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CVE-2026-65623— Quadratic CPU blow-up reassembling fragmented WebSocket messages in Bandit

Quick assessment

Affected
mtrudel bandit
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

mtrudel bandit是mtrudel的安全检查工具。 mtrudel bandit 1.11.0版本至1.12.1之前版本存在资源管理错误漏洞，该漏洞源于WebSocket分片重组时的算法复杂性效率低下，可能导致未经身份验证的攻击者通过发送大量微小延续帧耗尽CPU，造成拒绝服务。

CVSS 8.7 · High EPSS 0.64% · P49

Affected Version Matrix 2

VendorProduct Version RangeStatus
mtrudel bandit 1.11.0< 1.12.1 affected
21612c7c7b1ce43eccd36d3af3a2299d23513667< 418ef7e906192a230ddba112f7a669c87b6b0e3a affected
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I. Basic Information for CVE-2026-65623

Vulnerability Information
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Vulnerability Title
Quadratic CPU blow-up reassembling fragmented WebSocket messages in Bandit
Source: CVE Program / CVE List V5
Vulnerability Description
Inefficient Algorithmic Complexity vulnerability in mtrudel bandit allows unauthenticated remote denial of service via CPU exhaustion during WebSocket fragment reassembly. The size guard 'Elixir.Bandit.WebSocket.Connection':oversize_message?/2 called from handle_frame/3 in lib/bandit/websocket/connection.ex appends each non-final continuation frame to a left-nested iolist and then re-measures the entire accumulated buffer with IO.iodata_length/1 on every frame. Because the buffer grows by one element per frame and is fully re-traversed each time, reassembly work is quadratic (O(n^2)) in the number of continuation frames. The max_fragmented_message_size limit (default 8 MB) bounds total bytes but not frame count, and each frame can carry as little as one payload byte, so an attacker can send millions of tiny continuation frames using modest bandwidth to pin a CPU core for minutes to hours. Many concurrent connections can starve the whole server of CPU, denying service to legitimate users. The WebSocket read timeout does not help, because it is an idle timeout evaluated between reads and cannot preempt the synchronous reassembly work spent inside a single callback. This issue affects bandit: from 1.11.0 before 1.12.1.
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
算法复杂性
Source: CVE Program / CVE List V5
Vulnerability Title
mtrudel bandit 资源管理错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
mtrudel bandit是mtrudel的安全检查工具。 mtrudel bandit 1.11.0版本至1.12.1之前版本存在资源管理错误漏洞，该漏洞源于WebSocket分片重组时的算法复杂性效率低下，可能导致未经身份验证的攻击者通过发送大量微小延续帧耗尽CPU，造成拒绝服务。
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 1.11.0 ~ 1.12.1 cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*
mtrudel bandit 21612c7c7b1ce43eccd36d3af3a2299d23513667 ~ 418ef7e906192a230ddba112f7a669c87b6b0e3a cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-65623

# POC Description Source Link Shenlong Link
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Qwen3.6-35B-A3B · 11898 chars
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III. Intelligence Information for CVE-2026-65623

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Patches & Fixes for CVE-2026-65623 (1)

Vendor Advisories for CVE-2026-65623 (3)

IV. Related Vulnerabilities

Same product: bandit
Same vendor: mtrudel
Same weakness: CWE-407

V. Comments for CVE-2026-65623

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