# CVE-2026-42786: WebSocket fragmented message reassembly unbounded in bandit

> Unauthenticated attackers can cause denial of service via memory exhaustion in the Bandit Elixir library by sending unbounded WebSocket continuation frames.

- **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:** 1 May 2026
- **Updated:** 27 May 2026
- **Also known as:** EEF-CVE-2026-42786, GHSA-pf94-94m9-536p

## What is CVE-2026-42786?

CVE-2026-42786 is a high-severity denial of service vulnerability in mtrudel bandit, published on 1 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion.

The fragment reassembly path in 'Elixir.Bandit.WebSocket.Connection':handle_frame/3 in lib/bandit/websocket/connection.ex appends every incoming Continuation{fin: false} frame's payload to a per-connection iolist with no cumulative size cap. The existing max_frame_size option only bounds individual frames; a peer that streams an unbounded number of continuation frames without ever setting fin=1 grows BEAM heap linearly until the OS or a supervisor kills the process.

Because the accumulation happens before WebSock.handle_in/2 is called, the application has no opportunity to interpose a size check. Phoenix Channels and LiveView both run over WebSock on Bandit, so a stock Phoenix application exposes this surface as soon as it accepts socket connections.

This issue affects bandit: from 0.5.0 before 1.11.0.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-42786?

3 open source packages ship an affected version, including bandit.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bandit` | Hex | From 0.5.0 | 1.11.0 |
| `bandit` | Hex | From 0.5.0 | 1.11.0 |
| `bandit` | Hex | From 0.5.0 | 1.11.0 |

## How do I fix CVE-2026-42786?

Upgrade bandit to 1.11.0 or later.

## Is CVE-2026-42786 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42786 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42786 and fix it if it is affected.

What it is: CVE-2026-42786 is a high-severity denial of service vulnerability in mtrudel bandit, published on 1 May 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-2026-42786 exists.

Affected packages and fixed versions:
- bandit (Hex): affected from 0.5.0; fixed in 1.11.0
- bandit (Hex): affected from 0.5.0; fixed in 1.11.0
- bandit (Hex): affected from 0.5.0; fixed in 1.11.0

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-2026-42786/ (Markdown: https://insights.sbomify.com/CVE-2026-42786/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42786
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42786
- OSV: https://osv.dev/vulnerability/CVE-2026-42786

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42786/
Summaries are generated and may contain mistakes; the official description above is the record.