CriticalCVE-2021-27102

    Accellion FTA OS Command Injection Vulnerability (CVE-2021-27102)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Accellion FTA 9_12_411 and earlier is affected by OS command execution via a local web service call. The fixed version is FTA_9_12_416 and later.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. 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
    • Accellion FTA lte *..9_12_411

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    Metrics

    7.8

    Priority

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

    Source: NVD, CVSS v3

    7.8

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org

    3.7 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 89.6

    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
    228.18 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.

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    7.8 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
    7.8
    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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    3.7 %
    Moderate — model estimates 1-10% 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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    89.6
    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.

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    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 27 May 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-78

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The CVSS rating classes this as locally exploitable with low privileges. In practice, it was chained with other FTA flaws (SQL injection, SSRF) into an internet-reachable path to full appliance takeover, with high impact on confidentiality, integrity and availability. CISA flags it as used in ransomware campaigns. The typical pattern was data theft followed by leak-site extortion, aimed at organisations that use FTA to exchange sensitive documents with partners: law firms, government bodies, universities, healthcare and finance. Current exploitation likelihood is elevated rather than top-tier. Even so, any FTA instance still running and internet-exposed should be treated as critical: the product is legacy, the attack chain is publicly documented, and the appliance usually holds exactly the data attackers want for extortion. If no FTA exists in your estate, a documented negative check closes the item.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Identify every Accellion FTA instance, including test, legacy and DMZ boxes, and check the installed build. FTA 9_12_411 and earlier are affected. The issue is fixed in FTA_9_12_416 and later. Treat any unpatched instance as compromised until proven otherwise.
      • Patch or pull the plug: Where possible, upgrade immediately to FTA_9_12_416 or newer. FTA is a legacy product. If you cannot update within hours, disconnect the appliance from the internet, since external access was the entry point of the observed attack chain.
      • Preserve evidence before remediation: Snapshot or image the appliance before patching or rebuilding. Collect Apache access/error logs, FTA application logs and the MySQL contents. Patching does not remove web shells that were already planted.
      • Hunt for web shells: Review the web root (e.g. /home/seos/courier/) for unknown or recently modified .html/.php files. A known pattern is a web shell at about.html that enabled bulk download of stored files.
      • Rotate credentials: Replace all accounts, API/OAuth tokens, LDAP/AD bind accounts and SMTP credentials stored on the FTA, and invalidate active sessions. If compromise is suspected, start a data-exposure assessment and check your NIS2/GDPR reporting obligations (72 h window).
      • Plan migration: Treat replacement with a supported platform as an emergency change, not a roadmap item.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Move the appliance into an isolated DMZ segment. Allow inbound 443/TCP only from defined partner ranges or via VPN. Restrict the appliance's egress to required destinations (mail relay, LDAP) and drop everything else to block exfiltration and reverse shells.
      • Admin access: Expose admin paths and SSH (22/TCP) only to a dedicated jump-host network. Internal web-service endpoints must never be reachable from outside.
      • WAF/reverse proxy: Block, or tightly allowlist, external requests to /courier/document_root.html, /courier/sftp_account_edit.php and /courier/oauth.api. Also reject parameters containing shell metacharacters (;, |, `, $(, &&) and SQL-injection patterns.
      • Host hardening: Mount the web root read-only where feasible and enable file-integrity monitoring (AIDE/Tripwire) on /home/seos/courier/. Minimise the execution rights of the web-server user.
      • Data minimisation: Shorten retention on the FTA and purge stale files. Every stored file becomes extortion leverage if the box is breached.

    3. Runbook step 3

      Detection rules

      • Web-server access log: Watch for anomalous hits on document_root.html, sftp_account_edit.php, oauth.api and about.html, especially from unfamiliar IPs or with long, encoded parameters. SPL example: index=fta sourcetype=access_combined uri_path IN ("/courier/document_root.html","/courier/sftp_account_edit.php","/courier/about.html") | stats count by src_ip, uri_path
      • Process ancestry (auditd/EDR): Alert when the web-server or PHP process spawns a shell or system tools. Sigma shape: parent_image|endswith: ['/httpd','/php','/php-fpm'] AND image|endswith: ['/sh','/bash','/perl','/python','/wget','/curl','/nc']. auditd: -a always,exit -F arch=b64 -S execve -F uid=<webuser> -k fta_exec
      • File integrity: Alert on new or modified files in the web root outside maintenance windows. auditd: -w /home/seos/courier/ -p wa -k fta_webroot
      • Network telemetry (Zeek/Suricata): Look for unusually large outbound HTTPS responses from the FTA to single clients (bulk download) and for new outbound connections from the appliance to unknown destinations. Zeek approach: filter conn.log on id.orig_h == <FTA-IP> with resp_bytes well above baseline.
      • Application and database logs: Search MySQL and FTA logs for malformed queries or unexpected changes in account and token management. New API clients or admin accounts without a change ticket are compromise indicators.

    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

    Accellion FTA 9_12_411 and earlier is affected by OS command execution via a local web service call. The fixed version is FTA_9_12_416 and later.

    Source: 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
    accellionfta
    • ≤ 9_12_411
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

      Base

      The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

      Entry at MITRE CWE-78

    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

    • accellion.comproducts/ftax_refsource_MISC, Product, Vendor advisory
    • github.comaccellion/CVEs/blob/main/CVE-2021-27102.txtx_refsource_MISC, Broken link, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27102government-resource, US government resource
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