CVE-2019-17023Public exploitNetwork attack vector

Connection stall via post-HelloRetryRequest protocol downgrade

Published Jan 8, 2020 · Updated Aug 5, 2024

Denial of service in NSS in Mozilla Firefox before 72 allows remote attackers to stall a TLS connection through a crafted protocol downgrade. After sending a TLS 1.3 HelloRetryRequest, NSS fails to retain that state and accepts TLS 1.2 or earlier, then drops every application-data record. A malicious TLS server must induce the downgrade; release builds stall only the affected connection rather than crashing the browser.

CVSS severityUnavailable
Unscored
EPSS probability1.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial of service in NSS in Mozilla Firefox before 72 allows remote attackers to stall a TLS connection through a crafted protocol downgrade. After sending a TLS 1.3 HelloRetryRequest, NSS fails to retain that state and accepts TLS 1.2 or earlier, then drops every application-data record. A malicious TLS server must induce the downgrade; release builds stall only the affected connection rather than crashing the browser.

The record

CVE
CVE-2019-17023
Published
Jan 8, 2020
Updated
Aug 5, 2024
Vendor
Mozilla
Product
Firefox
Classifications
Unavailable
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 8, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=before 72

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=before 72

Published CVSS scores

No CVSS assessment is available in this record.

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 exploitPublished

Weakness, pattern, technique

No sourced classifications are available.

Public exploit references

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo