# 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.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N`)
- **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:** Yes
- **Published:** 26 May 2026
- **Updated:** 26 May 2026
- **Also known as:** GHSA-hw27-4v2q-5qff, GO-2026-5440

## 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 (Permissive Cross-domain Security Policy with Untrusted Domains)

## Which versions are affected by CVE-2026-46431?

3 open source packages ship an affected version, including github.com/xyproto/algernon.


| Package | Ecosystem | 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-46431 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-46431 exists.

Affected packages and fixed versions:
- github.com/xyproto/algernon (Go): affected all versions; fixed in 1.17.7
- github.com/xyproto/algernon (Go): affected all versions; fixed in 1.17.7
- github.com/xyproto/algernon (Go): affected all versions; fixed in 1.17.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-46431/ (Markdown: https://insights.sbomify.com/CVE-2026-46431/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-46431
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-46431
- OSV: https://osv.dev/vulnerability/CVE-2026-46431

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-46431/
Summaries are generated and may contain mistakes; the official description above is the record.