CVE-2026-7830: uvnc ultravnc Inadequate Encryption Strength

Vulnerability summary

CVE-2026-7830 is a high (CVSS 7.4) Inadequate Encryption Strength vulnerability in uvnc ultravnc. EPSS estimates a 0.29% chance of exploitation in the next 30 days, higher than 19.7% of scored vulnerabilities.

Timeline

1. 01-07-2026
   PublishedVulnerability first disclosed.
2. 09-07-2026
   Last modifiedVulnerability information updated.

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.

• CWE-326Inadequate Encryption StrengthThe product stores or transmits sensitive data using an encryption scheme that is theoretically sound, but is not strong enough for the level of protection required.
• CWE-338Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)The product uses a Pseudo-Random Number Generator (PRNG) in a security context, but the PRNG's algorithm is not cryptographically strong.