CVE-2026-7840— UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
Possible ATT&CK Techniques 2AI
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I. Basic Information for CVE-2026-7840
Vulnerability Information
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Vulnerability Title
UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
Source: CVE Program / CVE List V5
Vulnerability Description
UltraVNC repeater through 1.8.2.2 contains a global buffer overflow in its embedded HTTP administration server. The functions wi_senderr() and wi_replyhdr() in repeater/webgui/webutils.c write the caller-supplied HTTP request URI into a fixed 1000-byte global buffer (hdrbuf) via unchecked sprintf calls. The HTTP receive buffer accepts URIs up to approximately 150 KB (WI_RXBUFSIZE = 153600), so an unauthenticated attacker who can reach the repeater HTTP port (default TCP 80) can overflow hdrbuf by at least 500 bytes with a single HTTP request containing a URI of 1500 bytes or longer, corrupting adjacent .bss-segment globals. The overflow occurs before any authentication check, making it reachable without credentials. A remote, unauthenticated attacker can achieve arbitrary code execution on the host running the repeater.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
跨界内存写
Source: CVE Program / CVE List V5
Vulnerability Title
UltraVNC 缓冲区错误漏洞
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及之前版本存在缓冲区错误漏洞，该漏洞源于嵌入式HTTP管理服务器中存在全局缓冲区溢出，函数wi_senderr()和wi_replyhdr()通过未检查的sprintf调用将调用者提供的HTTP请求URI写入固定1000字节的全局缓冲区hdrbuf，可能导致未经身份验证的攻击者通过发送长度至少1500字节的URI的HTTP请求溢出hdrbuf至少500字节，破坏相邻的.bss段全局变量，从
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
II. Public POCs for CVE-2026-7840
| # | POC Description | Source Link | Shenlong Link |
|---|
AI-Generated POCPremium
Qwen3.6-35B-A3B · 6843 chars
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III. Intelligence Information for CVE-2026-7840
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News Coverage for CVE-2026-7840 (1)
Vendor Pages for CVE-2026-7840 (1)
Same Patch Batch · uvnc · 2026-07-01 · 10 CVEs total
| 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 |
| CVE-2026-44042 | 3.7 LOW | UltraVNC repeater wi_uudecode off-by-one in base64 decode boundary check |
IV. Related Vulnerabilities
Same product: UltraVNC
- CVE-2026-7839
UltraVNC repeater ships hardcoded default admin password all - CVE-2026-7838
UltraVNC viewer heap buffer overflow via integer overflow in - CVE-2026-7831
UltraVNC viewer off-by-one stack overflow in ServerInit desk - CVE-2026-7830
UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded l - CVE-2026-7829
UltraVNC repeater authenticated out-of-bounds write in rule
Same vendor: uvnc
- CVE-2026-7839
UltraVNC repeater ships hardcoded default admin password all - CVE-2026-7838
UltraVNC viewer heap buffer overflow via integer overflow in - CVE-2026-7831
UltraVNC viewer off-by-one stack overflow in ServerInit desk - CVE-2026-7830
UltraVNC MS-Logon II uses 64-bit Diffie-Hellman and seeded l - CVE-2026-7829
UltraVNC repeater authenticated out-of-bounds write in rule
Same weakness: CWE-787
- CVE-2026-66041
FFmpeg 7.0 - 8.1.2 Heap Out-of-Bounds Write via vf_quirc Fil - CVE-2026-45813
Apache NimBLE: Incorrect data validation in BASS add/modify - CVE-2026-16807
Google Chrome 缓冲区错误漏洞 - CVE-2026-65706
FFmpeg 3.0 - 8.1.2 vf_swaprect Out-of-Bounds Write via NV12 - CVE-2026-65705
FFmpeg 3.4 - 8.1.2 vf_floodfill Out-of-Bounds Write via filt
V. Comments for CVE-2026-7840
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