# CVE-2026-39806: HTTP/1 chunked decoder infinite loop on requests with trailer fields in bandit

> Unauthenticated attackers can cause denial of service in Bandit by sending chunked requests with trailer fields, exhausting worker processes.

- **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-2026-39806, GHSA-rf5q-vwxw-gmrf

## What is CVE-2026-39806?

CVE-2026-39806 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

Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in mtrudel bandit allows unauthenticated remote denial of service via worker process exhaustion.

'Elixir.Bandit.HTTP1.Socket':do_read_chunked_data!/5 in lib/bandit/http1/socket.ex terminates only when the last-chunk line 0\r\n is followed immediately by the empty trailer line \r\n. RFC 9112 §7.1.2 permits zero or more trailer fields between them. When trailers are present, none of the match clauses fit: the catch-all arm computes a negative to_read, calls read_available!/2, receives <<>> on timeout, and tail-recurses with unchanged state. The worker process is pinned for the lifetime of the TCP connection.

A handful of concurrent connections sending RFC-conformant chunked requests with trailer fields is sufficient to exhaust the Bandit worker pool and render the server unresponsive to all further traffic. No authentication, special headers, or large payload is required. Proxies such as NGINX and HAProxy legitimately forward trailer-bearing requests, so servers behind such proxies may be affected without any malicious client involvement.

This issue affects bandit: from 1.6.1 before 1.11.1.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2026-39806?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bandit` | Hex | From 1.6.0 | 1.11.1 |
| `bandit` | Hex | From 1.6.0 | 1.11.1 |
| `bandit` | Hex | From 1.6.1 | 1.11.1 |

Products: Mtrudel Bandit.

## How do I fix CVE-2026-39806?

Upgrade bandit to 1.11.1 or later.

## Is CVE-2026-39806 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39806 exists.

## Other vulnerabilities in mtrudel bandit

- [CVE-2026-39803](https://insights.sbomify.com/CVE-2026-39803/): HTTP/1 chunked body reader ignores length cap in bandit (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39806 and fix it if it is affected.

What it is: CVE-2026-39806 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-2026-39806 exists.

Affected packages and fixed versions:
- bandit (Hex): affected from 1.6.0; fixed in 1.11.1
- bandit (Hex): affected from 1.6.0; fixed in 1.11.1
- bandit (Hex): affected from 1.6.1; 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-2026-39806/ (Markdown: https://insights.sbomify.com/CVE-2026-39806/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39806
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39806
- OSV: https://osv.dev/vulnerability/CVE-2026-39806

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39806/
Summaries are generated and may contain mistakes; the official description above is the record.