Mercusys AC12G Static Nonce & XOR-based Encoding Password Recovery Vulnerability (CVE-2026-36609)

Vulnerability Overview CVE ID: CVE-2026-36609 Vulnerability Name: Static nonces in authentication allow password recovery Description: The Mercusys AC12G(EU) V1 router firmware AC12G(EU)_V1_200909 uses a static authentication nonce that does not change across requests from the same source IP. Combined with predictable XOR-based password encoding (the function), an attacker can reverse captured authentication tokens to recover plaintext passwords. Impact Scope Severity: High CVSS v3.1 Score: 7.5 CWE: CWE-330: Use of Insufficiently Random Values Affected Products: - Vendor: Mercusys (Sub-brand of TP-Link) - Product: AC12G (EU) V1 - Tested Firmware: AC12G(EU)_V1_200909, AC12G(EU)_V1_210128 - Component: HTTP authentication mechanism, nonce generation - Status: No patch planned Mitigation Strategies 1. Generate a unique, random nonce for each authentication challenge. 2. Implement session token expiration and periodic rotation. 3. Use cryptographically secure session token generation (e.g., HMAC or similar). 4. Bind session tokens to the client IP address and timestamp. Technical Details TDOP Authentication Protocol: Returns a challenge containing a 32-character nonce and a 255-character group key. The nonce is generated once per boot based on the source IP. Session Token Calculation: , where is a fixed value and is the static nonce. The function uses simple XOR-based encoding, and the password salt (or ) is fully reversible. Impact: - Password recovery from a single captured authentication exchange. - Session replay – captured tokens remain valid indefinitely. - Offline precomputation of session tokens. - All devices with the same password generate identical session tokens. Discoverer Discoverer: TymbarK7372 References GitHub Link

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Mercusys AC12G Static Nonce & XOR-based Encoding Password Recovery Vulnerability (CVE-2026-36609)

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