External risk intelligence

Firefox JavaScript Engine Integer Overflow

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2025-49710

The vulnerability exists within the Firefox browser's JavaScript engine. Web browsers are client-side software applications used by individuals to access the internet, rather than internet-facing services, gateways, or infrastructure components that provide public-facing network services.

Integer Overflow

Mozilla Firefox

before 139.0.4

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 has been identified in the JavaScript engine used by the Firefox browser, specifically within a component called `OrderedHashTable`. This issue allows for an integer overflow, which could potentially lead to severe security compromises. While the primary concern is confirming relevance and exposure, such flaws in widely used software can have broad implications for user security.

Flaw in browser's JavaScript engine.
Affects user data and system integrity.
Confirm relevance and assess potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted data to a vulnerable version of Firefox. The JavaScript engine's `OrderedHashTable` component, when processing this data, could experience an integer overflow. This could allow an attacker to potentially compromise the integrity, confidentiality, and availability of the affected system.

No special access or authentication is needed.
Triggered by processing malicious input.
Risk of information disclosure and code execution.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could affect the behavior of the JavaScript engine within Firefox when processing specific code. When supported by the advisory, this could lead to code execution or denial of service.

JavaScript engine behavior.
Integer overflow in processing.
Potential for code execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

This critical vulnerability in Mozilla Firefox's JavaScript engine requires immediate attention from teams managing browser deployments and security. The first practical step is to identify all instances of affected Firefox versions, confirm their exposure and business criticality, and then coordinate remediation efforts with the relevant stakeholders, likely involving end-user computing or desktop support teams.

Browser deployment and security teams own this.
Verify all Firefox installations.
Plan coordinated upgrade and testing.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is Firefox and how does it use OrderedHashTable?

Firefox is a widely used web browser developed by Mozilla that allows users to navigate the internet. At its core, it includes a JavaScript engine responsible for executing the scripts that make modern websites interactive. OrderedHashTable is a specialized internal component within this engine designed to efficiently store and manage data structures. When you visit a website, the browser uses this component to organize information, making it essential for the browser's daily performance and speed.

What does CVE-2025-49710 mean by integer overflow?

This vulnerability is classified as an integer overflow (CWE-190). In programming, this happens when a calculation tries to store a number larger than the computer's memory can hold for that specific task. This 'overflow' can cause the system to wrap the number around to an incorrect value. In the context of CVE-2025-49710, this mistake confuses the browser's JavaScript engine, potentially allowing it to bypass security checks or access memory it shouldn't, which can lead to system compromises.

How is this integer overflow triggered?

The flaw is triggered when the Firefox JavaScript engine processes specifically crafted, malicious data provided by an attacker. When the engine attempts to perform operations using the faulty OrderedHashTable logic on this input, the overflow occurs. Importantly, standard web browsing or processing legitimate, non-malicious code will not trigger this error; it requires the browser to interact with the specific, malformed data structures designed to exploit the calculation error.

Is my network at risk from this browser vulnerability?

According to Halo Surface Signal, this vulnerability is considered 'Very unlikely' to impact your network infrastructure. This is because the flaw exists in client-side software—the browser itself—rather than in internet-facing servers, gateways, or public-facing network services. While an individual user's machine running an older version of Firefox is vulnerable to malicious web content, the bug does not represent a direct weakness in your network's perimeter or server-side architecture.

What is the first step to address CVE-2025-49710?

The primary step is to ensure all Firefox installations across your environment are updated to version 139.0.4 or newer, as this release contains the fix for the integer overflow. You should inventory your systems to identify any endpoints still running older versions. Coordinate with your desktop support or end-user computing teams to roll out the update, as they manage the application life cycle for browser software on individual workstations.

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.