CVE-2026-65623
Quadratic CPU blow-up reassembling fragmented WebSocket messages in Bandit
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.
INFO
Published Date :
July 24, 2026, 5:17 p.m.
Last Modified :
July 24, 2026, 6:18 p.m.
Remotely Exploit :
Yes !
Source :
6b3ad84c-e1a6-4bf7-a703-f496b71e49db
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 4.0 | HIGH | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | ||||
| CVSS 4.0 | HIGH | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db |
Solution
- Update Bandit to a version that includes the fix.
- Ensure WebSocket fragment reassembly is efficient.
- Implement rate limiting for incoming WebSocket frames.
- Monitor server CPU usage for anomalies.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-65623.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-65623 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-65623
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-65623 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-65623 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
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CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jul. 24, 2026
Action Type Old Value New Value Added Reference https://github.com/mtrudel/bandit/security/advisories/GHSA-vg8x-66vg-5pxh Added SSVC {'id': 'CVE-2026-65623', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-07-24T17:08:51.111665Z'} -
New CVE Received by 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
Jul. 24, 2026
Action Type Old Value New Value Added Affected [{'cpes': ['cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/mtrudel/bandit', 'vendor': 'mtrudel', 'modules': ['Elixir.Bandit.WebSocket.Connection'], 'product': 'bandit', 'versions': [{'status': 'affected', 'version': '1.11.0', 'lessThan': '1.12.1', 'versionType': 'semver'}], 'packageURL': 'pkg:hex/bandit', 'packageName': 'bandit', 'programFiles': ['lib/bandit/websocket/connection.ex'], 'collectionURL': 'https://repo.hex.pm', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': "'Elixir.Bandit.WebSocket.Connection':handle_frame/3"}, {'name': "'Elixir.Bandit.WebSocket.Connection':oversize_message?/2"}]}, {'cpes': ['cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*'], 'repo': 'https://github.com/mtrudel/bandit', 'vendor': 'mtrudel', 'modules': ['Elixir.Bandit.WebSocket.Connection'], 'product': 'bandit', 'versions': [{'status': 'affected', 'version': '21612c7c7b1ce43eccd36d3af3a2299d23513667', 'lessThan': '418ef7e906192a230ddba112f7a669c87b6b0e3a', 'versionType': 'git'}], 'packageURL': 'pkg:github/mtrudel/bandit', 'packageName': 'mtrudel/bandit', 'programFiles': ['lib/bandit/websocket/connection.ex'], 'collectionURL': 'https://github.com', 'defaultStatus': 'unaffected', 'programRoutines': [{'name': "'Elixir.Bandit.WebSocket.Connection':handle_frame/3"}, {'name': "'Elixir.Bandit.WebSocket.Connection':oversize_message?/2"}]}] Added 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. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CWE CWE-407 Added Reference https://cna.erlef.org/cves/CVE-2026-65623.html Added Reference https://github.com/mtrudel/bandit/commit/418ef7e906192a230ddba112f7a669c87b6b0e3a Added Reference https://github.com/mtrudel/bandit/security/advisories/GHSA-vg8x-66vg-5pxh Added Reference https://osv.dev/vulnerability/EEF-CVE-2026-65623