In March 2021, Microsoft disclosed a massive cyberattack that compromised its Exchange Server email software and affected tens of thousands of organizations worldwide. The attack, which was attributed to a Chinese state-sponsored hacking group known as Hafnium, exploited four previously unknown vulnerabilities in Exchange Server to gain access to email accounts, steal data and install malware.
But the attack also had another, more alarming consequence: the hackers managed to steal Microsoft’s signing key, which is used to verify the authenticity and integrity of software updates. This meant that hackers could use the stolen key to sign malicious code and distribute it as legitimate updates to unsuspecting users.
How did this happen? How could a sophisticated company like Microsoft lose control of such a critical asset? And what does this mean for the security of software supply chains?
The answer lies in a comedy of errors involving human error, technical failures and organizational failures. These are some of the key factors that contributed to this unprecedented breach:
– Microsoft used a self-signed certificate for its Exchange Server updates, rather than a certificate issued by a trusted external authority. This made it easy for hackers to impersonate Microsoft and trick users into installing malicious updates.
– Microsoft stored its signing key on a server connected to the Internet, rather than isolating it in a secure offline environment. This exposed the key to possible compromise by anyone who could access the server.
– Microsoft failed to implement adequate security controls on its signature server, such as encryption, authentication, logging and monitoring. This allowed hackers to access the server undetected and alter its configuration.
– Microsoft did not follow the principle of least privilege, which means granting only the minimum level of access necessary for each user or process. Instead, it granted its signature server full administrative privileges, which allowed hackers to execute arbitrary commands and steal the key.
– Microsoft did not have a robust incident response plan in place that would have helped it detect, contain and mitigate the attack. Microsoft took several weeks to discover the breach and notify its customers, giving the hackers ample time to exploit the stolen key.
These errors illustrate how a single weak link can compromise an entire software supply chain and put millions of users at risk. They also highlight the need for software developers and vendors to adopt best practices to protect their keys and signing processes, such as:
– Use certificates from accredited authorities that apply strict verification and revocation policies.
– Storage of signature keys in hardware security modules (HSMs) that provide physical and logical protection against unauthorized access.
– Implement security controls such as encryption, authentication, logging and monitoring on signature servers and networks.
– Follow the principle of least privilege and apply the principle of defense in depth, which means using multiple layers of security to protect critical assets.
– Have a clear and comprehensive incident response plan that includes regular testing and updates.
The theft of Microsoft’s signing key was a wake-up call for the software industry and a reminder of the importance of protecting software supply chains. By learning from this incident and applying these best practices, software developers and vendors can prevent similar attacks in the future and ensure the reliability of their products.
