uvnc UltraVNC up to 1.8.2.2 rfb/dh.cpp rng inadequate encryption

Summaryinfo

A vulnerability was found in uvnc UltraVNC up to 1.8.2.2. It has been declared as problematic. This affects the function rng of the file rfb/dh.cpp. The manipulation results in inadequate encryption. This vulnerability was named CVE-2026-7830. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability was found in uvnc UltraVNC up to 1.8.2.2. It has been classified as problematic. Affected is the function rng of the file rfb/dh.cpp. The manipulation with an unknown input leads to a inadequate encryption vulnerability. CWE is classifying the issue as CWE-326. The 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. This is going to have an impact on confidentiality. CVE summarizes:

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.

The weakness was presented by Arjun Basnet. This vulnerability is traded as CVE-2026-7830 since 05/05/2026. The exploitability is told to be difficult. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1600.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• Connectivity Software

Vendor

• uvnc

Name

• UltraVNC

Version

• 1.8.2.0
• 1.8.2.1
• 1.8.2.2

License

• open-source

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CPE 2.2info

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