CVE-2026-7830— UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception

UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception

UltraVNC through 1.8.2.2 uses inadequate cryptography in the MS-Logon II authentication scheme (rfbUltraVNC_MsLogonIIAuth). In rfb/dh.cpp the Diffie-Hellman key exchange is performed with parameters that fit in an unsigned 64-bit integer (DH_MAX_BITS controls the prime size). A 64-bit DH key can be broken by Pollard's rho algorithm in under one second on current hardware. Additionally, the private exponent is generated by the rng() function, which multiplies three libc rand() values seeded from time(NULL). With approximately 31 bits of internal state and a time-based seed, the private exponent is recoverable in under a minute by a passive observer. A network attacker who can observe the MS-Logon II handshake (via sniffing, recording, or man-in-the-middle) can derive the shared DH key and decrypt the encapsulated username and password, resulting in full credential disclosure. This affects legacy MS-Logon II connections; MS-Logon III (X25519 + AES-256-GCM) is unaffected.

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

不充分的加密强度

UltraVNC 加密问题漏洞

ultravnc是UltraVNC组织开源的一款用于Windows平台的开源远程终端控制软件。 UltraVNC 1.8.2.2及之前版本存在加密问题漏洞，该漏洞源于MS-Logon II认证方案中使用了不充分的加密，包括Diffie-Hellman密钥交换参数过小且私钥生成不随机，可能导致网络攻击者通过中间人攻击、嗅探或记录等方式恢复私钥并解密凭证，造成凭据泄露。

N/A

N/A