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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-39806?
3 open source packages ship an affected version, including bandit. It affects 1 product.
| Package | 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 include Mtrudel Bandit.
How do I fix CVE-2026-39806?
Upgrade bandit to 1.11.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
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
Sources
Also known as EEF-CVE-2026-39806, GHSA-rf5q-vwxw-gmrf.