CVE-2021-27102: Accellion FTA OS Command Injection Vulnerability

CVE-2021-27102 is an OS command injection vulnerability in Accellion FTA. It can be abused through a local web service call, giving an attacker a path to run operating-system commands on the host. CISA has noted known ransomware use of this issue, so organizations still running Accellion FTA should treat it as a high-priority remediation item and confirm all details against the vendor advisory.

Command injection flaws in file-transfer appliances matter because these systems often sit at the edge of the network, handle sensitive data in transit, and may have broad internal reach. Successful abuse can lead to full host compromise, lateral movement, and data theft or encryption.

How it works

The weakness is classified under CWE-78 (OS Command Injection) and CWE-20 (Improper Input Validation). In this class of flaw, the application passes attacker-influenced input into a shell or system command without adequate sanitization or parameterization. According to the CISA summary, the vulnerability in Accellion FTA is exploited via a local web service call.

An attacker who can reach the vulnerable local web service endpoint can supply crafted input that the application incorporates into an operating-system command. If the service runs with elevated privileges, the injected commands execute in that context. Exact request format, parameters, and preconditions are not detailed here; defenders must obtain those specifics from the vendor advisory and any accompanying technical analysis. No exploit code or step-by-step reproduction is provided in public high-level summaries, and inventing them is unnecessary for defensive work.

Am I affected? How to find it in your systems

Accellion FTA is a managed file-transfer appliance. It typically appears as a dedicated server, virtual appliance, or hardened host in DMZ or partner-facing network segments, often fronted by reverse proxies or load balancers. Inventory efforts should focus on:

• Asset and CMDB records listing “Accellion,” “FTA,” or related file-transfer products.
• Network scans and banner grabs on hosts that present file-transfer or web-service interfaces associated with the product.
• Configuration-management and vulnerability-scanner results that flag Accellion FTA installations.
• Administrative consoles, license servers, and support contracts that still reference the product.

Compare installed versions and build levels against the fixed releases named in the vendor advisory; do not rely on version numbers from secondary sources. Because the attack path involves a local web service call, also identify any processes or services that expose local HTTP/HTTPS endpoints on FTA hosts and whether those endpoints are reachable only from localhost or from broader network ranges.

Telemetry that may indicate exploitation attempts includes unusual process spawns from the web-service or application user (especially shells or scripting interpreters), unexpected outbound connections from the FTA host, authentication or access anomalies around the local web service, and sudden changes in file-transfer job behavior or system configuration. Correlate these with web-service access logs if they are retained. Absence of such signals does not prove the system is clean; it only means no clear evidence was observed in the logs you examined.

How to remediate

Patch first. CISA’s required action is to apply updates per vendor instructions. Obtain the official advisory and fixed software packages directly from the vendor, validate integrity, and schedule installation according to your change process. After patching, verify the running version matches the remediated release and re-scan with your vulnerability management tools.

Beyond the patch, harden the appliance class:

• Restrict management and local web-service interfaces to the minimum set of administrative hosts via host firewalls and network ACLs.
• Run the service under least privilege where the product architecture allows it.
• Disable unused features, connectors, and legacy protocols.
• Ensure centralized logging of application, web-service, and process-creation events, with retention sufficient for incident review.
• Keep the underlying OS and supporting components current in line with vendor guidance.

Confirm every configuration change against the vendor’s supported hardening documentation so you do not break supported functionality or void support.

If you can't patch immediately

If an immediate update is not possible, reduce exposure while you prepare the patch window:

• Network segmentation: isolate FTA hosts from general user and server networks; allow only required file-transfer and administrative flows.
• Access control: limit who can reach the local web service and management interfaces; prefer allow-lists over broad internal reachability.
• Virtual patching / WAF: if a reverse proxy or web application firewall sits in front of the service, apply vendor- or community-supplied rules that block known malicious patterns for this vulnerability class—only after validating they do not disrupt legitimate traffic. Treat this as temporary.
• Feature disablement: turn off any non-essential local web-service functionality the product permits.
• Heightened monitoring: alert on process creation from the application account, unexpected shell invocation, and anomalous outbound connections; increase log verbosity temporarily if performance allows.
• Credential and session hygiene: rotate administrative credentials and review active sessions after any suspicious activity.

These controls lower likelihood and impact but do not replace the vendor update. Schedule the official patch as soon as practicable.

If your data may have been exposed

Actively exploited vulnerabilities, especially those with documented ransomware use, frequently precede data theft or encryption. If FTA hosts were unpatched and reachable during the period of known exploitation, assume potential compromise until you can demonstrate otherwise through forensic review of the appliance, connected systems, and data-transfer logs. Preserve evidence, follow your incident-response plan, and engage legal, privacy, and communications stakeholders as required by your regulatory environment. As a simple additional check, individuals can run a free exposure scan of their work email addresses against known breach data sets to see whether those addresses already appear in public compilations of compromised credentials or leaks.