CVE-2026-2783: Mozilla Firefox Information Disclosure Flaw

CVE-2026-2783 Overview

CVE-2026-2783 is a high-severity information disclosure vulnerability caused by JIT (Just-In-Time) miscompilation in the JavaScript Engine JIT component of Mozilla Firefox and Thunderbird. The flaw allows attackers to leverage maliciously crafted JavaScript code to trigger incorrect compilation behavior, potentially exposing sensitive memory contents to unauthorized parties.

Critical Impact

This vulnerability enables remote attackers to disclose sensitive information through network-accessible JavaScript execution without requiring user interaction or authentication.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Firefox ESR versions prior to 140.8
• Mozilla Thunderbird versions prior to 148 and prior to 140.8

Discovery Timeline

• 2026-02-24 - CVE-2026-2783 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-2783

Vulnerability Analysis

The vulnerability resides in Mozilla's SpiderMonkey JavaScript engine, specifically within the JIT compiler component. JIT compilation transforms JavaScript bytecode into optimized machine code at runtime for improved performance. However, a flaw in the JIT compilation logic causes type confusion (CWE-843), where the compiler incorrectly infers or propagates type information during optimization passes.

When the JIT compiler mishandles certain JavaScript constructs, it may generate machine code that accesses memory using incorrect type assumptions. This miscompilation creates a scenario where an attacker can craft JavaScript that causes the engine to read memory beyond intended boundaries or interpret data structures incorrectly, leading to information exposure (CWE-200).

Root Cause

The root cause is a type confusion vulnerability in the JIT compiler's optimization pipeline. During the compilation of JavaScript to native code, the compiler makes assumptions about variable types to generate optimized machine instructions. When these assumptions are violated or incorrectly propagated through optimization passes, the resulting native code may perform operations on values as if they were different types than they actually are. This type confusion allows attackers to construct JavaScript payloads that leak arbitrary memory contents.

Attack Vector

The attack is network-based and requires no special privileges or user interaction. An attacker can exploit this vulnerability by:

1. Hosting malicious JavaScript on a website or injecting it into a compromised page
2. When a victim visits the page using a vulnerable Firefox or Thunderbird client, the JavaScript executes
3. The malicious code triggers the JIT miscompilation condition
4. The resulting type confusion allows reading memory that should not be accessible
5. Sensitive data such as credentials, cookies, or other browser memory contents may be exfiltrated

The vulnerability is particularly dangerous in the context of email clients like Thunderbird, where HTML emails with embedded JavaScript could potentially be leveraged in targeted attacks.

Detection Methods for CVE-2026-2783

Indicators of Compromise

• Unusual JavaScript execution patterns involving rapid type coercion or polymorphic function calls designed to trigger JIT compilation paths
• Network connections to unknown external servers following JavaScript execution in the browser or email client
• Unexpected memory access patterns in browser processes that may indicate exploitation attempts
• Presence of obfuscated JavaScript payloads in web pages or email content that target JIT optimization behaviors

Detection Strategies

• Monitor for abnormal browser process behavior including unexpected memory read operations
• Deploy network monitoring to identify exfiltration attempts following suspicious JavaScript execution
• Implement endpoint detection rules for known JIT exploitation patterns in browser environments
• Analyze JavaScript content in emails and web traffic for obfuscated code targeting JIT vulnerabilities

Monitoring Recommendations

• Enable detailed logging for browser and email client processes to capture potential exploitation indicators
• Monitor for connections to known malicious domains following JavaScript execution
• Track browser version inventory to ensure vulnerable versions are identified and prioritized for patching
• Implement Content Security Policy headers on internal web applications to reduce JavaScript injection risks

How to Mitigate CVE-2026-2783

Immediate Actions Required

• Update Mozilla Firefox to version 148 or later immediately
• Update Mozilla Firefox ESR to version 140.8 or later
• Update Mozilla Thunderbird to version 148 or 140.8 (ESR) or later
• Restrict access to untrusted websites on systems where immediate patching is not possible
• Consider disabling JavaScript in Thunderbird for email rendering until patches are applied

Patch Information

Mozilla has released security updates addressing this vulnerability across multiple product lines. Organizations should reference the official Mozilla Security Advisories for complete details:

• Mozilla Security Advisory MFSA-2026-13
• Mozilla Security Advisory MFSA-2026-15
• Mozilla Security Advisory MFSA-2026-16
• Mozilla Security Advisory MFSA-2026-17

Additional technical details are available in Mozilla Bug Report #2010943.

Workarounds

• Disable JavaScript execution in Firefox via about:config by setting javascript.enabled to false (note: this significantly impacts web functionality)
• In Thunderbird, disable remote content and JavaScript in emails via Options > Privacy & Security
• Use browser isolation technologies to contain potential exploitation in sandboxed environments
• Deploy web filtering solutions to block access to known malicious sites targeting this vulnerability

# Firefox configuration to disable JIT compilation as temporary mitigation
# Access about:config and set:
# javascript.options.ion -> false
# javascript.options.baselinejit -> false
# Note: This may impact browser performance but disables vulnerable JIT paths