uvnc UltraVNC up to 1.8.2.2 on Windows rfb/vncauth.c vncRandomBytes weak prng

Summaryinfo

A vulnerability, which was classified as problematic, has been found in uvnc UltraVNC up to 1.8.2.2 on Windows. The affected element is the function vncRandomBytes of the file rfb/vncauth.c. Performing a manipulation results in weak prng. This vulnerability is reported as CVE-2026-44040. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability was found in uvnc UltraVNC up to 1.8.2.2 on Windows. It has been classified as problematic. This affects the function vncRandomBytes of the file rfb/vncauth.c. The manipulation with an unknown input leads to a weak prng vulnerability. CWE is classifying the issue as CWE-338. The product uses a Pseudo-Random Number Generator (PRNG) in a security context, but the PRNG's algorithm is not cryptographically strong. This is going to have an impact on confidentiality. The summary by CVE is:

UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.

The weakness was disclosed by Arjun Basnet. This vulnerability is uniquely identified as CVE-2026-44040 since 05/05/2026. The exploitability is told to be difficult. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1600.001 for this issue.

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

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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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