Applying the STRIDE Framework to Email Server Security

Email servers serve as the backbone of communication for organizations worldwide, making them prime targets for cyber threats. By employing the STRIDE threat modeling framework, organizations can systematically identify and mitigate risks. This article explores how STRIDE can be applied to bolstering email server security, with real-world examples illustrating its effectiveness.

Spoofing

Many vendors remain highly vulnerable to spoofing attempts. Research by Stellastra showed that as of May 2024, only 38.1% of cyber security companies had implemented a DMARC policy with at least a partial rejection of quarantining command for emails spoofing their domain, with 35.9% having not even implemented a DMARC policy at all. The same study showed that only 91% of companies had implemented a secure SPF record for their domain, with 9% having either no record or an insecure SPF record. There is, however strong growth in email authentication, emboldening email providers like Yahoo and Google to now require senders to implement SPF, DKIM, and DMARC.

Mitigation Strategies:

• Authentication Protocols: Implement SPF, DKIM, and DMARC to verify the legitimacy of email senders and protect against forged headers. If an email host requires SPF, DKIM, and DMARC from senders, as Google has done, then it can implement security at the core of its architecture.
• Two-Factor Authentication (2FA): Companies like Microsoft have found that enabling 2FA can block 99.9% of account compromise attacks, offering an added layer of security even if credentials are stolen.

Tampering

• CVE-2020-2671: A vulnerability in Oracle’s Email Center component allows attackers to manipulate email content.
• CVE-2024-39929: A flaw in Exim permits a local attacker to exploit an RCE vulnerability, potentially leading to unauthorized modifications of email transmissions.

Mitigation Strategies:

• Data Integrity Checks: Use hashing algorithms such as SHA-256, applied by signed DKIM keys to ensure email content hasn’t been tampered with.

Repudiation

Any vulnerability that allows unauthorized access to someone else’s account undermines non-repudiation. If multiple people, including hackers and legitimate users, can access an account, then non-repudiation is compromised. The CVE-2024-42009 XSS vulnerability in Roundcube Webmail allows for the hijacking of email-sending capabilities. Another vulnerability allowed hackers to fully compromise a system through the opening of an email, allowing remote-code execution, allowing the hacker arbitrary code execution, including the ability to impersonate emails.

Mitigation Strategies:

• Comprehensive Logging: Maintain detailed, tamper-proof logs to track user activities and system events. These records are invaluable during forensic investigations.
• Digital Signatures: The use of cryptographic signatures, such as those provided by PGP (Pretty Good Privacy), helps confirm the identities of senders and recipients. S/MIME certificates can also be used to strengthen non-repudiation.

Information Disclosure

Emails are particularly vulnerable to unauthorized disclosure in attacks such as Man-in-the-Middle (MitM) primarily due to insufficient encryption practices. When emails are transmitted over networks without encryption, they can be intercepted and read by malicious actors positioned between the sender and recipient. This lack of encryption in transit makes it easier for attackers to access sensitive information. Similarly, when emails are stored at rest, on servers without proper encryption, they become easy targets for data breaches, allowing unauthorized parties to access and extract private data. Even when emails are not actively being used, they remain susceptible to threats if they lack encryption. This persistent vulnerability underscores the necessity for robust encryption protocols and security measures to protect email communications against interception and unauthorized access in an increasingly digital world. Prone to MitM attacks

Mitigation Strategies:

• Encryption: Organizations like ProtonMail employ end-to-end encryption to ensure email content remains confidential, even if intercepted.
• Access Controls: Implement robust access control lists (ACLs) and role-based access control (RBAC) to confine data access strictly to authorized users, as seen in industries dealing with highly sensitive information, like healthcare and finance.

Denial of Service (DoS)

In 2021, a vulnerability allowed an attacker to prevent a user from sending encrypted emails. In 2024, the vulnerability CVE-2024-27359 meant that an infinite loop could affect WithSecure products, including WithSecure Email.

Mitigation Strategies:

• Rate Limiting: Services like Cloudflare provide rate limiting features to filter excessive requests and protect against DoS attacks.
• Redundancy and Failover: Companies like Amazon Web Services (AWS) advocate for layered redundancy and automated failover systems to maintain email service continuity during disruptions.

Elevation of Privilege

A real-world example of an elevation of privilege in an email server is CVE-2023-23397, a critical elevation of privilege vulnerability affecting Microsoft Outlook for Windows. It allows attackers to steal NTLM credentials without any user interaction by sending specially crafted malicious emails. The vulnerability CVE-2014-2781, caused by the incorrect exchange of keyboard and mouse data, allowed elevation of privilege through an on-screen keyboard. In 2017 the vulnerability CVE-2017-8758 was caused by improper handling of web requests, allowing for an elevation of privilege.

Mitigation Strategies:

• Least Privilege Principle: Adopt the principle of least privilege to ensure users only access what is necessary. Organizations, such as Facebook, regularly review permissions to minimize risk.
• Regular Audits: Conduct regular security audits to identify and close privilege escalation vulnerabilities. Penetration testing firms like Offensive Security provide services to simulate attacks and uncover weaknesses before they can be exploited.

Conclusion

The STRIDE framework is an effective tool for assessing risks in software, and when applied to an email server, server security can be increased. By learning from real-world incidents and applying targeted mitigation strategies, organizations can protect their critical communication infrastructure from diverse threats. With proactive threat modeling, email servers can be hardened, safeguarding data, reputation, and trust.