CVE-2026-7830 | UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception

UltraVNC 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.

Published: 2026-07-01 Last update: 2026-07-09 Assigner: 33c584b5-0579-4c06-b2a0-8d8329fcab9c Source: 33c584b5-0579-4c06-b2a0-8d8329fcab9c
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-7830 is rated Low Risk (32.4/100): CVSS High severity, with low exploitation likelihood (EPSS 0.18%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-7830

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-07-01 — 0.18% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-7830

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.4 3.1 HIGH
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 2.2 5.2 33c584b5-0579-4c06-b2a0-8d8329fcab9c

Weakness enumeration for CVE-2026-7830

GitHub Security Advisory for CVE-2026-7830

GHSA-gf4x-6rw3-q4x6 · Severity: high — UltraVNC through 1.8.2.2 uses inadequate cryptography in the MS-Logon II authentication scheme ...

Affected software / configurations for CVE-2026-7830

Vendor Product Version Raw CPE
uvnc ultravnc <= 1.8.2.2 cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:*

References for CVE-2026-7830

cvelogic Threat Intelligence