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CVE-2026-44042— UltraVNC repeater wi_uudecode off-by-one in base64 decode boundary check

Quick assessment

Affected
uvnc UltraVNC
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

ultravnc是UltraVNC组织开源的一款用于Windows平台的开源远程终端控制软件。 UltraVNC 1.8.2.2及之前版本存在数字错误漏洞，该漏洞源于Base64解码辅助函数中存在差一错误，可能导致在受限条件下向1024字节栈缓冲区边界进行一字节写入。

CVSS 3.7 · Low EPSS 0.49% · P40

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
uvnc UltraVNC ≤ 1.8.2.2 affected
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I. Basic Information for CVE-2026-44042

Vulnerability Information
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Vulnerability Title
UltraVNC repeater wi_uudecode off-by-one in base64 decode boundary check
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
Off-by-one错误
Source: CVE Program / CVE List V5
Vulnerability Title
UltraVNC 数字错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
ultravnc是UltraVNC组织开源的一款用于Windows平台的开源远程终端控制软件。 UltraVNC 1.8.2.2及之前版本存在数字错误漏洞，该漏洞源于Base64解码辅助函数中存在差一错误，可能导致在受限条件下向1024字节栈缓冲区边界进行一字节写入。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
uvnc UltraVNC 0 ~ 1.8.2.2 -

II. Public POCs for CVE-2026-44042

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III. Intelligence Information for CVE-2026-44042

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Vendor Advisories for CVE-2026-44042 (1)

Vendor Pages for CVE-2026-44042 (1)

Same Patch Batch · uvnc · 2026-07-01 · 10 CVEs total

CVE-2026-7840 9.8 CRITICAL UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
CVE-2026-7839 9.1 CRITICAL UltraVNC repeater ships hardcoded default admin password allowing unauthenticated admin ac
CVE-2026-7838 8.8 HIGH UltraVNC viewer heap buffer overflow via integer overflow in RFB connection-failure reason
CVE-2026-7831 7.5 HIGH UltraVNC viewer off-by-one stack overflow in ServerInit desktop name parsing
CVE-2026-7830 7.4 HIGH UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded libc rand() enabling credential
CVE-2026-7829 7.2 HIGH UltraVNC repeater authenticated out-of-bounds write in rule parser via oversized token
CVE-2026-7828 5.3 MEDIUM UltraVNC repeater integer overflow in win_log malloc leading to heap overflow
CVE-2026-44040 4.8 MEDIUM UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge b
CVE-2026-44041 4.3 MEDIUM UltraVNC vncWc2Mb calls wcslen() before validating that the wide string is NUL-terminated

IV. Related Vulnerabilities

Same product: UltraVNC
Same vendor: uvnc
Same weakness: CWE-193

V. Comments for CVE-2026-44042

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