Vulnerability · Published Jul 1, 2026 · Assigned by securin
CVE-2026-7830UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception
High 7.4
In plain English
A high-severity security flaw in UltraVNC from uvnc. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 21% of all known vulnerabilities).
Official description
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.
Affected products (as reported)
Weakness: CWE-326, CWE-338 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N