CVE-2026-46431 Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *
An attacker can read live filename streams from the SSE event server via cross-origin requests due to a wildcard CORS header.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-46431?
CVE-2026-46431 is a medium-severity vulnerability in xyproto algernon, published on 26 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
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.
Weakness: CWE-942
Which versions are affected by CVE-2026-46431?
3 open source packages ship an affected version, including github.com/xyproto/algernon.
| Package | Affected | Fixed in |
|---|---|---|
github.com/xyproto/algernon Go | All versions | 1.17.7 |
github.com/xyproto/algernon Go | All versions | 1.17.7 |
github.com/xyproto/algernon Go | All versions | 1.17.7 |
How do I fix CVE-2026-46431?
Upgrade github.com/xyproto/algernon to 1.17.7 or later.
Is CVE-2026-46431 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-46431 exists.
Sources
Also known as GHSA-hw27-4v2q-5qff, GO-2026-5440.