Fast Facts

• Researchers found critical security flaws in LINE’s Letter Sealing v2 encryption protocol.
• Vulnerabilities include message replay, plaintext data leaks, and impersonation attacks.
• LINE is a “super app” central to daily life in Japan, Taiwan, Thailand, and Indonesia.
• No comprehensive fixes are planned by LINE, leaving users exposed.
• Threats range from personal privacy invasion to state-sponsored cyberespionage.

The Cracks Beneath Asia’s Favorite Messenger

Imagine your daily life wrapped around a single app - one that handles your chats, bank transfers, news, and even government business. For millions in East Asia, LINE is that indispensable digital companion. But beneath its cheerful stickers and encrypted facade, researchers have discovered a security house of cards, primed for collapse.

This December at Black Hat Europe, cryptographers Thomas Mogensen and Diego De Freitas Aranha will reveal how LINE’s custom-built encryption, called Letter Sealing v2, contains several fatal flaws. Their findings, verified by hands-on attacks against genuine LINE apps, suggest that billions of private messages could be vulnerable to prying eyes - from petty cybercriminals to nation-state spies.

How Attackers Slip In: From Replays to Impersonation

So, what’s wrong with LINE’s security? First, its stateless design means a malicious server can replay any encrypted message - think of a mischievous butler handing you the same letter over and over, whenever they please. A simple “yes” sent today could be resent a year later, out of context, leading to confusion or even manipulation.

Second, LINE’s beloved stickers and web link previews quietly leak what you’re typing. When you type a message, the app checks with LINE’s servers to suggest stickers, sending your words in plain text. Similarly, sharing a link for a private meeting or document? The server sees the full URL - potentially including sensitive information.

Most troubling is the impersonation flaw: in group chats, any participant can forge messages as if they came from someone else, provided they’re working with a rogue server. It’s a digital ventriloquist act, with users none the wiser.

The Bigger Picture: Geopolitics and Old Lessons Unlearned

While these attacks require a compromised server - usually via social engineering or insider threats - the stakes are enormous. In countries like Japan and Taiwan, where LINE is woven into government and business infrastructure, a breach could ripple through civil society. And with tensions simmering in the region, the possibility of state-sponsored snooping isn’t far-fetched.

Worryingly, these aren’t new mistakes. Similar flaws plagued other messaging apps a decade ago. The researchers argue that LINE’s decision to build its own encryption, rather than adopting well-tested industry standards, left it open to old, avoidable errors. Despite being notified, LINE has offered only minor workarounds, with no major overhaul in sight.

WIKICROOK

• End: End-to-end encryption is a security method where only the sender and recipient can read messages, keeping data private from service providers and hackers.
• Man: A Man-in-the-Middle attack occurs when a hacker secretly intercepts and possibly alters communication between two parties, posing as each to the other.
• Replay Attack: A replay attack is when an attacker resends intercepted messages to trick systems or users, exploiting security flaws to gain unauthorized access.
• Impersonation Attack: An impersonation attack is when a cybercriminal poses as a trusted person or brand to trick victims and gain access to sensitive information.
• Social Engineering: Social engineering is the use of deception by hackers to trick people into revealing confidential information or providing unauthorized system access.