engine.io Uncaught Exception vulnerability

Answer in brief

CVE-2023-31125 records a Medium severity (CVSS 6.5) vulnerability in engine.io Uncaught Exception vulnerability. The current sources do not mark it as known exploited. The current feed maps engine.io (npm). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published May 3, 2023Updated Sep 10, 2026Source checked Sep 10, 2026First seen by HOL Sep 10, 2026Material review Sep 10, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 6.5
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 6.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps engine.io (npm). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
engine.ionpm	>=5.1.0 <6.4.2	6.4.2

Recommended response

Check inventory. Check lockfiles and deployed manifests for engine.io.
Review the reported fix. Update engine.io to 6.4.2 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
May 3, 2023
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Sep 10, 2026

Sources and claim methodology

Upstream source description

### Impact A specially crafted HTTP request can trigger an uncaught exception on the Engine.IO server, thus killing the Node.js process. ``` TypeError: Cannot read properties of undefined (reading 'handlesUpgrades') at Server.onWebSocket (build/server.js:515:67) ``` This impacts all the users of the [`engine.io`](https://www.npmjs.com/package/engine.io) package, including those who uses depending packages like [`socket.io`](https://www.npmjs.com/package/socket.io). ### Patches A fix has been released today (2023/05/02): [6.4.2](https://github.com/socketio/engine.io/releases/tag/6.4.2) This bug was introduced in version 5.1.0 and included in version 4.1.0 of the `socket.io` parent package. Older versions are not impacted. For `socket.io` users: | Version range | `engine.io` version | Needs minor update? | |-----------------------------|---------------------|--------------------------------------------------------------------------------------------------------| | `[email protected]` | `~6.4.0` | `npm audit fix` should be sufficient | | `[email protected]` | `~6.2.0` | Please upgrade to `[email protected]` | | `[email protected]` | `~6.1.0` | Please upgrade to `[email protected]` | | `[email protected]` | `~6.0.0` | Please upgrade to `[email protected]` | | `[email protected]` | `~5.2.0` | Please upgrade to `[email protected]` | | `[email protected]` | `~5.1.1` | Please upgrade to `[email protected]` | | `[email protected]` | `~5.0.0` | Not impacted | | `[email protected]` | `~4.1.0` | Not impacted | | `[email protected]` | `~4.0.0` | Not impacted | | `[email protected]` | `~3.6.0` | Not impacted | | `[email protected]` and below | `~3.5.0` | Not impacted | ### Workarounds There is no known workaround except upgrading to a safe version. ### For more information If you have any questions or comments about this advisory: * Open an issue in [`engine.io`](https://github.com/socketio/engine.io) Thanks to Thomas Rinsma from Codean for the responsible disclosure.

Quoted source text, attributed separately from HOL analysis.