CVE-2021-28363Network attack vector

Traffic interception via unchecked HTTPS proxy hostname

Published Mar 15, 2021 · Updated Aug 3, 2024

Improper certificate validation in urllib3 1.26.0 through 1.26.3 allows man-in-the-middle attackers to impersonate HTTPS proxies. The _connect_tls_proxy method leaves check_hostname disabled when it creates the default SSLContext, so a trusted certificate for another host is accepted. Exploitation requires HTTPS-to-HTTPS proxy use without a caller-supplied SSLContext and exposes proxied traffic to interception or modification.

CVSS severity6.5
Medium
EPSS probability2.11%
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 urllib3 1.26.0 through 1.26.3 allows man-in-the-middle attackers to impersonate HTTPS proxies. The _connect_tls_proxy method leaves check_hostname disabled when it creates the default SSLContext, so a trusted certificate for another host is accepted. Exploitation requires HTTPS-to-HTTPS proxy use without a caller-supplied SSLContext and exposes proxied traffic to interception or modification.

The record

CVE
CVE-2021-28363
Published
Mar 15, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-295, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 15, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

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)
Vectornetwork
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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