CVE-2021-3116: before_upstream_connection in AuthPlugin in http/proxy/auth.py in proxy.py before 2.3.1 accepts incorrect Prox

Q: How do I fix CVE-2021-3116?

proxy.py: upgrade to 2.3.1. Exact commands for each OS are on this page.

Q: Is CVE-2021-3116 being exploited?

CVE-2021-3116 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-3116 affect?

CVE-2021-3116 affects proxy.py. Severity is high (CVSS 7.5). before_upstream_connection in AuthPlugin in http/proxy/auth.py in proxy.py before 2.3.1 accepts incorrect Proxy-Authorization header data because of a boolean confusion (and versus or).

Q: How urgent is CVE-2021-3116?

Priority 56/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

This week

HIGH 7.5REMOTEUPGRADE

CVE-2021-3116 is a high-severity flaw (CVSS 7.5) affecting proxy.py. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

before_upstream_connection in AuthPlugin in http/proxy/auth.py in proxy.py before 2.3.1 accepts incorrect Proxy-Authorization header data because of a boolean confusion (and versus or).

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.7% (top 23%)
CISA exploited list	No
Weakness	CWE-697
Published	2021-01-11
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: proxy.py project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-01-11 05:15 UTCPublished CVE record published
2026-06-17 04:04 UTCUpdated Record last modified

proxy.py project proxy.py UPGRADE

Affected: < 2.3.1

Fixed in: 2.3.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy proxy.py || rpm -q proxy.py || proxy.py --version

2. UPGRADE to 2.3.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade proxy.py

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade proxy.py

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3116?

proxy.py: upgrade to 2.3.1. Exact commands for each OS are on this page.

Is CVE-2021-3116 being exploited?

CVE-2021-3116 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3116 affect?

CVE-2021-3116 affects proxy.py. Severity is high (CVSS 7.5). before_upstream_connection in AuthPlugin in http/proxy/auth.py in proxy.py before 2.3.1 accepts incorrect Proxy-Authorization header data because of a boolean confusion (and versus or).

How urgent is CVE-2021-3116?

Priority 56/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

References

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