How severe is CVE-2019-9791?

CVE-2019-9791 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2019-9791 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 20% (97th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2019-9791?

CVE-2019-9791 primarily affects Mozilla Firefox. In total, 10 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2019-9791?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2019-9791 published?

CVE-2019-9791 was published on 2019-04-26 and last updated on 2026-06-17.

CVE-2019-9791

CVE-2019-9791 is a critical-severity vulnerability in Mozilla Firefox with a CVSS 3.x base score of 9.8. Its EPSS exploit-prediction score of 20% places it in the 97th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-843.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 20% (97th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-843
Affected product: Mozilla Firefox
Published: 2019-04-26
Last modified: 2026-06-17

Description

The type inference system allows the compilation of functions that can cause type confusions between arbitrary objects when compiled through the IonMonkey just-in-time (JIT) compiler and when the constructor function is entered through on-stack replacement (OSR). This allows for possible arbitrary reading and writing of objects during an exploitable crash. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.

Frequently asked questions

What is CVE-2019-9791?: The type inference system allows the compilation of functions that can cause type confusions between arbitrary objects when compiled through the IonMonkey just-in-time (JIT) compiler and when the constructor function is entered through on-stack replacement (OSR). This allows for possible arbitrary reading and writing of objects during an exploitable crash. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.
How severe is CVE-2019-9791?: CVE-2019-9791 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2019-9791 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 20% (97th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2019-9791?: CVE-2019-9791 primarily affects Mozilla Firefox. In total, 10 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2019-9791?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2019-9791 published?: CVE-2019-9791 was published on 2019-04-26 and last updated on 2026-06-17.

References

Affected products (10)

cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux:8.0:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_eus:8.1:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_eus:8.2:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_eus:8.4:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_server_aus:8.2:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_server_aus:8.4:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_server_tus:8.2:*:*:*:*:*:*:*
cpe:2.3:o:redhat:enterprise_linux_server_tus:8.4:*:*:*:*:*:*:*

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