What is CVE-2019-17026?

CVE-2019-17026 is a high-severity vulnerability in Mozilla Firefox, classified under Access of Resource Using Incompatible Type (Type Confusion). CVSS score: 8.8/10. Published 2020-03-02.

How severe is CVE-2019-17026?

High severity. CVSS v3 base score is 8.8 out of 10.

Is CVE-2019-17026 known to be exploited?

Yes. CVE-2019-17026 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 18 public proof-of-concept repositories are indexed.

Vulnerability in Mozilla Firefox

CVE-2019-17026

Incorrect alias information in IonMonkey JIT compiler for setting array elements could lead to a type confusion. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Firefox ESR < 68.4.1, Thunderbird < 68.4.1, and Firefox < 72.0.1.

Published 2020-03-02 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.463 (98.8th percentile), scored 2026-10-18.

Moderate probability of exploitation in the next 30 days — watch for active exploitation reports. 99th percentile — 98.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.463 · newest: 0.463 · change: 0.000

Affected products

Mozilla Firefox — versions unspecified
Mozilla Firefox Esr — versions unspecified
Mozilla Thunderbird — versions unspecified
Canonical Ubuntu Linux — versions 16.04

Weakness classification (CWE)

CWE-843 — Access of Resource Using Incompatible Type (Type Confusion)

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2021-11-03. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2022-05-03.

Required action: Apply updates per vendor instructions.

Public proof-of-concept exploits

References

www.mozilla.org/security/advisories/mfsa2020-04/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2020-03/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Issue Tracking)
security.gentoo.org/glsa/202003-02 (vendor-advisory, Third Party Advisory)
usn.ubuntu.com/4335-1/ (vendor-advisory, Third Party Advisory)
packetstormsecurity.com/files/162568/Firefox-72-IonMonkey-JIT-Type-Confusion.ht… (Exploit, VDB Entry, Third Party Advisory)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)

Frequently asked questions

What is CVE-2019-17026?: CVE-2019-17026 is a high-severity vulnerability in Mozilla Firefox, classified under Access of Resource Using Incompatible Type (Type Confusion). CVSS score: 8.8/10. Published 2020-03-02.
How severe is CVE-2019-17026?: High severity. CVSS v3 base score is 8.8 out of 10.
Is CVE-2019-17026 known to be exploited?: Yes. CVE-2019-17026 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 18 public proof-of-concept repositories are indexed.