CVE-2026-46431: Xyproto Algernon

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, the SSE event server's Access-Control-Allow-Origin response header was hardcoded to the wildcard * regardless of the caller's Origin. Because EventSource does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin EventSource to the SSE port and read the live filename stream from JavaScript. This vulnerability is fixed in 1.17.7.

Affected products

• Xyproto Algernon: before 1.17.7 (fixed in 1.17.7)

Published 2026-05-26. Last modified 2026-07-24.