How severe is CVE-2019-16791?

CVE-2019-16791 has a CVSS score of 5.9/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2019-16791?

CVE-2019-16791 affects the following packages: postfix-mta-sts-resolver (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2019-16791?

Update postfix-mta-sts-resolver to 0.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-16791 is resolved across your whole dependency graph.

How do I detect CVE-2019-16791 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for postfix-mta-sts-resolver, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2019-16791 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2019-16791 actively exploited in the wild?

No public exploit code has been indexed for CVE-2019-16791 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2019-16791?

CVE-2019-16791 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 58th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2019-16791?

CVE-2019-16791 is classified as CWE-757 (CWE-757). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2019-16791 published, and has it been updated?

CVE-2019-16791 was published on January 22, 2020 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2019-16791 — postfix-mta-sts-resolver

MEDIUM

CVE-2019-16791 is a medium-severity (CVSS 5.9) CWE-757 vulnerability in postfix-mta-sts-resolver. A fix is available for postfix-mta-sts-resolver — see the affected versions and patch details below.

postfix-mta-sts-resolver Algorithm Downgrade vulnerability

Also known asGHSA-h92m-42h4-82f6PYSEC-2020-174

Published

Jan 22, 2020

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.9%probability of exploitation in next 30 days

Lower Risk+0.07%

Lower risk than most CVEs58th percentile — riskier than 58% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐍postfix-mta-sts-resolver

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Incorrect query parsing

Impact

All users of versions prior to 0.5.1 can receive incorrect response from daemon under rare conditions, rendering downgrade of effective STS policy.

Patches

Problem has been patched in version 0.5.1

Workarounds

Users may remediate this vulnerability without upgrading by applying these patches to older suppoorted versions.

For more information

If you have any questions or comments about this advisory:

Open an issue in postfix-mta-sts-resolver repo
Email me at vladislav at vm-0 dot com

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`postfix-mta-sts-resolver`	all versions	0.5.1`pip install --upgrade 'postfix-mta-sts-resolver==0.5.1'`

Affected Products

1 product · 1 configurations

Application

postfix-mta-sts-resolverpostfix-mta-sts-resolver_project

< 0.5.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for postfix-mta-sts-resolver, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update postfix-mta-sts-resolver to 0.5.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-16791 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

## Incorrect query parsing ### Impact All users of versions prior to 0.5.1 can receive incorrect response from daemon under rare conditions, rendering downgrade of effective STS policy. ### Patches Problem has been patched in version 0.5.1 ### Workarounds Users may remediate this vulnerability without upgrading by applying [these patches](https://gist.github.com/Snawoot/b9da85d6b26dea5460673b29df1adc6b) to older suppoorted versions. ### For more information If you have any questions or comments about this advisory: * Open an issue in [postfix-mta-sts-resolver repo](https://github.com/Snawo