CVE-2026-44040
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedUltraVNC 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceUltraVNC through 1.8.2.2 uses a cryptographically weak PRNG in vncRandomBytes() seeded with time(0) + getpid() + rand(), producing only ~31 bits of entropy. Since the seed relies on publicly observable values (wall-clock time and process ID), an attacker who intercepts the VNC authentication exchange can enumerate the seed space and predict the 16-byte challenge within seconds, enabling authentication forgery or offline brute-forcing of responses.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data<= 1.8.2.2CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check UltraVNC installation and versionLook for UltraVNC installation directory (commonly C:\Program Files\UltraVNC or C:\Program Files (x86)\UltraVNC on Windows). Check the version by right-clicking winvnc.exe or uvnc_service.exe, selecting Properties, and viewing the File Version tab. Also check the built-in version info via 'winvnc.exe -version' or 'uvnc_service.exe -version' if supported.Affected if The installed version is 1.8.2.2 or any earlier version (1.8.x series). The version number in the file properties will be less than or equal to 1.8.2.2.
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Verify VNC service or process is runningOpen Task Manager or use command 'tasklist /FI "IMAGENAME eq winvnc.exe"' or 'sc query winvnc' to check if the UltraVNC server (winvnc.exe) or service (uvnc_service) is active. Also check for listening TCP port 5900 (or custom VNC port) using 'netstat -an | findstr 5900'.Affected if The UltraVNC server process is running and accepting VNC connections on port 5900 (or a configured alternative port). This is required for the weak PRNG to be used in authentication challenges.
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Identify vulnerable vncRandomBytes usage in binariesExamine the UltraVNC binary (winvnc.exe or uvnc_service.exe) for the presence of the weak PRNG function. If you have access to the binary or debug symbols, search for the function name 'vncRandomBytes' or its call pattern involving time(), getpid(), and rand(). On Windows, you can use 'strings winvnc.exe | findstr -i random' to look for related function references.Affected if The binary contains the vncRandomBytes() implementation using time(0) + getpid() + rand() for seeding. This can be confirmed by binary analysis or source code review if accessible.
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Test authentication challenge predictability (if permitted in your environment)Capture multiple VNC authentication handshakes by connecting to the UltraVNC server and recording the 16-byte challenge sent by the server (the first 16 bytes of the VNC authentication protocol). Compare challenges at different times - if the same timestamp produces identical challenges, the PRNG is weak. Note: This is an observational check and may require multiple connection attempts.Affected if The authentication challenge can be reproduced or predicted based on known timestamps. If challenges repeat predictably or can be enumerated within seconds using the known seed formula, the system is vulnerable.
You are affected if UltraVNC version 1.8.2.2 or earlier is installed and the VNC server service is running, using the weak vncRandomBytes() PRNG for authentication challenge generation.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataReplace the weak vncRandomBytes() implementation with a cryptographically secure PRNG (CSPRNG) such as CryptGenRandom on Windows, Arc4Random on macOS, or /dev/urandom on Unix/Linux systems. Ensure the challenge generation uses at least 128 bits of entropy from the system's entropy pool.
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- Implementation6.0 h
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- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-44040 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-44040 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
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- Version or environment caveats, and links to real fixes
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