External risk intelligence

Integer Overflow in Firefox and Thunderbird JavaScript Component

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-2762

This vulnerability affects client-side applications (web browsers and email clients). These products are end-user software, not network-facing services, gateways, or appliances. While they access the internet to function, the software itself is not a public-facing service and lacks the characteristics of an externally reachable attack surface as defined by this rubric.

Integer Overflow

Mozilla Firefox

before 140.8.0before 148.0

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability exists in the JavaScript Standard Library component of Firefox and Thunderbird. This integer overflow issue could allow for severe impacts if exploited. The main concern is confirming relevance and exposure within your environment.

Integer overflow in JavaScript library.
Critical impact if exploited.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker can exploit this vulnerability by sending specially crafted data over the network to the vulnerable JavaScript: Standard Library component within affected Mozilla products. This could lead to an integer overflow, potentially allowing the attacker to gain significant control over the affected system.

Requires no authentication or user interaction.
Triggered by network-accessible component.
High risk of compromise.

Live Threat

Current exploitation, exposure, and threat context

An integer overflow in the JavaScript standard library component could allow an attacker to execute arbitrary code. This could occur when processing specific JavaScript code within affected applications.

Affects application integrity and execution.
Code execution via specially crafted JavaScript.
Potential for full system compromise.

Operational Fix

Recommended remediation, mitigation, and detection steps

The affected technologies, Firefox and Thunderbird, are typically managed by end-user computing, desktop support, or platform teams, with potential involvement from security teams for policy enforcement and vendor management for release coordination. The immediate first step is to identify all instances of the affected software, determine their exposure and criticality, and confirm the responsible owner for remediation planning.

Ownership: End-user computing or platform teams.
Verify first: Identify and confirm affected software instances.
Action: Plan and coordinate vendor-supported updates.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the JavaScript Standard Library in Firefox and Thunderbird?

This component is a fundamental part of the engine that executes JavaScript code within the browser and email client. It manages essential functions, including data processing and memory operations, allowing these applications to run web content and complex scripts reliably. Because it handles various inputs, any weakness here can impact how the entire application manages data, making it a critical foundation for security.

What does an integer overflow mean for CVE-2026-2762?

This vulnerability is classified as CWE-190, or integer overflow. In simple terms, the software performs a calculation that results in a number too large for its assigned memory space. When this happens, the program can behave unpredictably, potentially allowing an attacker to overwrite sensitive memory areas or execute unauthorized code.

How is this vulnerability triggered?

The flaw is triggered when the application processes specially crafted JavaScript code designed to force an invalid arithmetic result. It does not require user interaction or authentication to initiate. Importantly, simply having the browser or email client open does not trigger it; the application must be actively processing malicious script content provided through network data.

Why is this CVE considered relevant for my environment?

While Halo Surface Signal notes that browsers and email clients are end-user software rather than public-facing servers, they are still prone to threats from the internet. If you use Firefox or Thunderbird, your systems are potential targets if they navigate to or receive malicious content. It is important to assess if your organization has systems that interact with untrusted web or email traffic.

How do I respond to CVE-2026-2762?

The primary response is to update your software to the patched versions provided by the vendor. Start by identifying all instances of Firefox and Thunderbird within your environment to understand your footprint. Coordinate with your desktop management or platform teams to ensure the latest versions—Firefox 148 or 140.8 ESR and Thunderbird 148 or 140.8 ESR—are deployed as quickly as possible to mitigate risk.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.