WebSocket fragmented message reassembly unbounded in bandit
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- None
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Fixed
Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub. Impact type from CWE-770.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- Poc
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| mtrudel | bandit | 0.5.0 to before 1.11.0 | 1.11.0 | Patch available | CNA |
| — | bandit (Hex) | 0.5.0 to before 1.11.0 | 1.11.0 | Patch available | GHSA-pf94-94m9-536p |
- bandit Vendormtrudel Affected 0.5.0 to before 1.11.0 Fixed in 1.11.0 Patch available CNA
- bandit (Hex) Vendor— Affected 0.5.0 to before 1.11.0 Fixed in 1.11.0 Patch available GHSA-pf94-94m9-536p
Remediation
- Fix · CNA bandit (mtrudel) Fixed in 1.11.0
- Fix · GHSA-pf94-94m9-536p bandit (Hex) Fixed in 1.11.0 Advisory →
Fixed in the same release
Records fixed by the same vendor advisory or release, as the CNA lists them.
- Release ·
mtrudel bandit 1.11.0
5 records Highest priority Priority HIGH 5 HIGH
Other records, highest priority first
- TI-2026-28687 WebSocket permessage-deflate inflate has no output-size cap in banditPriority HIGH 75
- TI-2026-31008 HTTP/2 frame size limit checked after body is buffered in banditPriority HIGH 70
- TI-2026-28690 Client-supplied URI scheme trusted without transport verification in banditPriority HIGH 67
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2026-42786
- 2026-10-03 Alias added: GHSA-pf94-94m9-536p
- 2026-10-21 EPSS moved to the 50th–90th percentile
- 2026-10-25 EPSS moved to the below the 50th percentile
- 2026-10-27 EPSS moved to the 50th–90th percentile
References
Advisory 2
More mtrudel vulnerabilities
- TI-2026-81989 HTTP/2 request bodies made of 1-byte DATA frames amplify memory use in Bandit CVSS 4.0HIGH 8.7
- TI-2026-74311 Bandit HTTP/2 send timers reset, letting trickles pin stream processes CVSS 4.0HIGH 8.7
- TI-2026-52571 HTTP/2 connection-window starvation pins Plug processes indefinitely in Bandit CVSS 4.0HIGH 8.7
- TI-2026-46780 Quadratic CPU blow-up reassembling fragmented WebSocket messages in Bandit CVSS 4.0HIGH 8.7
- TI-2026-28689 HTTP/1 chunked decoder infinite loop on requests with trailer fields in bandit CVSS 4.0HIGH 8.7
- TI-2026-28686 HTTP/1 chunked body reader ignores length cap in bandit CVSS 4.0HIGH 8.7
Sources
- CVE-2026-42786 · CVE Program
- CVE-2026-42786 · NVD
- FIRST EPSS
- GHSA-pf94-94m9-536p · GitHub Advisory Database · CC BY 4.0
- CVE-2026-42786 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences