CVE-2026-46431

Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *

State: Published
Severity: 4.3 Medium
Published: May 26, 2026
Updated: May 26, 2026
Reserved: May 13, 2026

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.

Scores

CVSS	Score	Vector	Source
3.1	4.3 Medium	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N	GitHub_M

Weaknesses

CWE-942 Permissive Cross-domain Policy with Untrusted Domains

Affected Products

Product	Versions
algernonxyproto	< 1.17.7Affected

References

https://github.com/xyproto/algernon/security/advisories/GHSA-hw27-4v2q-5qff

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