CVE-2026-65623 Quadratic CPU blow-up reassembling fragmented WebSocket messages in Bandit
Unauthenticated attackers can cause CPU exhaustion and denial of service by sending fragmented WebSocket messages to the Bandit server.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-65623?
CVE-2026-65623 is a high-severity denial of service vulnerability in mtrudel bandit, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Inefficient Algorithmic Complexity vulnerability in mtrudel bandit allows unauthenticated remote denial of service via CPU exhaustion during WebSocket fragment reassembly.
The size guard 'Elixir.Bandit.WebSocket.Connection':oversize_message?/2 called from handle_frame/3 in lib/bandit/websocket/connection.ex appends each non-final continuation frame to a left-nested iolist and then re-measures the entire accumulated buffer with IO.iodata_length/1 on every frame. Because the buffer grows by one element per frame and is fully re-traversed each time, reassembly work is quadratic (O(n^2)) in the number of continuation frames.
The max_fragmented_message_size limit (default 8 MB) bounds total bytes but not frame count, and each frame can carry as little as one payload byte, so an attacker can send millions of tiny continuation frames using modest bandwidth to pin a CPU core for minutes to hours. Many concurrent connections can starve the whole server of CPU, denying service to legitimate users. The WebSocket read timeout does not help, because it is an idle timeout evaluated between reads and cannot preempt the synchronous reassembly work spent inside a single callback.
This issue affects bandit: from 1.11.0 before 1.12.1.
Weakness: CWE-407
Which versions are affected by CVE-2026-65623?
1 open source package ship an affected version, including bandit.
| Package | Affected | Fixed in |
|---|---|---|
bandit Hex | From 1.11.0 | 1.12.1 |
How do I fix CVE-2026-65623?
Upgrade bandit to 1.12.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-65623 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-65623 exists.
Sources
Also known as EEF-CVE-2026-65623, GHSA-vg8x-66vg-5pxh.