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CVE-2026-7838— UltraVNC viewer heap buffer overflow via integer overflow in RFB connection-failure reason length

Quick assessment

Affected
uvnc UltraVNC
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

uvnc UltraVNC是uvnc个人开发者开源的一款Windows远程控制软件。 UltraVNC 1.8.2.2及之前版本存在安全漏洞，该漏洞源于RFB协议失败响应解析路径中的整数溢出导致堆缓冲区溢出，具体涉及vncviewer/ClientConnection.cpp中reasonLen参数的处理，可能导致恶意VNC服务器或中间人攻击者在用户连接时触发远程代码执行。

CVSS 8.8 · High EPSS 1.87% · P79

Affected Version Matrix 1

VendorProduct Version RangeStatus
uvnc UltraVNC ≤ 1.8.2.2 affected
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I. Basic Information for CVE-2026-7838

Vulnerability Information
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Vulnerability Title
UltraVNC viewer heap buffer overflow via integer overflow in RFB connection-failure reason length
Source: CVE Program / CVE List V5
Vulnerability Description
UltraVNC viewer through 1.8.2.2 contains an integer overflow leading to a heap buffer overflow in the RFB protocol failure-response parsing path. In vncviewer/ClientConnection.cpp, the 4-byte network-supplied reasonLen field (type CARD32) is passed as reasonLen+1 to CheckBufferSize(). Because both operands are unsigned 32-bit, a reasonLen of 0xFFFFFFFF overflows to 0, causing CheckBufferSize to allocate only 256 bytes. The subsequent ReadString(m_netbuf, reasonLen) call then performs ReadExact for the original 4 GiB length into that 256-byte heap buffer. This overflow is reachable via rfbConnFailed (auth-scheme negotiation) and rfbVncAuthFailed (post-handshake) message types without successful authentication. A malicious VNC server, or any man-in-the-middle on the RFB stream, can trigger this condition when the victim viewer connects, potentially resulting in remote code execution as the user running the viewer. The crash was confirmed with AddressSanitizer on a portable reproduction harness (heap-buffer-overflow WRITE at offset 256).
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
整数溢出或超界折返
Source: CVE Program / CVE List V5
Vulnerability Title
UltraVNC 数字错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
uvnc UltraVNC是uvnc个人开发者开源的一款Windows远程控制软件。 UltraVNC 1.8.2.2及之前版本存在安全漏洞，该漏洞源于RFB协议失败响应解析路径中的整数溢出导致堆缓冲区溢出，具体涉及vncviewer/ClientConnection.cpp中reasonLen参数的处理，可能导致恶意VNC服务器或中间人攻击者在用户连接时触发远程代码执行。
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-2026-7838

# POC Description Source Link Shenlong Link
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Qwen3.6-35B-A3B · 8346 chars
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III. Intelligence Information for CVE-2026-7838

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Vendor Advisories for CVE-2026-7838 (1)

Vendor Pages for CVE-2026-7838 (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-2026-7831 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
CVE-2026-44042 3.7 LOW UltraVNC repeater wi_uudecode off-by-one in base64 decode boundary check

IV. Related Vulnerabilities

Same product: UltraVNC
Same vendor: uvnc
Same weakness: CWE-190

V. Comments for CVE-2026-7838

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