# CVE-2026-39803: HTTP/1 chunked body reader ignores length cap in bandit

> Unauthenticated attackers can cause denial of service via memory exhaustion in the Bandit Elixir library by sending large chunked HTTP requests.

- **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:** 13 May 2026
- **Updated:** 27 May 2026
- **Also known as:** EEF-CVE-2026-39803, GHSA-9q9q-324x-93r2

## What is CVE-2026-39803?

CVE-2026-39803 is a high-severity denial of service vulnerability in mtrudel bandit, published on 13 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 chunked clause of 'Elixir.Bandit.HTTP1.Socket':read_data/2 in lib/bandit/http1/socket.ex ignores the caller-supplied :length option when reading HTTP/1 chunked request bodies. Instead of capping the accumulated body at the configured limit (e.g. Plug.Parsers' default 8 MB), do_read_chunked_data!/5 buffers every received chunk into an iolist unconditionally and materializes the entire body as a single binary. The function always returns {:ok, body, ...}, so callers cannot interpose a 413 response.

Because Plug.Parsers runs before routing and authentication in the standard Phoenix endpoint, an unauthenticated attacker needs no valid route or credentials. Sending a single Transfer-Encoding: chunked POST request with an arbitrarily large body to any path causes the BEAM process to exhaust available memory and be terminated by the OS OOM killer.

The content-length path in the same function correctly enforces the limit and is not affected.

This issue affects bandit: from 1.4.0 before 1.11.1.

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

## Which versions are affected by CVE-2026-39803?

3 open source packages ship an affected version, including bandit. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bandit` | Hex | From 1.4.0 | 1.11.1 |
| `bandit` | Hex | From 1.4.0 | 1.11.1 |
| `bandit` | Hex | From 1.4.0 | 1.11.1 |

Products: Mtrudel Bandit.

## How do I fix CVE-2026-39803?

Upgrade bandit to 1.11.1 or later.

## Is CVE-2026-39803 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39803 exists.

## Other vulnerabilities in mtrudel bandit

- [CVE-2026-39806](https://insights.sbomify.com/CVE-2026-39806/): HTTP/1 chunked decoder infinite loop on requests with trailer fields in bandit (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39803 and fix it if it is affected.

What it is: CVE-2026-39803 is a high-severity denial of service vulnerability in mtrudel bandit, published on 13 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-2026-39803 exists.

Affected packages and fixed versions:
- bandit (Hex): affected from 1.4.0; fixed in 1.11.1
- bandit (Hex): affected from 1.4.0; fixed in 1.11.1
- bandit (Hex): affected from 1.4.0; fixed in 1.11.1

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-2026-39803/ (Markdown: https://insights.sbomify.com/CVE-2026-39803/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39803
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39803
- OSV: https://osv.dev/vulnerability/CVE-2026-39803

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39803/
Summaries are generated and may contain mistakes; the official description above is the record.