python-socketio EUVD-2026-56894

Exploitation requires that the server accept binary EVENT or ACK messages from the attacker - in vulnerable versions (<= 5.16.1) this path is reachable without authentication, which is why the 5.16.2 fix limits binary packets to authenticated clients. The concrete trigger is sending a binary message that declares N attachments while intentionally omitting one or more attachment frames, leaving the message and received attachments buffered in memory; sustained impact requires many such incomplete messages or connections to accumulate memory. Limiting factors: deployments that already require authentication before accepting events, or that disallow/limit binary payloads, substantially reduce exposure; impact is availability-only (no data exposure), and effectiveness depends on the server's memory headroom and any external connection/size/timeout limits already in place.

The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, base 7.5) reflects a remotely reachable, low-complexity, availability-only impact with no confidentiality or integrity loss - consistent with a resource-exhaustion DoS rather than data compromise. The fact that the official fix restricts binary packets to authenticated clients implies the vulnerable default path was reachable pre-auth, supporting PR:N. No EPSS score, KEV entry, or POC was supplied, so exploitation probability cannot be quantified from the input; the absence of these signals plus an availability-only impact suggests this is a genuine but moderate-priority hardening fix rather than an emergency. The realistic risk is highest for internet-facing Socket.IO servers that accept binary events from unauthenticated or untrusted clients; servers that already gate connections behind authentication are far less exposed even before patching.

An attacker connects to an internet-facing python-socketio server and emits a binary EVENT packet declaring multiple attachments but sends only some (or none) of the attachment frames, then opens many such connections and repeats. Each incomplete message and its buffered attachments remain resident in server memory, and with enough concurrent partial messages the worker's memory is exhausted, degrading or crashing the service. No public POC is referenced, but the attack is low-complexity and requires only the ability to speak the Socket.IO protocol.