CVE-2026-44042 UltraVNC Repeater Base64 Decode Off-by-One
UltraVNC repeater through 1.8.2.2 contains an off-by-one error in the Base64 decode helper used for HTTP Basic authentication. In repeater/webgui/webutils.c:817, the wi_uudecode() function checks whether the input length exceeds the output buffer with a strict greater-than comparison (>), while the correct check should be greater-than-or-equal (>=). When strlen(authdata) equals sizeof(decode), the decoded output length (approximately 3/4 of input) does not overflow the buffer in current practice because the outer HTTP request bounds constrain the Authorization header. However, the defective check leaves a latent off-by-one condition that could become exploitable if the buffering constraints change. The current risk is limited to a one-byte write at the boundary of a 1024-byte stack buffer under constrained conditions.
Quick answer
UltraVNC Repeater should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- <= 1.8.2.2
Fixed versions
- No vendor fix published yet
How to fix it
UltraVNC Repeater is affected and no public fixed version is listed in the local advisory data yet. Reduce access, disable risky features where possible, and apply the vendor fix when it is released.
- Check where UltraVNC Repeater is installed and whether it is reachable from the internet.
- Limit access to trusted users, trusted networks, or a VPN until a vendor fix is available.
- Disable the affected feature or module if the advisory points to a specific upload, login, admin, chat, or network service path.
- Watch the vendor or advisory source and apply the first supported fix as soon as it is released.
- Review logs for strange access, failed attempts, crashes, file uploads, or unexpected account changes.
Scan now. Google sign-in is only needed to unlock fix guidance.
Verify the fix
- Confirm the installed version is no longer in the affected range.
- Retest the affected page, API, upload, login, chat, container, or network flow.
- Check logs after the change for blocked attempts, crashes, or errors.
- Run a fresh Fixnx scan and save the report.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-44042?
UltraVNC Repeater versions listed as affected should be reviewed: <= 1.8.2.2.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.