Algernon: Auto-refresh SSE event server binds to all interfaces with Access-Control-Allow-Origin: * and no authentication

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary When auto-refresh is enabled, Algernon spins up an SSE handler that streams a data: line for every filesystem event under the watched directory. The handler performs no authentication of any kind — no shared token, no cookie check against the permissions2 userstate, no IP allow-list, no path-prefix permission. Any client that can complete a TCP connection to the listener address receives the stream. This advisory covers the authentication gap in isolation. The cross-origin browser-reach (advisory #2b) and the network-reach (advisory #2c) amplify the impact, but each is independently fixable; this finding addresses the case where a same-origin or LAN-local client connects directly to the SSE port and reads the stream without proving anything about its identity. Details Root cause — the SSE handler does not consult permissions2 or any other auth Note the handler signature: func(w http.ResponseWriter, http.Request). The request is discarded — no Cookie, Authorization,…

Published by the source

19 May 2026, 14:36 UTC

Last modified by the source

19 May 2026, 14:36 UTC

Source last checked

26 Oct 2026, 15:22 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-9v4j-7g44-qcqw

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# Patch Next advisory brief: GHSA-9v4j-7g44-qcqw

Advisory: Algernon: Auto-refresh SSE event server binds to all interfaces with Access-Control-Allow-Origin: * and no authentication
IDs: GHSA-9v4j-7g44-qcqw
Record: https://patchnext.com/advisories/ghsa-9v4j-7g44-qcqw (revision 1, source checked 26 Oct 2026, 15:22 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/xyproto/algernon 1.17.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/xyproto/algernon: affected <= 1.17.6; fixed in 1.17.7

## Suggested inspection (read-only)
- Go github.com/xyproto/algernon: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/xyproto/algernon.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-9v4j-7g44-qcqw (GitHub-reviewed advisory GHSA-9v4j-7g44-qcqw lists affected ranges: Go github.com/xyproto/algernon >=0 <=1.17.6)

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