CVE-2021-27104: Accellion FTA RCE Vulnerability

CVE-2021-27104 Overview

CVE-2021-27104 is a critical OS command injection vulnerability affecting Accellion FTA (File Transfer Appliance) version 9_12_370 and earlier. The vulnerability allows remote attackers to execute arbitrary operating system commands via crafted POST requests to various administrative endpoints. This flaw enables unauthenticated attackers to gain complete control over affected systems, potentially leading to data exfiltration, lateral movement, and full system compromise.

Critical Impact

This vulnerability is actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Attackers can execute arbitrary OS commands without authentication, enabling complete system takeover and data theft.

Affected Products

• Accellion FTA version 9_12_370 and earlier
• All Accellion FTA deployments prior to version FTA_9_12_380

Discovery Timeline

• 2021-02-16 - CVE-2021-27104 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-27104

Vulnerability Analysis

This vulnerability falls under CWE-78 (Improper Neutralization of Special Elements used in an OS Command), commonly known as OS Command Injection. The Accellion FTA appliance contains multiple administrative endpoints that fail to properly sanitize user-supplied input before passing it to system shell commands.

When a malicious POST request is crafted and sent to vulnerable admin endpoints, the attacker-controlled data is concatenated directly into shell commands without adequate input validation or sanitization. This allows threat actors to inject arbitrary OS commands that execute with the privileges of the web application process, typically with elevated system permissions.

The vulnerability is particularly severe because it requires no authentication, can be exploited remotely over the network, and has been actively weaponized by threat actors in real-world attacks targeting organizations using Accellion FTA for secure file transfers.

Root Cause

The root cause of CVE-2021-27104 is the improper handling of user-supplied input in administrative endpoint handlers. The application fails to implement proper input validation, output encoding, or parameterized command execution when processing POST request data. Instead of treating user input as data, the vulnerable code incorporates it directly into shell command strings, allowing metacharacters and command separators to break out of the intended command context.

Attack Vector

The attack vector is network-based, requiring the attacker to send specially crafted HTTP POST requests to the vulnerable administrative endpoints of the Accellion FTA appliance. The attack requires no user interaction and no prior authentication.

An attacker would craft a POST request containing OS command injection payloads using shell metacharacters such as semicolons (;), pipes (|), backticks (`), or command substitution syntax ($(command)). When the vulnerable endpoint processes this request, the injected commands execute on the underlying operating system.

The vulnerability has been actively exploited by threat actors, leading to its inclusion in CISA's Known Exploited Vulnerabilities catalog. Organizations should treat any unpatched Accellion FTA instance as potentially compromised.

Detection Methods for CVE-2021-27104

Indicators of Compromise

• Unusual POST requests to Accellion FTA administrative endpoints containing shell metacharacters or command injection patterns
• Unexpected processes spawned by the web server process on the FTA appliance
• Evidence of data exfiltration or unusual outbound network connections from the FTA system
• Web access logs showing requests with encoded or obfuscated payloads targeting admin endpoints
• Filesystem modifications or creation of unauthorized files in web-accessible directories

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block command injection patterns in POST request bodies
• Monitor HTTP access logs for suspicious requests targeting Accellion FTA administrative paths
• Deploy endpoint detection and response (EDR) solutions to identify anomalous process execution chains on FTA appliances
• Utilize network intrusion detection systems (IDS) with signatures for known command injection attack patterns

Monitoring Recommendations

• Enable comprehensive logging on Accellion FTA appliances and forward logs to a centralized SIEM
• Monitor for process execution anomalies, particularly shell processes spawned by web server components
• Track file integrity on critical system directories and web application paths
• Alert on unusual network traffic patterns from FTA systems, especially connections to unknown external hosts

How to Mitigate CVE-2021-27104

Immediate Actions Required

• Upgrade Accellion FTA to version FTA_9_12_380 or later immediately
• If immediate patching is not possible, take the FTA appliance offline until it can be updated
• Conduct forensic analysis on potentially exposed systems to identify signs of compromise
• Review and audit all data that may have been accessible through the FTA appliance
• Reset credentials for all accounts with access to the FTA system

Patch Information

Accellion has released version FTA_9_12_380 which addresses this command injection vulnerability. Organizations should upgrade to this version or later immediately. For detailed patch information and guidance, refer to the Accellion FTA Product Information page and the GitHub CVE-2021-27104 Details.

Given the critical nature of this vulnerability and its active exploitation, CISA has included it in their Known Exploited Vulnerabilities catalog, mandating federal agencies to remediate within specified timeframes.

Workarounds

• Restrict network access to Accellion FTA administrative interfaces using firewall rules, allowing only trusted IP addresses
• Implement network segmentation to isolate FTA appliances from critical internal networks
• Deploy a web application firewall (WAF) with rules to filter malicious command injection payloads
• Disable or remove unnecessary administrative endpoints if supported by your deployment
• Consider migrating to alternative secure file transfer solutions if patching cannot be performed promptly

# Example: Restrict access to FTA admin interfaces via iptables
# Allow only trusted management network
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP