UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception

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Summary

CVECVE-2026-7830
StatePUBLISHED
Assignersecurin
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-01 05:16:24 UTC
Updated2026-07-02 15:18:24 UTC
DescriptionUltraVNC through 1.8.2.2 uses inadequate cryptography in the MS-Logon II authentication scheme (rfbUltraVNC_MsLogonIIAuth). In rfb/dh.cpp the Diffie-Hellman key exchange is performed with parameters that fit in an unsigned 64-bit integer (DH_MAX_BITS controls the prime size). A 64-bit DH key can be broken by Pollard's rho algorithm in under one second on current hardware. Additionally, the private exponent is generated by the rng() function, which multiplies three libc rand() values seeded from time(NULL). With approximately 31 bits of internal state and a time-based seed, the private exponent is recoverable in under a minute by a passive observer. A network attacker who can observe the MS-Logon II handshake (via sniffing, recording, or man-in-the-middle) can derive the shared DH key and decrypt the encapsulated username and password, resulting in full credential disclosure. This affects legacy MS-Logon II connections; MS-Logon III (X25519 + AES-256-GCM) is unaffected.

Risk And Classification

Primary CVSS: v3.1 7.4 HIGH from 33c584b5-0579-4c06-b2a0-8d8329fcab9c

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

EPSS: 0.001830000 probability, percentile 0.080670000 (date 2026-07-02)

Problem Types: CWE-326 | CWE-338 | CWE-326 Inadequate Encryption Strength | CWE-338 Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

VersionSourceTypeScoreSeverityVector
3.133c584b5-0579-4c06-b2a0-8d8329fcab9cSecondary7.4HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
3.1CNACVSS7.4HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

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