Mozilla Firefox and Thunderbird memory safety vulnerabilities
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Multiple memory safety bugs in Firefox and Thunderbird could allow arbitrary code execution; no active exploitation reported.
Mozilla has released security updates addressing multiple vulnerabilities across Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR products. The most severe issues are memory safety bugs that could lead to arbitrary code execution if exploited, potentially allowing an attacker to install programs, modify or delete data, or create new accounts with full user privileges depending on the compromised account's permission level. The vulnerabilities affect Firefox versions prior to 150.0.1, Firefox ESR prior to 140.10.1 and 115.35.1, Thunderbird prior to 150.0.1, and Thunderbird ESR prior to 140.10.1. Additionally, a sandbox escape vulnerability in WebRTC and information disclosure issues in the Audio/Video component have been identified. CIS reports no current exploitation in the wild, but defenders should apply patches immediately after testing, enforce principle of least privilege to limit impact, and maintain updated browser and email client versions across enterprise environments.
Mentioned in this report
Detection guidance
Firefox or Thunderbird Spawning Shell or Script Interpreter
Firefox or Thunderbird launching cmd, PowerShell, script hosts or common LOLBins, a post-exploitation pattern after browser or mail client memory-corruption code execution. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.
title: Firefox or Thunderbird Spawning Shell or Script Interpreter
id: 834cc992-863a-50c3-a7cc-3e8d5162476f
status: experimental
description: Detects Firefox or Thunderbird spawning a command shell, PowerShell,
script host or common living-off-the-land binary. This is a typical post-exploitation
pattern after a drive-by memory-safety exploit achieves code execution in the browser
or mail client. Generalises on the parent/child relation, not on any specific exploit.
tags:
- attack.initial-access
- attack.t1189
- attack.execution
logsource:
category: process_creation
product: windows
detection:
selection:
ParentImage|endswith:
- \firefox.exe
- \thunderbird.exe
Image|endswith:
- \cmd.exe
- \powershell.exe
- \pwsh.exe
- \wscript.exe
- \cscript.exe
- \mshta.exe
- \rundll32.exe
- \regsvr32.exe
- \certutil.exe
- \bitsadmin.exe
- \msiexec.exe
condition: selection
falsepositives:
- Users configuring an external protocol handler or helper application that launches
a script through Firefox or Thunderbird
- Browser extensions or native messaging hosts implemented as batch or PowerShell
scripts
level: medium
author: Vorant
references:
- https://www.cisecurity.org/advisory/multiple-vulnerabilities-in-mozilla-products-could-allow-for-arbitrary-code-execution_2026-039
Firefox or Thunderbird Spawning Executable From User-Writable Path
Firefox or Thunderbird creating a child process whose image lies in a user-writable location such as Temp, AppData or Public, excluding the products' own binaries. Auto-generated starting point — validate and tune in your environment before deploying. IOC matches can false-positive on shared infrastructure and decay as adversary infrastructure rotates.
title: Firefox or Thunderbird Spawning Executable From User-Writable Path
id: d5ba4330-09c2-58b3-93ff-9c938c05a710
status: experimental
description: Detects Firefox or Thunderbird starting a child process from a user-writable
directory (Temp, AppData, Users\Public), excluding the products' own helper binaries.
May indicate a dropped payload executed after a browser or mail client exploit.
tags:
- attack.initial-access
- attack.t1189
- attack.execution
logsource:
category: process_creation
product: windows
detection:
selection:
ParentImage|endswith:
- \firefox.exe
- \thunderbird.exe
Image|contains:
- \AppData\
- \Temp\
- \Users\Public\
filter_own_binaries:
Image|endswith:
- \firefox.exe
- \thunderbird.exe
- \plugin-container.exe
- \updater.exe
- \crashreporter.exe
- \pingsender.exe
- \default-browser-agent.exe
- \minidump-analyzer.exe
- \maintenanceservice.exe
condition: selection and not filter_own_binaries
falsepositives:
- User opening a downloaded installer directly from the browser download panel
- Per-user installed helper applications launched from Thunderbird attachments
level: medium
author: Vorant
references:
- https://www.cisecurity.org/advisory/multiple-vulnerabilities-in-mozilla-products-could-allow-for-arbitrary-code-execution_2026-039
Behavioural rules are generated from public reporting — validate in your environment before deploying.
Source reporting: https://www.cisecurity.org/advisory/multiple-vulnerabilities-in-mozilla-products-could-allow-for-arbitrary-code-execution_2026-039
What this brief leaves out
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