Bulletproof TLS Newsletter 
Bulletproof TLS Newsletter is a free periodic newsletter bringing you commentary and news surrounding SSL/TLS and Internet PKI, designed to keep you informed about the latest developments in this space. Maintained by Hanno Böck.
Current Issue
Vulnerabilities show fragility of STARTTLS
31 Aug 2021
A paper published at the USENIX security conference showed a large number of vulnerabilities in STARTTLS implementations. The author of this newsletter participated in this research.
STARTTLS is a mechanism that allows upgrading plaintext protocol connections to TLS. The research focused on STARTTLS in the communication between email clients and servers (SMTP, IMAP, POP3). It turns out this upgrading mechanism is fragile and can lead to a number of security problems.
In 2011, Postfix developer Wietse Venema discovered that when sending additional content together with the STARTTLS-initiating command in the same TCP packet, many servers would interpret this as part of the encrypted connection even though it was plaintext and could thus be injected by a man-in-the-middle attacker.
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