In this study, we discover a new class of unknown side channels — “sequence-number-dependent” host packet counters — that exist in Linux/Android and BSD/Mac OS to enable TCP sequence number inference attacks. It allows a piece of unprivileged on-device malware to collaborate with an off-path attacker to infer the TCP sequence numbers used between a client and a server, leading to TCP injection and hijacking attacks. We show that the inference takes, in common cases, under a second to complete and is quick enough for attackers to inject malicious Javascripts into live Facebook sessions and to perform malicious actions on behalf of a victim user. Since supporting unprivileged access to global packet counters is an intentional design choice, we believe our findings provide important lessons and offer insights on future system and network design.