous writes on the storage path via NTFS ACLs.
- Endpoint hardening: EDR on the controller with script-block logging and AMSI enabled; block w3wp.exe child processes (cmd.exe, powershell.exe, certutil.exe) via EDR policy.
- IAM / least privilege: the Storage Zones Controller service account must not hold domain-admin or local-admin rights on neighbouring hosts; use a separate pull-only backup account for shadow copies of the shares.
Runbook step 3
Detection rules
- IIS access logs (W3SVC): POST requests to
/upload-threaded-2.aspxor/upload.aspxwithout a preceding authenticated session flow — Sigma shape:cs-method: POST AND cs-uri-stem contains "upload" AND cs(Referer) is null. - Sysmon EID 1 on the controller: w3wp.exe spawning cmd.exe, powershell.exe, cscript.exe or nltest.exe — the classic webshell execution chain. KQL:
DeviceProcessEvents | where InitiatingProcessFileName == "w3wp.exe" and FileName in~ ("cmd.exe","powershell.exe","cscript.exe"). - Filesystem monitoring (Sysmon EID 11) on ShareFile storage paths: new
.aspx,.ashx,.asmxfiles appearing outside scheduled deployments — strong webshell indicator. - Network telemetry (Zeek/Suricata): unexpected outbound connections from the controller (anything other than Citrix cloud FQDNs, in particular TOR exits or VPS providers like DigitalOcean / Hetzner), plus SMB lateral movement from the controller into internal subnets.
- Windows Security EID 4624/4672 with Logon Type 3 or 10 on the controller from unexpected source IPs; EID 4688 for
whoami,net group /domain,tasklistwithin short windows following an HTTP upload burst.
- IIS access logs (W3SVC): POST requests to
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
Improper Access Control in Citrix ShareFile storage zones controller before 5.11.20 may allow an unauthenticated attacker to remotely compromise the storage zones controller.
Source: 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 versions | Fixed in | ||
|---|---|---|---|
| — | Citrix ShareFile storage zones controller |
|
|
| citrix | sharefile_storagezones_controller |
|
|
Vendors and products
Weakness classes (CWE)
CWE-284Improper Access Control
Pillar
The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor.
Entry at MITRE CWE-284
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.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited |
References & sources
- support.citrix.comarticle/CTX328123x_refsource_MISC, Broken link, Vendor advisory
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-22941government-resource, US government resource
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-22941"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-22941.svg?lang=en" alt="CVE-2021-22941 – NEOSEC Intel" height="20"></a>