UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes

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Summary

CVECVE-2026-44040
StatePUBLISHED
Assignersecurin
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-01 05:16:20 UTC
Updated2026-07-02 14:58:09 UTC
DescriptionUltraVNC 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.

Risk And Classification

Primary CVSS: v3.1 6.5 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS: 0.002190000 probability, percentile 0.123970000 (date 2026-07-02)

Problem Types: CWE-338 | CWE-338 Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
3.133c584b5-0579-4c06-b2a0-8d8329fcab9cSecondary4.8MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
3.1CNACVSS4.8MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Uvnc Ultravnc All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Uvnc UltraVNC affected 1.8.2.2 custom Not specified

References

ReferenceSourceLinkTags
uvnc.com 33c584b5-0579-4c06-b2a0-8d8329fcab9c uvnc.com Product, Release Notes
github.com/ultravnc/UltraVNC 33c584b5-0579-4c06-b2a0-8d8329fcab9c github.com Product
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Arjun Basnet, Securin ([email protected]) (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-06-02T00:00:00.000ZVulnerability discovered during security audit
CNA2026-06-17T00:00:00.000ZReported to vendor (coordinated disclosure)
CNA2026-09-15T00:00:00.000ZPlanned public disclosure (90-day window)
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