---
id: CVE-2019-16791
aliases:
  - GHSA-h92m-42h4-82f6
  - PYSEC-2020-174
title: postfix-mta-sts-resolver Algorithm Downgrade vulnerability
summary: postfix-mta-sts-resolver Algorithm Downgrade vulnerability
severity: medium
cvss: 6.9
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:H/UI:R/S:C/C:H/I:H/A:N'
vendor: postfix-mta-sts-resolver
product: postfix-mta-sts-resolver
ecosystem: pip
affected:
  - postfix-mta-sts-resolver < 0.5.1
patched:
  - postfix-mta-sts-resolver 0.5.1
published: '2019-07-05'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:47:52.688559963Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-h92m-42h4-82f6'
references:
  - url: >-
      https://github.com/Snawoot/postfix-mta-sts-resolver/security/advisories/GHSA-h92m-42h4-82f6
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2019-16791'
  - url: 'https://gist.github.com/Snawoot/b9da85d6b26dea5460673b29df1adc6b'
  - url: 'https://github.com/Snawoot/postfix-mta-sts-resolver'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/postfix-mta-sts-resolver/PYSEC-2020-174.yaml
tags:
  - osv
  - pip
epss: 0.00784
epssPercentile: 0.54117
ingestedAt: '2026-09-12T03:13:01.699Z'
---

## Overview

## 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/Snawoot/postfix-mta-sts-resolver)
* Email me at [vladislav at vm-0 dot com](mailto:vladislav-ex-gh-advisory@vm-0.com)

## Affected packages

- `postfix-mta-sts-resolver < 0.5.1`

## Remediation

Upgrade to a patched release:

- `postfix-mta-sts-resolver 0.5.1`