CVE-2025-13476 Overview
CVE-2025-13476 is a cryptographic vulnerability affecting Rakuten Viber's Cloak mode feature on Android and Windows platforms. The vulnerability stems from the use of a static and predictable TLS ClientHello fingerprint that lacks extension diversity. This implementation flaw enables Deep Packet Inspection (DPI) systems to trivially identify and block proxy traffic, effectively undermining the censorship circumvention capabilities that Cloak mode is designed to provide.
Critical Impact
Users relying on Viber's Cloak mode for privacy and censorship circumvention may be identified and their traffic blocked by surveillance systems, potentially exposing them to tracking or censorship in restrictive network environments.
Affected Products
- Rakuten Viber for Android v25.7.2.0g
- Rakuten Viber for Windows v25.6.0.0 through v25.8.1.0
Discovery Timeline
- 2026-03-05 - CVE-2025-13476 published to NVD
- 2026-03-05 - Last updated in NVD database
Technical Details for CVE-2025-13476
Vulnerability Analysis
This vulnerability is classified as CWE-327 (Use of a Broken or Risky Cryptographic Algorithm). The core issue lies in how Viber's Cloak mode implements its TLS handshake when establishing proxy connections. Rather than employing randomized or diverse TLS extensions in the ClientHello message, the implementation uses a static, predictable fingerprint pattern.
TLS fingerprinting is a well-known technique used by DPI systems to identify specific applications or protocols based on the unique characteristics of their TLS handshake. When an application uses a consistent, unchanging pattern in its ClientHello extensions, ordering, and cipher suite preferences, it creates a distinctive signature that network monitoring equipment can easily detect and categorize.
The practical impact of this vulnerability is significant for users in environments where network traffic is monitored or censored. The Cloak mode feature is specifically designed to disguise proxy traffic and help users bypass censorship. However, this static fingerprint defeats that purpose entirely, as DPI systems can identify Viber Cloak traffic with high reliability and either block or flag it for further inspection.
Root Cause
The root cause of CVE-2025-13476 is the implementation of TLS ClientHello messages with insufficient extension diversity. The Cloak mode feature fails to implement proper fingerprint randomization techniques that would make the traffic appear as generic HTTPS traffic. This includes static cipher suite ordering, predictable extension lists, and consistent TLS parameters that create a unique and identifiable signature.
Attack Vector
The attack vector involves passive network monitoring capabilities. An adversary with access to network infrastructure (such as an ISP, government agency, or network administrator) can deploy DPI systems to analyze TLS handshake patterns. When Viber Cloak mode traffic passes through these monitoring points, the static ClientHello fingerprint allows the DPI system to:
- Identify that the connection is using Viber Cloak mode
- Block the connection to prevent circumvention
- Log the connection for surveillance purposes
- Potentially de-anonymize users attempting to bypass censorship
This is a passive attack that does not require any interaction with the victim's device and can be performed at scale across entire network segments.
Detection Methods for CVE-2025-13476
Indicators of Compromise
- TLS connections from Viber applications exhibiting static ClientHello fingerprint patterns
- Network traffic analysis revealing consistent cipher suite ordering and extension combinations unique to Viber Cloak mode
- Blocked or disrupted Cloak mode connections in environments with active DPI monitoring
Detection Strategies
- Monitor network traffic for TLS ClientHello patterns matching known Viber Cloak fingerprints
- Implement TLS fingerprinting tools (such as JA3/JA3S) to identify vulnerable application versions
- Review network logs for patterns indicating Cloak mode traffic being flagged or blocked by DPI systems
- Audit application versions across enterprise environments to identify vulnerable installations
Monitoring Recommendations
- Deploy network monitoring solutions capable of TLS fingerprint analysis at network egress points
- Establish baseline traffic patterns to detect when Viber Cloak mode is being used in the environment
- Monitor for vendor updates and security advisories related to Viber applications
- Review CERT Vulnerability Report #772695 for additional technical indicators
How to Mitigate CVE-2025-13476
Immediate Actions Required
- Update Rakuten Viber to the latest available version on both Android and Windows platforms
- Advise users in high-risk environments to avoid relying solely on Viber Cloak mode for censorship circumvention until a fix is available
- Consider alternative privacy tools with proven fingerprint randomization capabilities
- Review organizational security policies regarding the use of circumvention tools
Patch Information
Users should obtain the latest version of Rakuten Viber from the official Viber Download Page. Check for updates that address the TLS fingerprinting issue described in this vulnerability. Monitor the CERT Vulnerability Report #772695 for vendor response and patch availability information.
Workarounds
- Use alternative censorship circumvention tools with robust fingerprint randomization (such as Tor Browser or properly configured VPN solutions)
- Layer Viber Cloak mode traffic through additional obfuscation mechanisms if Cloak mode must be used
- Avoid using Cloak mode in environments where DPI monitoring is known or suspected to be active
- Consider using Viber over a separate VPN connection to mask the application-level fingerprint
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
