CVE Intelligence
Medium Cryptography

CVE-2021-28363 — Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection

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.

Published Mar 15, 2021 · Updated Oct 14, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: mitre

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.
How the score was calculated
  • EPSS 2.1% (30-day exploitation probability)+12
  • Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Not confirmed
Not in CISA KEV — an absence of record, not proof it is unexploited.
EPSS FIRST
2.11%
ahead of 81% of scored CVEs · Oct 14, 2026
Affects
Fedoraproject
Fedora and 2 more
CVSS base
6.5
Medium · 2 source(s)

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

  • Documented patch — github.com (NIST National Vulnerability Database)
  • Documented patch — github.com (NIST National Vulnerability Database)
  • Documented mitigation — github.com (NIST National Vulnerability Database)
  • Documented patch — www.oracle.com (NIST National Vulnerability Database)
  • Pip Urllib3: fixed in version 1.26.4 — github.com (GitHub Security Advisories)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Fedoraproject Fedora, Oracle Peoplesoft Enterprise Peopletools, Python Urllib3…).
  2. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
  3. Apply the fixed version or the patch documented under Remediation.
  4. Search available logs for exploit probes matching the vulnerability class (cryptography), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Fedoraproject Fedora version 34 Vulnerable NIST National Vulnerability Database
Oracle Peoplesoft Enterprise Peopletools version 8.59 Vulnerable NIST National Vulnerability Database
Python Urllib3 ≥ 1.26.0 and < 1.26.4 Vulnerable NIST National Vulnerability Database
Pip Urllib3 ≥ 1.26.0 and < 1.26.4 Vulnerable GitHub Security Advisories

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
6.5 CVSS 3.1 Medium 3.9 2.5 nvd@nist.gov via NIST National Vulnerability Database
6.5 CVSS 3.1 Medium — — GitHub, Inc. via GitHub Security Advisories
6.4 CVSS 2.0 Medium 10.0 4.9 nvd@nist.gov via NIST National Vulnerability Database
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N (score shown at the top: nvd@nist.gov)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-295 · Improper Certificate Validation

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 2.11%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
  • Check TLS negotiations and unexpected certificates in the component’s traffic.
web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

  • 2026
  • Fix documented
    Oct 14, 2026 · GitHub Security Advisories
  • Affected products added · pip/urllib3
    Oct 14, 2026 · GitHub Security Advisories
  • Record published
    Oct 14, 2026
  • Fix documented
    Oct 14, 2026 · NIST National Vulnerability Database
  • Affected products added · python/urllib3, fedoraproject/fedora, oracle/peoplesoft enterprise peopletools
    Oct 14, 2026 · NIST National Vulnerability Database
  • CVSS changed · 6.5
    Oct 14, 2026 · NIST National Vulnerability Database
  • Source status changed · → Modified
    Oct 14, 2026 · NIST National Vulnerability Database
  • Record created in this database
    Oct 14, 2026 · Alertas do Telegram
  • 2021
  • CVE published
    Mar 15, 2021 · mitre

Source activity

Repeated readings are counted rather than listed.

  • 2× References added · Oct 14, 2026 – Oct 14, 2026
  • 1× Official description updated · Oct 14, 2026 – Oct 14, 2026

References

20 on the record

Elsewhere on this site

Fedoraproject
every CVE for this vendor
Cryptography
same class
CWE-295
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-97689 urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory 16d
  • CVE-2026-97687 urllib3: HTTPS proxy TLS configuration may be ignored or overridden 16d
  • CVE-2026-97688 urllib3: Chunked Deflate streaming can enter an infinite loop 16d
  • CVE-2026-87264 Improper Access Control in Oracle Peoplesoft Enterprise Peopletools Sep 15, 2026
  • CVE-2026-83018 Improper Access Control in Oracle Peoplesoft Enterprise Peopletools Sep 15, 2026
  • CVE-2026-83014 Improper Access Control in Oracle Peoplesoft Enterprise Peopletools Sep 15, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No CISA SSVC assessment.
  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2021-28363 is "Using default SSLContext for HTTPS requests in an HTTPS proxy doesn't verify certificate hostname for proxy connection", rated Medium. It affects Fedora (Fedoraproject); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.5 describes technical severity. EPSS 2.11% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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