Vulnerability · Published May 26, 2026 · Assigned by GitHub_M
CVE-2026-46431Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *
Medium 4.3
In plain English
A medium-severity security flaw in algernon from xyproto. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 12% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/xyproto/algernon | 1.17.7 | GHSA-hw27-4v2q-5qff |
| Go | github.com/xyproto/algernon | 1.17.7 | GO-2026-5440 |
Affected products (as reported)
Weakness: CWE-942 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
References
- https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff x_refsource_CONFIRM