CVE-2026-44040
UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes
Description
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.
INFO
Published Date :
July 1, 2026, 5:16 a.m.
Last Modified :
July 9, 2026, 6:16 a.m.
Remotely Exploit :
Yes !
Source :
33c584b5-0579-4c06-b2a0-8d8329fcab9c
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 33c584b5-0579-4c06-b2a0-8d8329fcab9c | ||||
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update UltraVNC to a patched version.
- Verify secure random number generation is used.
- If possible, migrate to a more secure authentication method.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-44040.
| URL | Resource |
|---|---|
| https://github.com/ultravnc/UltraVNC | Product |
| https://uvnc.com/ | Product Release Notes |
| https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-44040 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-44040
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-44040 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-44040 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Modified by 33c584b5-0579-4c06-b2a0-8d8329fcab9c
Jul. 09, 2026
Action Type Old Value New Value Added Reference https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc -
Initial Analysis by [email protected]
Jul. 02, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N Added CPE Configuration OR *cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:* versions up to (including) 1.8.2.2 Added Reference Type securin: https://github.com/ultravnc/UltraVNC Types: Product Added Reference Type securin: https://uvnc.com/ Types: Product, Release Notes -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jul. 01, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-44040', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-07-01T12:44:56.012420Z'} -
New CVE Received by 33c584b5-0579-4c06-b2a0-8d8329fcab9c
Jul. 01, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'uvnc', 'modules': ['winvnc', 'vncviewer'], 'product': 'UltraVNC', 'versions': [{'status': 'affected', 'version': '0', 'versionType': 'custom', 'lessThanOrEqual': '1.8.2.2'}], 'defaultStatus': 'unaffected'}] Added Description 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N Added CWE CWE-338 Added Reference https://github.com/ultravnc/UltraVNC Added Reference https://uvnc.com/