# CVE-2026-7830: UltraVNC weak cryptography in MS-Logon II authentication

Source: https://junglewise.ai/threats/cve-2026-7830-ultravnc-weak-cryptography-in-ms-logon-ii-authentication  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-01. Updated: 2026-07-01  
License: CC BY 4.0 with a link back

CVE-2026-7830 · Severity: high · CVSS 7.4 · Published 2026-07-01

UltraVNC, a popular remote desktop tool used for technical support and remote server management, contains a security weakness in its legacy login system. An attacker who can monitor network traffic between a user and the server can quickly crack the encryption used during login. This allows the attacker to steal usernames and passwords, potentially leading to unauthorized access to the remote computer and sensitive data.

Technologies: [UltraVNC](https://junglewise.ai/threats/technologies/ultravnc). Vendors: [UltraVNC](https://junglewise.ai/threats/vendors/ultravnc).

## Quick answers

**Is CVE-2026-7830 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-7830 being exploited in the wild.

**How severe is CVE-2026-7830?** High severity, with a CVSS score of 7.4, published 1 July 2026.

**Which products and versions are affected?** UltraVNC through 1.8.2.2.

**Is there a fix for CVE-2026-7830?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-7830 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-7830?** Check whether you run UltraVNC at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

UltraVNC (through version 1.8.2.2) implements the MS-Logon II authentication scheme (rfbUltraVNC_MsLogonIIAuth) using weak cryptographic primitives. Specifically, the Diffie-Hellman (DH) key exchange in rfb/dh.cpp is restricted to a 64-bit prime size (DH_MAX_BITS), which is vulnerable to Pollard's rho algorithm in under one second. Furthermore, the private exponent is generated using a weak PRNG seeded by time(NULL), resulting in only ~31 bits of entropy. A passive network observer or man-in-the-middle attacker can recover the private exponent and shared key to decrypt the encapsulated credentials. This vulnerability does not affect the more modern MS-Logon III scheme.

## Affected products

- uvnc UltraVNC through 1.8.2.2

## Timeline

- 2026-07-01: advisory: NVD publication date

## References

- [https://github.com/ultravnc/UltraVNC](https://github.com/ultravnc/UltraVNC)
- [https://uvnc.com/](https://uvnc.com/)

## Related threats

- [UltraVNC viewer heap buffer overflow in RFB failure-response parsing](https://junglewise.ai/threats/cve-2026-7838-ultravnc-viewer-heap-buffer-overflow-in-rfb-failure-response)
- [UltraVNC out-of-bounds read in vncWc2Mb conversion helper](https://junglewise.ai/threats/cve-2026-44041-ultravnc-out-of-bounds-read-in-vncwc2mb-conversion-helper)
- [UltraVNC weak PRNG in VNC authentication challenge](https://junglewise.ai/threats/cve-2026-44040-ultravnc-weak-prng-in-vnc-authentication-challenge)