Vulnerability record · CVE-2026-44042 · published 1 July 2026
CVE-2026-44042: Uvnc ultravnc vulnerability
Uvnc · Ultravnc
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.
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.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Affected products
1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.
References
| Link | Tags |
|---|---|
| https://github.com/ultravnc/UltraVNC | Product |
| https://uvnc.com/ | ProductRelease Notes |
| https://www.securin.io/zero-days/cve-2026-44042-wi-uudecode-off-by-one-bounded-ultravnc-repeater |
Track CVE-2026-44042 inside VULONE
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Related vulnerabilities
Same products first, then exploited flaws of the same weakness class.
Source: NIST National Vulnerability Database (record CVE-2026-44042), CISA KEV, FIRST EPSS (scores of 2026-10-07). This page is refreshed as NVD updates the record.
