CVE-2021-28363

mediumpriority · track · 27EPSS 0.022 · 82 %ileDEEURUUS
grimoire priorityP3 · Track
Exploitable?
—
no known exploitation
Likely / severe?
2.17% (82nd) EPSS
CVSS 6.4 · medium
Is it mine?
29 products
Check your exposure
Who says so?
7 sources
cnnvd · enisa_euvd · fstec_bdu · osv

EPSS · scored 2026-10-27

The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies. The initial connection to the HTTPS proxy (if an SSLContext isn't given via proxy_config) doesn't verify the hostname of the certificate. This means certificates for different servers that still validate properly with the default urllib3 SSLContext will be silently accepted.

01

remediations

Vendor fix, mitigation, or workaround guidance, as published by each source.

A vendor fix is available.

02

cvss

Technical severity, 0–10. One row per scoring source.

6.5
010

base score 0–10 · higher = more severe · hover a vector to decode

v2.06.4mediumAV:N/AC:L/Au:N/C:P/I:P/A:Nprimary · NVD / NIST USexploitability 10 · impact 4.9
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:Nprimary · NVD / NIST USexploitability 3.9 · impact 2.5
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N/E:P/RL:O/RC:Csecondary · CISA US
v3.16.5mediumAV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:Nsecondary · FSTEC (BDU) RU
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:Nsecondary · OSV
v3.16.5mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N/E:P/RL:O/RC:Csecondary · Siemens ProductCERT DE
v3.16mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N/E:Usecondary · CVSS-BT (t0sche)
03

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-295USprimaryCWE-295USCWE-295USCWE-295USCWE-295US
04

affected products (29)

Products and version ranges known to be affected.

showing 20 of 29
05

advisories (4)

Vendor and ecosystem advisories describing this.

  • SSA-398330SSA-398330: Vulnerabilities in the additional GNU/Linux subsystem of the SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP >= V3.1.0 and < V3.1.52023-12-12csaf · siemens_psirtDE

    Multiple vulnerabilities have been identified in the additional GNU/Linux subsystem of the firmware version >= V3.1.0 and < V3.1.5 for the SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP (incl. SIPLUS variant). These GNU/Linux vulnerabilities have been externally identified. Siemens has released new versions for the affected products and recommends to update to the latest versions. SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP >= V3.1.0 and < V3.1.5 are no longer maintained and there won't be any update to the CVE list anymore. Please update to the latest version. Note: This SSA advises vulnerabilities for firmware version V3.1 only; for versions < V3.1 refer to Siemens Security Bulletin SSB-439005 (https://cert-portal.siemens.com/productcert/html/ssb-439005.html).

  • GHSA-5phf-pp7p-vc2rUsing default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection2021-03-19ghsa · osvUS

    ### Impact Users who are using an HTTPS proxy to issue HTTPS requests and haven't configured their own SSLContext via `proxy_config`. Only the default SSLContext is impacted. ### Patches [urllib3 >=1.26.4 has the issue resolved](https://github.com/urllib3/urllib3/releases/tag/1.26.4). urllib3<1.26 is not impacted due to not supporting HTTPS requests via HTTPS proxies. ### Workarounds Upgrading is recommended as this is a minor release and not likely to break current usage. Configuring an `SSLContext` with `check_hostname=True` and passing via `proxy_config` instead of relying on the default `SSLContext` ### For more information If you have any questions or comments about this advisory: * Email us at [sethmichaellarson@gmail.com](mailto:sethmichaellarson@gmail.com)

  • ICSA-23-348-10Siemens SIMATIC S7-15002023-12-12icsa · cisa_csaf_otUS

    Multiple vulnerabilities have been identified in the additional GNU/Linux subsystem of the firmware version >= V3.1.0 and < V3.1.5 for the SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP (incl. SIPLUS variant). These GNU/Linux vulnerabilities have been externally identified. Siemens has released new versions for the affected products and recommends to update to the latest versions. Note: This SSA advises vulnerabilities for firmware version V3.1 only; for versions < V3.1 refer to Siemens Security Bulletin SSB-439005 ( https://cert-portal.siemens.com/productcert/html/ssb-439005.html).

  • PYSEC-2021-592021-03-15vendor · osvUS

    The urllib3 library 1.26.x before 1.26.4 for Python omits SSL certificate validation in some cases involving HTTPS to HTTPS proxies. The initial connection to the HTTPS proxy (if an SSLContext isn't given via proxy_config) doesn't verify the hostname of the certificate. This means certificates for different servers that still validate properly with the default urllib3 SSLContext will be silently accepted.

06

also known as (7)

Other names and identifiers for this entity.

07

sources

Which sources assert this record, and when.

cnnvdenisa_euvdfstec_bduosvcisa_csaf_otnvdsiemens_psirtNVDCVE.orgMITRE
published 2021-03-15modified 2026-06-17
09

references (35)

External citations — patches, advisories, analysis.

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