CVE-2026-5734

Modified Modified - Updated After Analysis

Memory Corruption in Firefox and Thunderbird Enables Code Execution

Publication date: 2026-04-07

Last updated on: 2026-06-30

Assigner: Mozilla Corporation

Description

Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 149.0.2, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

CVSS Scores

EPSS Scores

Probability:
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Meta Information

Published

2026-04-07

Last Modified

2026-06-30

Generated

2026-07-06

AI Q&A

2026-04-07

EPSS Evaluated

2026-07-05

NVD

CVE-2026-5734

EUVD

EUVD-2026-19614

Affected Vendors & Products

Vendor	Product	Version / Range
mozilla	firefox	to 149.0.2 (exc)
mozilla	firefox	to 140.9.1 (exc)
mozilla	thunderbird	to 149.0.2 (exc)
mozilla	thunderbird	to 140.9.1 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-120	The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
CWE-787	The product writes data past the end, or before the beginning, of the intended buffer.

Attack-Flow Graph

Executive Summary

This vulnerability involves memory safety bugs found in specific versions of Firefox and Thunderbird, including Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1, and Thunderbird 149.0.1.

Some of these bugs showed evidence of memory corruption, which means that the software could access or modify memory in unintended ways.

It is presumed that with enough effort, some of these bugs could be exploited by attackers to run arbitrary code on affected systems.

The vulnerability affects Firefox versions earlier than 149.0.2 and Firefox ESR versions earlier than 140.9.1.

Impact Analysis

Exploitation of these memory safety bugs could allow an attacker to execute arbitrary code on your system.

This means that an attacker might be able to take control of your browser or email client, potentially leading to unauthorized access, data theft, or further system compromise.

Mitigation Strategies

To mitigate this vulnerability, update Firefox and Thunderbird to versions that are not affected. Specifically, upgrade Firefox to version 149.0.2 or later, and Firefox ESR to version 140.9.1 or later.

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