CVE-2023-4056: Mozilla Firefox RCE Vulnerability

CVE-2023-4056 Overview

CVE-2023-4056 covers memory safety bugs reported by Mozilla developers and community members in Firefox 115, Firefox ESR 115.0, Firefox ESR 102.13, Thunderbird 115.0, and Thunderbird 102.13. Mozilla observed evidence of memory corruption in some of these bugs and assessed that, with sufficient effort, an attacker could leverage them to run arbitrary code. The defect is tracked under [CWE-787] (Out-of-bounds Write). Affected releases include Firefox before 116, Firefox ESR before 102.14, and Firefox ESR before 115.1.

Critical Impact
A network-based attacker can deliver crafted web content that triggers memory corruption in the browser process, potentially leading to arbitrary code execution within the renderer context.

Affected Products

Mozilla Firefox versions prior to 116
Mozilla Firefox ESR versions prior to 102.14 and prior to 115.1
Debian GNU/Linux 10, 11, and 12 (firefox-esr and thunderbird packages)

Discovery Timeline

2023-08-01 - CVE-2023-4056 published to the National Vulnerability Database
2023-08-01 - Mozilla publishes advisories MFSA-2023-29, MFSA-2023-30, and MFSA-2023-31
2023-08-01 - Debian publishes DSA-5464 and DSA-5469
2024-11-21 - Last updated in the NVD database

Technical Details for CVE-2023-4056

Vulnerability Analysis

This CVE bundles several distinct memory safety defects identified across the Firefox 115 and ESR 102.13/115.0 codebases. Mozilla disclosed the issues collectively because they share a common class of weakness: out-of-bounds memory access during processing of untrusted content. The associated Bugzilla entries (1820587, 1824634, 1839235, 1842325, 1843847) document the individual root causes across different browser subsystems.

Firefox processes complex, attacker-controlled inputs such as HTML, CSS, JavaScript, WebAssembly, media, and graphics. Defects in C++ memory management within any of these subsystems can corrupt heap or stack structures. Mozilla noted active evidence of memory corruption in the reported bugs, which is the standard precursor to control-flow hijacking.

Root Cause

The underlying weakness is classified as [CWE-787] Out-of-bounds Write. These defects originate from native C++ code paths where bounds checks, lifetime tracking, or type assumptions fail under specific input conditions. Mozilla did not publish per-bug technical write-ups for each entry rolled into this CVE.

Attack Vector

Exploitation requires the victim to load malicious web content in an affected Firefox or Thunderbird build. No authentication or user interaction beyond browsing is needed. A successful exploit operates within the renderer process and would typically be chained with a sandbox escape to achieve full system compromise.

// No verified public proof-of-concept exists for CVE-2023-4056.
// Refer to Mozilla Bugzilla entries 1820587, 1824634, 1839235,
// 1842325, and 1843847 for the underlying technical reports.

Detection Methods for CVE-2023-4056

Indicators of Compromise

Unexpected Firefox or Thunderbird process crashes with access violation or segmentation fault signatures during web content rendering
Firefox or thunderbird processes spawning child processes such as cmd.exe, powershell.exe, bash, or sh
Outbound network connections from the browser process to unfamiliar hosts following a crash event

Detection Strategies

Inventory installed browser versions and flag any Firefox build below 116 or Firefox ESR below 102.14 or 115.1
Correlate browser crash telemetry with subsequent process creation or file write events to surface possible exploitation attempts
Monitor Debian package state on Linux endpoints to confirm firefox-esr and thunderbird updates from DSA-5464 and DSA-5469 are applied

Monitoring Recommendations

Forward endpoint process and crash telemetry to a centralized analytics platform for retroactive hunting
Alert on browser processes loading unsigned modules or writing to autorun locations
Track egress traffic from browser processes to newly registered or low-reputation domains

How to Mitigate CVE-2023-4056

Immediate Actions Required

Upgrade Firefox to version 116 or later on all managed endpoints
Upgrade Firefox ESR to 102.14 or 115.1 (or later) where ESR is required for compatibility
Upgrade Thunderbird beyond 115.0 and 102.13 to receive the same memory safety fixes
On Debian systems, apply updates referenced in DSA-5464 and DSA-5469

Patch Information

Mozilla addressed these defects in Firefox 116, Firefox ESR 102.14, Firefox ESR 115.1, Thunderbird 115.1, and Thunderbird 102.14. Fix details are tracked in Mozilla Bugzilla and in advisories MFSA-2023-29, MFSA-2023-30, and MFSA-2023-31. Debian shipped corresponding package updates in DSA-5464 and DSA-5469.

Workarounds

No vendor-supplied workaround exists; patching is the only supported remediation
Restrict browsing to trusted sites and disable JavaScript in untrusted contexts where upgrade is delayed
Enforce least-privilege user accounts to limit post-exploitation impact if the renderer is compromised

bash

# Debian / Ubuntu: apply vendor security updates
sudo apt update
sudo apt install --only-upgrade firefox-esr thunderbird

# Verify installed versions meet the fixed baseline
apt list --installed 2>/dev/null | grep -E 'firefox-esr|thunderbird'