PatchTheCVECVE-2026-7840: UltraVNC repeater through 1.8.2.2 contains a global buffer overflow in its embedded HTTP administration server
CVE-2026-7840 is a critical-severity flaw (CVSS 9.8) affecting ultravnc. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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 repeate
Why this priority
- Critical severity
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 9.8 (Critical) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 2.6% (top 15%) |
| CISA exploited list | No |
| Weakness | CWE-787 |
| Published | 2026-07-01 |
| Last modified | 2026-10-25 |
Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-7840?
ultravnc: upgrade to newer than 1.8.2.2. Exact commands for each OS are on this page.
Is CVE-2026-7840 being exploited?
CVE-2026-7840 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-7840 affect?
CVE-2026-7840 affects ultravnc. Severity is critical (CVSS 9.8). 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 fix
How urgent is CVE-2026-7840?
Priority 61/100. Critical severity; Remotely exploitable, no login, no user action
Related CVEs
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- CVE-2026-7828 MEDIUM 5.3 UPGRADE UltraVNC repeater through 1.8.2.2 contains an integer overflow in the HTTP request logging path
- CVE-2026-7829 HIGH 7.2 UPGRADE UltraVNC repeater through 1.8.2.2 contains a post-authentication out-of-bounds write in the allow/deny rule pa
- CVE-2026-7831 HIGH 7.6 UPGRADE UltraVNC viewer through 1.8.2.2 contains an off-by-one stack buffer overflow in the RFB ServerInit message han