Attack Chain

1. The attacker gains access to modify the configuration of an Emissary "place", typically through compromised credentials or a separate vulnerability allowing configuration changes.
2. The attacker sets the IN_FILE_ENDING or OUT_FILE_ENDING configuration value for a vulnerable place (e.g., UnixCommandPlace) to a malicious string containing shell metacharacters, such as backticks for command substitution.
3. The Emissary server is restarted or reloads the modified configuration, applying the attacker-controlled IN_FILE_ENDING or OUT_FILE_ENDING value.
4. A file is placed into the Emissary pickup directory, triggering the pipeline processing.
5. The file is routed through the configured places, eventually reaching the place with the malicious IN_FILE_ENDING or OUT_FILE_ENDING value.
6. The Executrix.getCommand() method constructs a shell command that includes the unsanitized IN_FILE_ENDING or OUT_FILE_ENDING value. The injected shell metacharacters are interpreted by the shell.
7. The constructed command is executed via /bin/sh -c, resulting in the attacker-specified commands being executed on the system with the privileges of the Emissary process.
8. The attacker achieves arbitrary code execution, potentially leading to data exfiltration, system compromise, or denial of service.

Impact

Successful exploitation of this vulnerability allows an attacker to execute arbitrary OS commands on the Emissary server, leading to full system compromise. The vulnerability affects all Emissary deployments where place configurations can be modified. The impact includes potential data breaches, malware installation, and complete control over the affected server. Given the nature of Emissary as a data processing framework, successful exploitation could allow attackers to compromise sensitive data being processed.

Recommendation

• Apply input validation and sanitization to the IN_FILE_ENDING and OUT_FILE_ENDING configuration parameters to prevent shell injection.
• Deploy the Sigma rule Detect Emissary InfileEnding OutfileEnding Injection Attempt to detect attempts to exploit this vulnerability by monitoring for shell metacharacters in configuration files.
• Implement strict access controls to limit who can modify place configurations in Emissary, as this is the primary attack vector.
• Enable DEBUG level logging in Emissary to expose the assembled shell command strings. This allows for forensic analysis and detection of injection attempts.