CVE-2025-13476

Executive Summary

CVE-2025-13476 identifies a critical weakness in Rakuten Viber's "Cloak mode" feature, designed to obfuscate proxy traffic for censorship circumvention. The vulnerability stems from the use of a static and predictable TLS ClientHello fingerprint, which lacks extension diversity. This allows Deep Packet Inspection (DPI) systems to trivially identify and block Viber Cloak mode traffic, completely undermining its intended purpose. The affected versions include Android v25.7.2.0g and Windows v25.6.0.0 through v25.8.1.0. As of this report, no official patch has been released, and the vendor has not provided a timeline for remediation.

Supply Chain & Package Security

This vulnerability is not related to an NPM, PyPI, Go, or Ruby package. It is a client-side application vulnerability in Rakuten Viber, a proprietary messaging application. The risk is primarily to end-users relying on Cloak mode for privacy and censorship circumvention, as their traffic can now be easily identified and blocked by network-level inspection systems.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder or an error in the NVD data. Based on the technical details, a more accurate assessment would be:

• Attack Vector (AV): Network (N) – The attack is performed remotely by a DPI system on the network path.
• Attack Complexity (AC): Low (L) – The predictable fingerprint is trivial to detect.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – The user does not need to perform any action beyond using Cloak mode.
• Scope (S): Unchanged (U) – The vulnerable component is the Viber client itself.
• Confidentiality (C): None (N) – No direct data breach.
• Integrity (I): None (N) – No data manipulation.
• Availability (A): Low (L) – The primary impact is denial of service (blocking of proxy traffic).

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L Estimated Score: 4.3 (Medium)

Threat Level: Medium. The vulnerability does not expose user data but completely negates the privacy feature of Cloak mode, making it ineffective against censorship.

Deep Dive & Mechanics

The vulnerability lies in the TLS ClientHello message generated by Viber's Cloak mode. In standard TLS connections, the ClientHello includes a variety of extensions (e.g., supported groups, signature algorithms, ALPN, etc.) that vary between implementations and even between different connections from the same client. This diversity makes it difficult for DPI systems to fingerprint specific applications.

Viber's Cloak mode, however, uses a static and predictable set of TLS extensions. This means every Cloak mode connection produces an identical ClientHello fingerprint. A DPI system can be configured with a simple rule to detect this specific fingerprint and block or throttle the traffic. This effectively nullifies the censorship circumvention capability, as the proxy traffic is now easily identifiable.

The root cause is a violation of CWE-327 (Use of a Broken or Risky Cryptographic Algorithm), specifically the use of a static, non-randomized TLS fingerprint. A secure implementation would randomize the order and selection of TLS extensions to mimic a standard browser or application, making detection significantly harder.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, the vulnerable behavior can be described conceptually:

Vulnerable Code (Conceptual):

# Pseudo-code for vulnerable Cloak mode TLS handshake
def create_client_hello():
    # Static, predictable fingerprint
    client_hello = {
        "version": "TLS 1.3",
        "cipher_suites": ["TLS_AES_128_GCM_SHA256"],
        "extensions": [
            {"type": "supported_groups", "data": "x25519"},
            {"type": "signature_algorithms", "data": "rsa_pss_rsae_sha256"},
            # ... other static extensions
        ]
    }
    return client_hello

Patched Code (Conceptual):

# Pseudo-code for patched Cloak mode TLS handshake
import random

def create_client_hello():
    # Randomized fingerprint to evade DPI
    client_hello = {
        "version": random.choice(["TLS 1.2", "TLS 1.3"]),
        "cipher_suites": random.sample(ALL_CIPHERS, k=random.randint(3, 6)),
        "extensions": random.sample(ALL_EXTENSIONS, k=random.randint(5, 10))
    }
    return client_hello

The patched version would introduce randomness in the TLS ClientHello, making it indistinguishable from other TLS traffic.

Exploitation & Lab Setup

Red Team

Objective: Demonstrate that Viber Cloak mode traffic can be identified and blocked by a DPI system.

Lab Setup:

1. Attacker Machine: Linux with tshark or tcpdump and a DPI tool like nDPI or Bro/Zeek.
2. Target Machine: Windows or Android device with Viber installed and Cloak mode enabled.
3. Network: Both machines on the same network segment (e.g., a lab switch).

Steps:

1. Start packet capture on the attacker machine: tcpdump -i eth0 -w viber_cloak.pcap
2. On the target machine, enable Viber Cloak mode and initiate a connection (e.g., send a message).
3. Stop the capture.
4. Analyze the TLS ClientHello fingerprint:

   tshark -r viber_cloak.pcap -Y "tls.handshake.type == 1" -T fields -e tls.handshake.extensions_server_name -e tls.handshake.ciphersuite
   

5. Observe the static nature of the extensions. Create a DPI rule to block this specific fingerprint:

   # Example using iptables + string match (simplified)
   iptables -A FORWARD -m string --string "ViberCloakFingerprint" --algo bm -j DROP
   

6. Confirm that the Cloak mode connection is now blocked.

Blue Team

Objective: Detect and alert on the use of Viber Cloak mode within the network.

Detection Strategy:

• Monitor for TLS ClientHello messages with a static, predictable set of extensions.
• Use network monitoring tools (e.g., Zeek, Suricata) to fingerprint TLS connections.
• Correlate with known Viber server IPs or domains.

Lab Setup:

1. Deploy a network sensor (e.g., Security Onion) with Suricata or Zeek.
2. Configure a custom signature to detect the Viber Cloak fingerprint.

Steps:

1. Create a Suricata rule (see Detection Engineering section).
2. Deploy the rule to the sensor.
3. Generate test traffic from a Viber client with Cloak mode enabled.
4. Verify that the sensor generates an alert.
5. Implement a response action (e.g., block the IP, notify the SOC).

Indicators of Compromise (IOCs)

• Network IOCs:

  • Static TLS ClientHello fingerprint (specific byte sequence). Example (hex): 16 03 01 02 00 01 00 01 fc 03 03 ... (This is a placeholder; the actual fingerprint would need to be extracted from a live capture.)
  • Destination IPs associated with Viber proxy servers (e.g., 91.121.86.0/24, 185.45.48.0/22 – these are examples, verify with threat intel).
  • JA3/JA3S hash: A static JA3 hash for the Cloak mode client. Example: 6734f37431670b3ab4292b8f60f29984 (placeholder).

• Host IOCs:

  • Presence of Viber application with Cloak mode enabled.
  • Registry keys (Windows): HKCU\Software\Viber\CloakMode (example).
  • File paths: %APPDATA%\Viber\cloak_config.json (example).

Detection Engineering

YARA Rule

rule Detect_Viber_Cloak_TLS_Fingerprint {
    meta:
        description = "Detects Viber Cloak mode TLS ClientHello static fingerprint"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-13476"
        hash = "d41d8cd98f00b204e9800998ecf8427e"
    strings:
        $tls_client_hello = { 16 03 01 02 00 01 00 01 fc 03 03 }  // Placeholder hex
        $viber_ja3 = "6734f37431670b3ab4292b8f60f29984" ascii wide
    condition:
        $tls_client_hello or $viber_ja3
}

Sigma Rule

title: Detection of Viber Cloak Mode TLS Fingerprint
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects network traffic matching the static TLS ClientHello fingerprint used by Viber Cloak mode.
author: Valters IT
date: 2025/04/10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-13476
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        ja3.hash: "6734f37431670b3ab4292b8f60f29984"
    condition: selection
falsepositives:
    - None expected due to static fingerprint
level: high

Suricata Rule

alert tls $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-13476 Viber Cloak Mode TLS Fingerprint Detected"; ja3_hash; content:"6734f37431670b3ab4292b8f60f29984"; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Cloak Mode: Advise users to disable Viber's Cloak mode until a patch is released.
2. Network Monitoring: Implement the provided detection rules to identify and alert on Cloak mode traffic.
3. Alternative Tools: Recommend users switch to verified censorship circumvention tools (e.g., Tor, Psiphon) that use randomized TLS fingerprints.

Long-Term Fixes:

1. Vendor Patch: Rakuten must release an update that randomizes the TLS ClientHello fingerprint in Cloak mode.
2. Code Review: Conduct a security audit of the Cloak mode implementation to ensure no other static identifiers are used.
3. User Education: Inform users about the risks of relying on a single obfuscation technique.

Workarounds:

• Use a VPN in addition to Viber to add an extra layer of encryption and traffic obfuscation.
• Configure a custom proxy that performs TLS fingerprint randomization before forwarding traffic to Viber servers.

Valters IT
Securing Digital Frontiers