CVE-2026-44042 UltraVNC repeater wi_uudecode off-by-one in base64 decode boundary check
An off-by-one error in UltraVNC repeater's Base64 decoder allows a one-byte stack buffer overflow, potentially leading to code execution or denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-44042?
CVE-2026-44042 is a low-severity memory corruption vulnerability in uvnc UltraVNC, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-193
Which versions are affected by CVE-2026-44042?
It affects 1 product.
Products include Uvnc Ultravnc.
How do I fix CVE-2026-44042?
No fix has been published for CVE-2026-44042 yet. Limit exposure of the affected software until one is available.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-44042 being exploited?
There is no known exploitation of CVE-2026-44042 in the wild and no public exploit code.
Other vulnerabilities in uvnc UltraVNC
Sources
Also known as GHSA-gh7j-9w79-xm55.