Common Weakness Enumeration
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CWE-326
Inadequate Encryption Strength
The product stores or transmits sensitive data using an encryption scheme that is theoretically sound, but is not strong enough for the level of protection required.
CVE-2026-7830 (GCVE-0-2026-7830)
Vulnerability from cvelistv5 – Published: 2026-07-01 03:33 – Updated: 2026-07-09 04:35
VLAI
EPSS
VEX
Title
UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception
Summary
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.
Severity
7.4 (High)
SSVC
Exploitation: none
Automatable: no
Technical Impact: total
CISA Coordinator (v2.0.3)
CWE
Assigner
References
3 references
| URL | Tags |
|---|---|
| https://uvnc.com/ | vendor-advisory |
| https://github.com/ultravnc/UltraVNC | product |
| https://www.securin.io/zero-days/cve-2026-7830-31… |
Credits
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"role": "CISA Coordinator",
"timestamp": "2026-07-01T13:13:10.627838Z",
"version": "2.0.3"
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"title": "CISA ADP Vulnrichment"
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"cna": {
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"modules": [
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"product": "UltraVNC",
"vendor": "uvnc",
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"status": "affected",
"version": "0",
"versionType": "custom"
}
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"credits": [
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"value": "Arjun Basnet, Securin (arjun.basnet@securin.io)"
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"value": "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\u0027s 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."
}
],
"metrics": [
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"cvssV3_1": {
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"baseScore": 7.4,
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"confidentialityImpact": "HIGH",
"integrityImpact": "HIGH",
"privilegesRequired": "NONE",
"scope": "UNCHANGED",
"userInteraction": "NONE",
"vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
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"description": "Inadequate Encryption Strength",
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},
{
"descriptions": [
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"cweId": "CWE-338",
"description": "Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)",
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"providerMetadata": {
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"references": [
{
"name": "UltraVNC project page",
"tags": [
"vendor-advisory"
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"url": "https://uvnc.com/"
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{
"name": "UltraVNC source repository",
"tags": [
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"url": "https://github.com/ultravnc/UltraVNC"
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"url": "https://www.securin.io/zero-days/cve-2026-7830-31-bit-dh-weak-rng-ms-logon-credential-interception-ultravnc"
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"source": {
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"discovery": "EXTERNAL"
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"time": "2026-06-02T00:00:00.000Z",
"value": "Vulnerability discovered during security audit"
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"lang": "en",
"time": "2026-06-17T00:00:00.000Z",
"value": "Reported to vendor (coordinated disclosure)"
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"time": "2026-09-15T00:00:00.000Z",
"value": "Planned public disclosure (90-day window)"
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"title": "UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential interception"
}
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"cveMetadata": {
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"datePublished": "2026-07-01T03:33:24.106Z",
"dateReserved": "2026-05-05T03:40:59.002Z",
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Mitigation
Phase: Architecture and Design
Description:
- Use an encryption scheme that is currently considered to be strong by experts in the field.
CAPEC-112: Brute Force
In this attack, some asset (information, functionality, identity, etc.) is protected by a finite secret value. The attacker attempts to gain access to this asset by using trial-and-error to exhaustively explore all the possible secret values in the hope of finding the secret (or a value that is functionally equivalent) that will unlock the asset.
CAPEC-192: Protocol Analysis
An adversary engages in activities to decipher and/or decode protocol information for a network or application communication protocol used for transmitting information between interconnected nodes or systems on a packet-switched data network. While this type of analysis involves the analysis of a networking protocol inherently, it does not require the presence of an actual or physical network.
CAPEC-20: Encryption Brute Forcing
An attacker, armed with the cipher text and the encryption algorithm used, performs an exhaustive (brute force) search on the key space to determine the key that decrypts the cipher text to obtain the plaintext.