EUVD-2026-40879
GHSA-j37g-g7hg-vhg6
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Successful auth bypass grants full desktop visibility and control (C:H/I:H); AC:H retained because attacker must observe a live challenge exchange to enumerate the seed.
Primary rating from Vendor (securin).
CVSS VectorNVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Lifecycle Timeline
3DescriptionNVD
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.
AnalysisAI
UltraVNC through 1.8.2.2 exposes a cryptographically weak VNC authentication challenge generator that an attacker can predict by observing network traffic and enumerating a roughly 31-bit seed space derived from wall-clock time and process ID. Successful seed reconstruction allows the attacker to forge or brute-force valid VNC authentication responses, effectively bypassing the RFB challenge-response mechanism and gaining unauthorized remote desktop access. No public exploit has been identified at time of analysis, and the vulnerability is not currently listed in the CISA KEV catalog; however, the mathematical basis for exploitation is straightforward given the small seed space.
Technical ContextAI
The vulnerability is in rfb/vncauth.c lines 119-129, where vncRandomBytes() seeds libc's rand() using time(0) + getpid() + rand(). The combined effective entropy is approximately 31 bits - both seed components (wall-clock seconds and process ID) are publicly observable, reducing the challenge generation to a deterministic, enumerable function. The VNC RFB protocol's security type 2 (VNC Authentication) transmits a 16-byte nonce challenge in cleartext; that nonce is then encrypted with the DES-based VNC password and returned by the client. If the nonce is predictable, the entire challenge-response scheme collapses. CWE-338 (Use of Cryptographically Weak Pseudo-Random Number Generator) is the precise root cause class - rand() is explicitly not suitable for cryptographic nonce generation. CPE cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:* covers all versions through 1.8.2.2. A significant open question per the CVE description is whether shipped Windows binaries actually reach vncauth.c or instead execute vncEncryptBytes2.cpp, which calls CryptGenRandom - a cryptographically secure Windows API - making the Windows code path exposure unconfirmed.
RemediationAI
No vendor-released patch has been identified at time of analysis - the available references point only to the vendor homepage (https://uvnc.com/) and the GitHub repository (https://github.com/ultravnc/UltraVNC) without specifying a fixed release version. Monitor the repository for commits addressing vncRandomBytes() in rfb/vncauth.c. As the primary compensating control, place UltraVNC behind a VPN or SSH tunnel so that the cleartext RFB authentication exchange is not observable by third parties - this directly eliminates the network observation prerequisite encoded in the CVSS AC:H metric. Alternatively, use SSH port forwarding to wrap VNC traffic, preventing passive capture of the challenge-response exchange. Restrict VNC port exposure at the firewall to trusted source IPs only, reducing the attacker's ability to position for traffic observation. Organizations unable to apply network-layer controls should evaluate migration to remote desktop solutions using cryptographically strong authentication (e.g., protocols seeded with OS-level CSPRNG). Each mitigating control trades operational access convenience for reduced exposure; VPN/SSH tunneling is the most robust option with minimal side effects.
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