Mem safety bug in Firefox <149.0.2 (ESR 115/140)
CVE-2026-5731 Published on April 7, 2026
Memory safety bugs fixed in Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird ESR 140.9.1, Firefox 149.0.2 and Thunderbird 149.0.2
Memory safety bugs present in Firefox ESR 115.34.0, 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 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.
Vulnerability Analysis
CVE-2026-5731 can be exploited with network access, requires user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to be very high.
Weakness Types
What is a Buffer Overflow Vulnerability?
The software performs operations on a memory buffer, but it can read from or write to a memory location that is outside of the intended boundary of the buffer.
CVE-2026-5731 has been classified to as a Buffer Overflow vulnerability or weakness.
What is a Memory Corruption Vulnerability?
The software writes data past the end, or before the beginning, of the intended buffer. Typically, this can result in corruption of data, a crash, or code execution. The software may modify an index or perform pointer arithmetic that references a memory location that is outside of the boundaries of the buffer. A subsequent write operation then produces undefined or unexpected results.
CVE-2026-5731 has been classified to as a Memory Corruption vulnerability or weakness.
Products Associated with CVE-2026-5731
You can be notified by email with stack.watch whenever vulnerabilities like CVE-2026-5731 are published in these products:
Affected Versions
Mozilla Firefox:- Version 115.34.1, <= 115.* is unaffected.
- Version 140.9.1, <= 140.* is unaffected.
- Version 149.0.2, <= * is unaffected.
- Version 140.9.1, <= 140.* is unaffected.
- Version 149.0.2, <= * is unaffected.
- Version 0:140.9.1-1.el10_1 and below * is unaffected.
- Version 0:140.9.1-1.el10_1 and below * is unaffected.
- Version 0:140.9.1-1.el10_0 and below * is unaffected.
- Version 0:140.9.1-1.el10_0 and below * is unaffected.
- Version 0:140.9.1-2.el7_9 and below * is unaffected.
- Version 0:140.9.1-1.el8_10 and below * is unaffected.
- Version 0:140.9.1-1.el8_10 and below * is unaffected.
- Version 0:140.9.1-1.el8_2 and below * is unaffected.
- Version 0:140.9.1-1.el8_4 and below * is unaffected.
- Version 0:140.9.1-1.el8_4 and below * is unaffected.
- Version 0:140.9.1-1.el8_4 and below * is unaffected.
- Version 0:140.9.1-1.el8_4 and below * is unaffected.
- Version 0:140.9.1-1.el8_6 and below * is unaffected.
- Version 0:140.9.1-1.el8_6 and below * is unaffected.
- Version 0:140.9.1-1.el8_6 and below * is unaffected.
- Version 0:140.9.1-1.el8_6 and below * is unaffected.
- Version 0:140.9.1-1.el8_6 and below * is unaffected.
- Version 0:140.9.1-1.el8_6 and below * is unaffected.
- Version 0:140.9.1-1.el8_8 and below * is unaffected.
- Version 0:140.9.1-1.el8_8 and below * is unaffected.
- Version 0:140.9.1-1.el8_8 and below * is unaffected.
- Version 0:140.9.1-1.el8_8 and below * is unaffected.
- Version 0:140.9.1-1.el9_7 and below * is unaffected.
- Version 0:140.9.1-1.el9_7 and below * is unaffected.
- Version 0:140.9.1-1.el9_0 and below * is unaffected.
- Version 0:140.9.1-1.el9_0 and below * is unaffected.
- Version 0:140.9.1-1.el9_2 and below * is unaffected.
- Version 0:140.9.1-1.el9_2 and below * is unaffected.
- Version 0:140.9.1-1.el9_4 and below * is unaffected.
- Version 0:140.9.1-1.el9_4 and below * is unaffected.
- Version 0:140.9.1-1.el9_6 and below * is unaffected.
- Version 0:140.9.1-1.el9_6 and below * is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.