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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-42786?
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
Which versions are affected by CVE-2026-42786?
3 open source packages ship an affected version, including bandit.
| Package | 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.
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-2026-42786 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42786 exists.
Sources
Also known as EEF-CVE-2026-42786, GHSA-pf94-94m9-536p.