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CVE-2026-42786— WebSocket fragmented message reassembly unbounded in bandit

Quick assessment

Affected
mtrudel bandit
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Bandit是Mat Trudel个人开发者的一款高性能HTTP与WebSocket服务器。 Bandit 0.5.0版本至1.11.0之前版本存在安全漏洞，该漏洞源于WebSocket连接中片段重组路径未对累积的续帧有效载荷设置大小上限，可能导致未经身份验证的远程拒绝服务。

AI Predicted 7.5 Difficulty: Easy EPSS 0.64% · P49

Affected Version Matrix 2

VendorProduct Version RangeStatus
mtrudel bandit 0.5.0< 1.11.0 affected
8909391f486d42138c5308410bc5ea49a65f4d46< 21612c7c7b1ce43eccd36d3af3a2299d23513667 affected
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I. Basic Information for CVE-2026-42786

Vulnerability Information
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Vulnerability Title
WebSocket fragmented message reassembly unbounded in bandit
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
不加限制或调节的资源分配
Source: CVE Program / CVE List V5
Vulnerability Title
Bandit 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Bandit是Mat Trudel个人开发者的一款高性能HTTP与WebSocket服务器。 Bandit 0.5.0版本至1.11.0之前版本存在安全漏洞，该漏洞源于WebSocket连接中片段重组路径未对累积的续帧有效载荷设置大小上限，可能导致未经身份验证的远程拒绝服务。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
mtrudel bandit 0.5.0 ~ 1.11.0 cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*
mtrudel bandit 8909391f486d42138c5308410bc5ea49a65f4d46 ~ 21612c7c7b1ce43eccd36d3af3a2299d23513667 cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-42786

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III. Intelligence Information for CVE-2026-42786

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Patches & Fixes for CVE-2026-42786 (1)

Vendor Advisories for CVE-2026-42786 (3)

Same Patch Batch · mtrudel · 2026-05-01 · 5 CVEs total

CVE-2026-39805 6.3 MEDIUM CL.CL HTTP request smuggling via duplicate Content-Length in bandit
CVE-2026-42788 HTTP/2 frame size limit checked after body is buffered in bandit
CVE-2026-39807 Client-supplied URI scheme trusted without transport verification in bandit
CVE-2026-39804 WebSocket permessage-deflate inflate has no output-size cap in bandit

IV. Related Vulnerabilities

Same product: bandit
Same vendor: mtrudel
Same weakness: CWE-770

V. Comments for CVE-2026-42786

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