A new SMTP Smuggling technique reportedly has the potential to bypass existing security protocols. Also it can enable attackers to send spoofed emails from seemingly legitimate addresses. This may breathe new life into email spam, despite its efficiency not decreasing throughout the last time.
What is SMTP Smuggling?
SMTP smuggling is a novice exploitation technique that manipulates the SMTP, a protocol used globally for sending emails since the inception of the Internet. This technique takes advantage of the differences in how outbound and inbound SMTP servers interpret the end-of-data sequence. It allows attackers to insert arbitrary SMTP commands and potentially send separate emails.
The core of SMTP smuggling lies in the discrepancies between how different servers handle the end-of-data sequence (<CR><LF>.<CR><LF>). By exploiting these differences, attackers can break out of the standard message data, smuggling in unauthorized commands. This technique requires the inbound server to accept multiple SMTP commands in a batch, a feature commonly supported by most servers today.
In-depth research into this vulnerability has revealed that SMTP servers of prominent email providers like Microsoft, GMX, and Cisco are susceptible to this exploit. While Microsoft and GMX have addressed these issues, Cisco has categorized the findings as a feature rather than a vulnerability, choosing not to alter the default configuration. Consequently, SMTP smuggling remains possible in Cisco Secure Email instances under default settings. Subsequently, the vulnerability was also identified in Microsoft’s Outlook SMTP server, further expanding the threat landscape.
What is the danger of SMTP vulnerability?
The implications of SMTP smuggling are far-reaching and alarming. Attackers can use this method to send forged emails that appear to be from credible sources, thereby circumventing checks designed to authenticate incoming messages, such as DomainKeys Identified Mail (DKIM), Domain-based Message Authentication, Reporting and Conformance (DMARC), and Sender Policy Framework (SPF).
In simple words, using this trick, frauds will be able to reach the corporate emails that were not receiving any spam before. Sure, the companies which opted for this security method are most likely aware of the dangers and have other protection methods running. But the very fact of them being exposed, too, creates a much bigger risk of cyberattacks.
Mitigating the effects of vulnerability
To mitigate the risks posed by SMTP smuggling, experts recommend several best practices. For Cisco users, changing settings from “Clean” to “Allow” is advised to avoid receiving spoofed emails with valid DMARC checks. Additionally, all email service providers and users should remain vigilant, regularly updating their systems and staying informed about the latest security developments.
Regularly monitor for unusual server activity and review security logs to detect potential breaches. Educate users about phishing and encourage skepticism about emails from unknown senders. Finally, consider consulting with cybersecurity professionals for advanced protective measures tailored to your specific infrastructure.
