CVE-2019-16791Network attack vector

MTA-STS downgrade via shared netstring decoder

Published Jan 22, 2020 · Updated Aug 5, 2024

Race condition in Snawoot postfix-mta-sts-resolver before 0.5.1 allows remote attackers to downgrade the effective MTA-STS policy. Each connection handler stores its netstring decoder on the shared responder object, so overlapping connections can replace the decoder while another request is still being parsed. The rare gate is concurrent daemon connections; an incorrect response can remove enforced TLS policy and expose subsequent SMTP delivery to an adversary-in-the-middle.

CVSS severity6.9
Medium
EPSS probability0.78%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Race condition in Snawoot postfix-mta-sts-resolver before 0.5.1 allows remote attackers to downgrade the effective MTA-STS policy. Each connection handler stores its netstring decoder on the shared responder object, so overlapping connections can replace the decoder while another request is still being parsed. The rare gate is concurrent daemon connections; an incorrect response can remove enforced TLS policy and expose subsequent SMTP delivery to an adversary-in-the-middle.

The record

CVE
CVE-2019-16791
Published
Jan 22, 2020
Updated
Aug 5, 2024
Vendor
Snawoot
Product
postfix-mta-sts-resolver
Classifications
CWE-757, CAPEC-620, CAPEC-220, CAPEC-606, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 22, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.5.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Snawoot · postfix-mta-sts-resolverversion=< 0.5.1

Published CVSS scores

6.9GitHub, Inc.CVSS:3.1/AV:L/AC:H/PR:H/UI:R/S:C/C:H/I:H/A:N
5.9NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-757Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade')
CAPEC-620Drop Encryption Level
CAPEC-220Client-Server Protocol Manipulation
CAPEC-606Weakening of Cellular Encryption
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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