# CVE-2026-39804: WebSocket permessage-deflate inflate has no output-size cap in bandit

> Unauthenticated attackers can cause memory exhaustion and denial of service via WebSocket compression if specific options are enabled.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/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-2026-39804, GHSA-frh3-6pv6-rc8j

## What is CVE-2026-39804?

CVE-2026-39804 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 when WebSocket permessage-deflate compression is enabled.

'Elixir.Bandit.WebSocket.PerMessageDeflate':inflate/2 in lib/bandit/websocket/permessage_deflate.ex calls :zlib.inflate/2 with no output-size cap, then materializes the entire decompressed payload as a single binary via IO.iodata_to_binary/1. The websocket_options.max_frame_size option only bounds the on-the-wire (compressed) frame size, not the decompressed output. A high-ratio compressed frame (e.g. uniform data at ~1024:1 ratio) can stay well under any wire-size limit while forcing GiB-scale heap allocations in the connection process before any application code runs.

An unauthenticated attacker who can open a WebSocket connection can send a single such frame to exhaust the BEAM node's memory and trigger an OOM kill.

This vulnerability requires both Bandit's server-level websocket_options.compress and the per-upgrade compress: true option passed to WebSockAdapter.upgrade/4 to be enabled. Stock Phoenix and LiveView applications are not affected as they default to compress: false.

This issue affects bandit: from 0.5.9 before 1.11.0.

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

## Which versions are affected by CVE-2026-39804?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bandit` | Hex | From 0.5.8 | 1.11.0 |
| `bandit` | Hex | From 0.5.8 | 1.11.0 |
| `bandit` | Hex | From 0.5.9 | 1.11.0 |

## How do I fix CVE-2026-39804?

Upgrade bandit to 1.11.0 or later.

## Is CVE-2026-39804 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39804 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39804 and fix it if it is affected.

What it is: CVE-2026-39804 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-2026-39804 exists.

Affected packages and fixed versions:
- bandit (Hex): affected from 0.5.8; fixed in 1.11.0
- bandit (Hex): affected from 0.5.8; fixed in 1.11.0
- bandit (Hex): affected from 0.5.9; 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-2026-39804/ (Markdown: https://insights.sbomify.com/CVE-2026-39804/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39804
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39804
- OSV: https://osv.dev/vulnerability/CVE-2026-39804

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39804/
Summaries are generated and may contain mistakes; the official description above is the record.