New Opossum Attack Allows Hackers to Compromise Secure TLS Channels with Malicious Messages

**Opossum** is a cross-protocol application-layer desynchronization vulnerability that leverages differences between implicit and opportunistic TLS to enable MITM attackers to inject messages, cause persistent client-server desynchronization, and perform session hijacking, content manipulation, and XSS across protocols (HTTP, FTP, SMTP, POP3, LMTP, NNTP). The report describes attack mechanics (redirecting implicit TLS to opportunistic TLS and abusing upgrade mechanisms like RFC 2817), notes internet-wide scanning identified millions of potentially vulnerable hosts, and recommends disabling opportunistic TLS and applying vendor patches as primary mitigations.