CriticalCVE-2021-27104

    Accellion FTA OS Command Injection Vulnerability (CVE-2021-27104)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Accellion FTA 9_12_370 and earlier is affected by OS command execution via a crafted POST request to various admin endpoints. The fixed version is FTA_9_12_380 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_370

    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: CVE.org

    56.7 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.06

    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
    252.50 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
    56.7 %
    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
    99.06
    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

    This flaw is network-reachable without authentication or user interaction and gives full control of the appliance. It ranks among the CVEs most likely to be exploited, and CISA flags it for known ransomware use. In practice it was used in large-scale data-theft-and-extortion campaigns against government bodies, universities, law firms, and financial and healthcare organisations that relied on FTA to exchange sensitive documents. Any FTA instance still running and internet-facing should be handled as an active incident at top priority. Patching alone is not enough: a compromise assessment and retirement of the discontinued product are mandatory.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Find every instance: Enumerate all Accellion FTA appliances using your CMDB, external attack-surface scans and DNS records (e.g. fta.*, securefile.*). Treat any instance on 9_12_370 or earlier as presumed compromised, not merely vulnerable.
      • Patch or pull it offline: Upgrade immediately to FTA_9_12_380 or later. FTA is a vendor-discontinued legacy product. If you cannot upgrade within hours, cut internet exposure by blocking inbound TCP 443/80 at the perimeter.
      • Preserve evidence first: Before rebooting or patching, take disk and memory images and export the web server, application and database logs. These logs live on the appliance and are lost on reinstall.
      • Rotate credentials: Reset all FTA admin accounts, service accounts, LDAP/AD bind credentials and API keys stored on the appliance. Assume anything held there has been exfiltrated.
      • Scope data exposure: Inventory the files exchanged via FTA and assess notification duties: GDPR Art. 33 (72 h) and the NIS2 early warning (24 h) to BSI or your national CSIRT. Both apply wherever exfiltration cannot be ruled out.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Move the appliance into an isolated DMZ with default-deny egress, so second-stage downloads and bulk exfiltration are blocked. Allow access to internal systems (AD, file servers) only through explicit, minimal rules.
      • Lock down admin interfaces: Make admin endpoints reachable only from a management network or via MFA-protected VPN. Enforce a source-IP allowlist on all admin paths at the reverse proxy or WAF.
      • WAF/IPS rules: Block POST requests to admin endpoints from non-allowlisted sources. Drop parameter values containing shell metacharacters (;, |, `, $(, &&) and common payload strings (wget, curl, /bin/sh, base64 -d). Enable any vendor signatures for CVE-2021-27104 in Suricata/Snort or your NGFW.
      • Integrity monitoring: Put file integrity monitoring on the appliance's web-root directories. Any new .php file should raise a high-severity alert.
      • Strategic replacement: Because the product is end-of-life, plan migration to a supported managed-file-transfer platform. Compensating controls only buy time.

    3. Runbook step 3

      Detection rules

      • Web access logs: Look for POSTs to admin paths from external or unknown IPs, especially with unusually large bodies or URL-encoded shell metacharacters. SPL example: index=web host=fta* method=POST uri_path="*admin*" NOT src_ip IN (<mgmt_ranges>) | stats count by src_ip, uri_path
      • Process ancestry (auditd/EDR): Alert when the web server (httpd/apache/php) spawns shells or network tools. auditd rule: -a always,exit -F arch=b64 -S execve -F uid=<webserver_uid> -k fta_exec, alerting on exe in (/bin/sh, /bin/bash, /usr/bin/wget, /usr/bin/curl, /usr/bin/perl, /usr/bin/python).
      • Web shell indicators: Watch for new or modified PHP files in the appliance's courier web directory; public reporting documented a web shell at about.php. Also check for requests to unusual .php files carrying file-download parameters, and for unexpected log deletions.
      • Network telemetry (Zeek/Suricata): Flag large outbound volumes from the FTA IP (conn.log orig_bytes well above baseline). Other signals are new egress to never-seen destinations and bursts of file-object downloads from a single source IP.
      • Database/application: Look for unknown admin accounts or altered configuration records in the FTA database. This matters because public incident reporting shows this flaw was chained with other FTA bugs (SQL injection, SSRF).

    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_370 and earlier is affected by OS command execution via a crafted POST request to various admin endpoints. The fixed version is FTA_9_12_380 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_370
    • —

    Vendors and products

    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-2021-27104.txtx_refsource_MISC, Broken link, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27104government-resource, US government resource
    Embed badge

    Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.

    Badge preview for CVE-2021-27104
    <a href="https://intel.neosec.eu/en/cve/CVE-2021-27104"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-27104.svg?lang=en" alt="CVE-2021-27104 – NEOSEC Intel" height="20"></a>