CVE-2026-3847: Mozilla Firefox RCE Vulnerability

CVE-2026-3847 Overview

CVE-2026-3847 is a memory corruption vulnerability affecting Mozilla Firefox 148.0.2 and earlier versions. Mozilla developers reported multiple memory safety bugs, some of which showed evidence of memory corruption during testing. Mozilla assesses that with sufficient effort, attackers could exploit these bugs to run arbitrary code in the context of the browser process.

The vulnerability is tracked under Mozilla Security Advisory MFSA-2026-19 and maps to [CWE-119] (improper restriction of operations within the bounds of a memory buffer). Mozilla addressed the issue in Firefox 148.0.2.

Critical Impact
Network-based exploitation requiring only user interaction (visiting a malicious page) can lead to arbitrary code execution with full impact to confidentiality, integrity, and availability.

Affected Products

Mozilla Firefox versions prior to 148.0.2
All supported operating systems (Windows, macOS, Linux)
Any application or extension that embeds vulnerable Firefox/Gecko components

Discovery Timeline

2026-03-10 - CVE-2026-3847 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-3847

Vulnerability Analysis

The advisory describes multiple memory safety bugs in the Firefox codebase identified by Mozilla developers and community contributors. The flaws are grouped under three Bugzilla entries (2017513, 2017622, 2019341) and consolidated into a single advisory because they share the same root cause class: improper memory buffer handling.

Mozilla states that some of the bugs showed evidence of memory corruption. This category typically includes use-after-free conditions, out-of-bounds reads and writes, and type confusion within the browser engine. Successful exploitation requires the user to interact with attacker-controlled content, such as visiting a crafted web page.

Because the bugs reside in the browser engine, exploitation runs within the content process. Sandbox escape would require chaining with a separate vulnerability, but in-process code execution alone exposes session cookies, stored credentials, and rendered DOM content.

Root Cause

The underlying defect class is [CWE-119]: improper restriction of operations within the bounds of a memory buffer. Firefox is a large C++ codebase, and complex object lifetimes across JavaScript, layout, and graphics subsystems frequently produce conditions where memory is read or written outside intended boundaries.

Attack Vector

An attacker hosts a malicious web page or compromises a legitimate site to deliver content that triggers the memory corruption. The user must load the page in a vulnerable Firefox build. Common delivery channels include phishing links, malvertising, and watering-hole attacks targeting specific user populations.

No public proof-of-concept exploit is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The EPSS probability is 0.049%, reflecting low observed exploitation activity at the time of disclosure. See the Mozilla Security Advisory MFSA-2026-19 for the underlying bug references.

Detection Methods for CVE-2026-3847

Indicators of Compromise

Firefox content process (firefox.exe or plugin-container) crashing repeatedly with access violation or segmentation fault signatures after visiting specific URLs
Unexpected child processes spawned by firefox.exe, such as cmd.exe, powershell.exe, or bash
Outbound network connections from the Firefox process to non-browsing infrastructure shortly after web navigation events

Detection Strategies

Inventory installed Firefox versions across the fleet and flag any host running a build older than 148.0.2
Monitor browser telemetry and EDR crash data for Gecko/SpiderMonkey exception patterns consistent with memory corruption
Correlate web proxy logs with endpoint crash events to identify potential exploitation attempts originating from specific domains

Monitoring Recommendations

Enable process-lineage logging for browser processes and alert on Firefox spawning shells, scripting hosts, or LOLBins
Track file writes by firefox.exe to autorun locations, scheduled task paths, and user startup folders
Ingest browser version inventory into the SIEM or data lake and build a dashboard for outdated installations

How to Mitigate CVE-2026-3847

Immediate Actions Required

Update Firefox to version 148.0.2 or later on all managed endpoints using your software distribution platform
Enforce automatic updates in policies.json or Group Policy to prevent users from running outdated builds
Audit Firefox ESR and embedded Gecko applications separately, as they may follow different release tracks

Patch Information

Mozilla released the fix in Firefox 148.0.2. Administrators should consult the Mozilla Security Advisory MFSA-2026-19 and the linked Mozilla Bug Reports for fix details. Verify deployed versions with firefox --version or by checking the About dialog.

Workarounds

Restrict browsing to trusted sites via web proxy or DNS filtering until patching completes
Disable JavaScript on untrusted origins using enterprise policy or extensions such as NoScript where operationally feasible
Route high-risk browsing through a remote browser isolation service to contain potential exploitation in an ephemeral environment

bash

# Verify Firefox version on Linux/macOS endpoints
firefox --version

# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion

# Enforce automatic updates via policies.json (place in distribution/ directory)
# {
#   "policies": {
#     "DisableAppUpdate": false,
#     "AppAutoUpdate": true
#   }
# }