CVE-2021-40828

TLS server impersonation via skipped hostname verification

Published Nov 22, 2021 · Updated Aug 4, 2024

Improper certificate validation in AWS IoT Device SDK v2 for Windows allows adjacent attackers to impersonate trusted TLS servers. The aws-c-io Windows TLS path skipped SNI hostname verification when an application replaced the default CA trust store, accepting a custom-CA certificate for the wrong host. Exploitation requires that custom trust configuration and an attacker positioned to intercept the connection, enabling disclosure or alteration of AWS IoT traffic.

CVSS severity8.8
High
EPSS probability0.41%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper certificate validation in AWS IoT Device SDK v2 for Windows allows adjacent attackers to impersonate trusted TLS servers. The aws-c-io Windows TLS path skipped SNI hostname verification when an application replaced the default CA trust store, accepting a custom-CA certificate for the wrong host. Exploitation requires that custom trust configuration and an attacker positioned to intercept the connection, enabling disclosure or alteration of AWS IoT traffic.

The record

CVE
CVE-2021-40828
Published
Nov 22, 2021
Updated
Aug 4, 2024
Vendor
Amazon Web Services
Product
AWS IoT Device SDK v2 for Python
Classifications
CWE-295, T1557
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 22, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <1.5.18

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Amazon Web Services · AWS IoT Device SDK v2 for Pythonversion=unspecified <1.5.18
Amazon Web Services · AWS IoT Device SDK v2 for C++version=unspecified <1.12.7
Amazon Web Services · AWS-C-IOversion=0.9.12
Amazon Web Services · AWS IoT Device SDK v2 for Javaversion=unspecified <1.3.3
Amazon Web Services · AWS IoT Device SDK v2 for Node.jsversion=unspecified <1.5.3

Published CVSS scores

5.4GitHub Advisory DatabaseCVSS:4.0/AV:A/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
5.8National Vulnerability DatabaseAV:A/AC:L/Au:N/C:P/I:P/A:P
8.8National Vulnerability DatabaseCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-295Improper Certificate Validation
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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