What is the CVSS score of CVE-2026-7830?

CVE-2026-7830 has a CVSS 3.1 base score of 7.4 (High). CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N. EPSS exploitation probability: 0.2%.

Which versions of UltraVNC are affected by CVE-2026-7830?

UltraVNC versions through 1.8.2.2 contain a critical weakness in MS-Logon II authentication that allows any attacker with network access to recover plaintext usernames and passwords through passive…

How to fix or mitigate CVE-2026-7830?

Within 24 hours: inventory all UltraVNC deployments and versions in use; assess exposure to untrusted networks. Within 7 days: migrate to MS-Logon III authentication (X25519 + AES-256-GCM) if current version supports it; alternatively, implement mandatory VPN access and network segmentation. Within 30 days: complete migration to a remote desktop solution using modern cryptography (e.g., transition away from affected UltraVNC versions entirely) or document persistent use with approved compensating controls.

UltraVNC CVE-2026-7830

EUVD-2026-40882 HIGH

Inadequate Encryption Strength (CWE-326)

2026-07-01 securin

GHSA-gf4x-6rw3-q4x6

Microsoft Information Disclosure Ultravnc

7.4

CVSS 3.1 · Vendor: securin

Severity by source

Vendor (securin) PRIMARY

7.4 HIGH

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

vuln.today AI

5.9 MEDIUM

AC:H because the attacker must observe the handshake (sniff/MITM); PR:N/UI:N as no auth or interaction is needed; primary impact is credential confidentiality (C:H), with integrity/availability not directly affected.

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

4.0 AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Primary rating from Vendor (securin).

CVSS VectorVendor: securin

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

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Lifecycle Timeline

Analysis Generated

Jul 01, 2026 - 05:22 vuln.today

CVE Published

Jul 01, 2026 - 03:33 cve.org

HIGH 7.4

DescriptionCVE.org

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.

AnalysisAI

Credential disclosure in UltraVNC through 1.8.2.2 lets a passive network observer break the MS-Logon II authentication handshake and recover plaintext usernames and passwords. The rfbUltraVNC_MsLogonIIAuth scheme relies on a Diffie-Hellman exchange whose prime fits in an unsigned 64-bit integer and a private exponent derived from time(NULL)-seeded libc rand(), both of which are trivially solvable, so an attacker who sniffs or man-in-the-middles the exchange derives the shared key in seconds to a minute. There is no public exploit identified at time of analysis and no EPSS/KEV signal supplied; CVSS is 7.4 (AC:H reflecting the need to observe the handshake), and MS-Logon III (X25519 + AES-256-GCM) is not affected.

Technical ContextAI

The flaw lives in UltraVNC's RFB protocol extension for MS-Logon II, implemented in rfb/dh.cpp. Diffie-Hellman key agreement is meant to protect the encapsulated credential blob, but DH_MAX_BITS caps the prime at a size that fits in a 64-bit unsigned integer; a 64-bit discrete-log problem is solvable by Pollard's rho in under a second on commodity hardware. Compounding this, the private exponent comes from a custom rng() that multiplies three libc rand() outputs seeded from time(NULL), yielding roughly 31 bits of effective entropy tied to a guessable timestamp, so the exponent itself is brute-forceable in under a minute. This maps to CWE-326 (Inadequate Encryption Strength): the cryptographic primitive is structurally correct but uses parameter sizes and a PRNG far below any meaningful security margin. The affected component per CPE is cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:* - the UltraVNC server/viewer stack on Windows.

RemediationAI

No vendor-released patch version is identified at time of analysis (the references point to the vendor homepage https://uvnc.com/ and the repository https://github.com/ultravnc/UltraVNC, not to a tagged fixed release), so confirm the current fixed build directly with UltraVNC before relying on an upgrade. The most effective mitigation is to stop using MS-Logon II and switch authentication to MS-Logon III (X25519 + AES-256-GCM), which is unaffected; where MS-Logon II must remain, tunnel all VNC traffic through an encrypted transport such as an SSH tunnel, IPsec, or a VPN so the DH handshake is never observable in cleartext on the wire, accepting the added configuration and latency overhead. Additionally restrict VNC (typically TCP 5900/5901 and the 5800 web port) to trusted management networks via firewall rules and disable it on untrusted or shared LAN segments, with the trade-off of reduced remote reachability for legitimate operators. Rotate any credentials that may have traversed MS-Logon II sessions on untrusted networks, since prior handshakes could have been recorded for offline recovery.