# 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.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Jul 2026
- **Updated:** 25 Jul 2026
- **Also known as:** EEF-CVE-2026-65623, GHSA-vg8x-66vg-5pxh

## 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 (Inefficient Algorithmic Complexity)

## Which versions are affected by CVE-2026-65623?

1 open source package ship an affected version, including bandit.


| Package | Ecosystem | 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.

## Is CVE-2026-65623 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-65623 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-65623 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-65623 exists.

Affected packages and fixed versions:
- bandit (Hex): affected from 1.11.0; fixed in 1.12.1

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-2026-65623/ (Markdown: https://insights.sbomify.com/CVE-2026-65623/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-65623
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-65623
- OSV: https://osv.dev/vulnerability/CVE-2026-65623

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-65623/
Summaries are generated and may contain mistakes; the official description above is the record.