Final answer:
A vulnerability in a bare-metal cloud server that allows attackers to implant a malicious backdoor in its firmware.
Step-by-step explanation:
A vulnerability that enables attackers to implant a malicious backdoor in a bare-metal cloud server is known as a "firmware vulnerability". Firmware is a type of software that is embedded in hardware devices. In this case, the attackers exploit a weakness in the server's firmware to gain unauthorized access and control over the server.
Once the attackers have implanted the backdoor in the firmware, they can remotely access the server, steal sensitive data, modify settings, or even launch further attacks on the system.
It is crucial for organizations to regularly update and patch the firmware of their server to address any known vulnerabilities and maintain a strong security posture.