LINE Messaging Flaw Susceptible to “Man-in-the-Middle” Attacks

By: Takanori Nishiyama, SVP APAC & Japan Country Manager, Keeper Security 

LINE messaging platform has raised urgent concerns for millions of users and the organisations that depend on its widely used “super-app” ecosystem. The analysis reveals serious flaws—including message replay, plaintext exposure via stickers and URL previews, and impersonation risks enabled through man-in-the-middle techniques—underscoring a critical truth: encryption alone is not enough to guarantee secure communication.

In LINE’s case, users have limited means to validate whether a server is genuinely behaving as intended. This creates opportunities for cybercriminals to exploit that implicit trust. The fact that encrypted message data could be replayed out of context illustrates how subtle protocol design flaws can be weaponised, even in systems marketed as secure. 

Enterprises and public-sector organisations should treat this as a timely warning. Relying on consumer messaging platforms for operational or mission-critical communication carries real risk unless the protocols and server architecture have been independently validated, continuously audited and engineered to withstand compromise.. A zero-trust approach must extend beyond internal systems and include any external service that handles sensitive information. 

A core principle of true zero-trust and zero-knowledge architecture is that encryption keys must be exclusively controlled by the organisation or end user. When providers lack access to those keys, the security model remains resilient, even if infrastructure, servers or administrative interfaces are compromised. For enterprises and government bodies, this level of assurance is essential and should be treated as a baseline, not an aspiration. 

Privileged access must also be governed with strict controls. Strong authentication, authorization and session monitoring reduce the risk of credential misuse, limit lateral movement and help maintain operational integrity when other parts of the environment are under pressure.The LINE vulnerabilities are a sharp reminder that secure messaging is not only about encrypting content at rest or in transit. It’s about how messages are handled before the leave a device, how they are processed across the service’s infrastructure and whether the system design prevents the provider itself from becoming the weakest link."