By leveraging man-in-the-middle positioning, attackers can inject unexpected messages into secure channels, causing persistent desynchronization between clients and servers and breaking the integrity assumptions of encrypted communications. The technical execution of Opossum involves intercepting client connections intended for implicit TLS endpoints and redirecting them through opportunistic TLS channels. The vulnerability affects protocols where subtle differences exist between implicit and opportunistic TLS variants after the handshake completion. Internet-wide scans identified over 3 million hosts supporting both implicit and opportunistic TLS, though practical exploitation remains limited for non-HTTP protocols. Apache servers can be configured vulnerably using the SSLEngine option, demonstrating the widespread applicability of this attack vector across different server implementations. Cross-protocol desynchronization targeting servers with both implicit TLS (secure ports) and opportunistic TLS (upgrade mechanisms). The most effective mitigation strategy involves disabling opportunistic TLS support entirely and migrating to implicit TLS implementations. For HTTP implementations, attackers intercept HTTPS traffic destined for port 443 and establish plaintext connections to port 80 with TLS upgrade headers. Man-in-the-middle attackers redirect connections between TLS methods, causing clients to receive wrong server responses permanently. When a client attempts a connection using one method, attackers can redirect traffic to utilize the alternative method, creating critical mismatches in protocol expectations. After TLS handshake completion, simultaneous message transmission creates persistent desynchronization where clients receive incorrect responses to their requests. The Opossum attack represents a sophisticated evolution of protocol-level vulnerabilities that target the authentication mechanisms within TLS implementations. The attack builds upon the foundational weaknesses identified in the ALPACA attack while introducing novel exploitation vectors that circumvent existing countermeasures. This application layer desynchronization persists throughout the connection lifecycle, enabling sustained manipulation of client-server communications. Vendors, including Apache Foundation (CVE-2025-49812), Cyrus IMAPD, and others, have begun implementing patches and deprecation strategies to address this vulnerability systematically.
This Cyber News was published on cybersecuritynews.com. Publication date: Thu, 10 Jul 2025 10:55:16 +0000