CVE-2026-7830 UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception
Weak Diffie-Hellman parameters and predictable random number generation in UltraVNC MS-Logon II allow attackers to intercept and decrypt user credentials.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-7830?
CVE-2026-7830 is a high-severity vulnerability in uvnc UltraVNC, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Which versions are affected by CVE-2026-7830?
It affects 1 product.
Products include Uvnc Ultravnc.
How do I fix CVE-2026-7830?
No fix has been published for CVE-2026-7830 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-7830 being exploited?
There is no known exploitation of CVE-2026-7830 in the wild and no public exploit code.
Other vulnerabilities in uvnc UltraVNC
Sources
Also known as GHSA-gf4x-6rw3-q4x6.