External risk intelligence

NSS Integer Overflow Vulnerability in Firefox

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-16389

This vulnerability affects the NSS library component within Firefox. Libraries of this nature are typically utilized in client-side software rather than acting as internet-facing services, gateways, or public-facing infrastructure. Exposure is generally limited to the local client environment.

Integer Overflow

Mozilla Firefox

before 153.0.0before 153.0

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 security vulnerability has been identified in the Libraries component of Firefox, which could allow an attacker to compromise systems without requiring user interaction or prior access. While the technical details involve issues with boundary conditions and integer overflow, the high-level concern is the potential for significant data compromise and system disruption. The primary focus for leadership is to confirm if this specific component is in use within the organization's environment.

Flaw in Firefox's internal library.
Potential for broad system compromise.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit a flaw in the NSS Libraries component of Firefox by sending specially crafted data over the network. This could allow them to trigger an integer overflow due to incorrect boundary conditions, potentially leading to a complete compromise of the affected system. The vulnerability is in a core library, and there is no indication of a specific entry point other than the network.

Network access required.
Vulnerable component handling network data.
Unauthenticated remote code execution risk.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in the NSS component of Firefox could allow an attacker to execute arbitrary code when a user visits a malicious website. This could lead to the compromise of the user's system or sensitive data.

Affects user data and system integrity.
Malicious websites could trigger the overflow.
Remote code execution and data theft.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in the NSS library affects Firefox. Ownership for addressing this issue likely falls to teams managing application deployments or client endpoint security, depending on whether Firefox is centrally managed or user-installed. The first practical step is to identify all systems running the affected version of Firefox, determine their business criticality, and confirm the scope of potential exposure.

Application and security teams own.
Verify Firefox deployment and reachability.
Plan Firefox updates within maintenance windows.

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 NSS library in Firefox?

NSS stands for Network Security Services. It is a set of libraries designed to support cross-platform development of security-enabled client and server applications. In Firefox, it handles critical tasks like implementing SSL, TLS, and other cryptographic standards to ensure secure communication when your browser connects to websites.

What does integer overflow mean for CVE-2026-16389?

An integer overflow is a weakness class (CWE-190) where a calculation produces a number too large for the system to handle, causing it to wrap around to an unexpected, smaller value. In this CVE, incorrect boundary conditions during this process can confuse the software's memory management, potentially allowing an attacker to overwrite data or execute malicious instructions.

How can an attacker trigger this vulnerability?

The flaw is triggered when the Firefox NSS library processes specially crafted data sent over the network. This typically happens when a user visits a malicious or compromised website. Simply having the browser installed does not trigger the bug; the system must actively receive and process malicious network traffic through the vulnerable library.

Is my system at risk according to Halo Surface Signal?

Halo Surface Signal notes that while this is a network-based vulnerability, the affected NSS library is primarily used in client software rather than internet-facing infrastructure. Risk is generally confined to local client environments where users browse the web, rather than server-side gateways. You should focus on endpoints where Firefox is used.

What should I do to protect my systems?

Start by identifying all endpoints in your environment running versions of Firefox prior to version 153. Once identified, prioritize these systems based on their business use and update the browser software. Since this is a client-side library issue, ensuring Firefox is updated to the latest version is the primary method to resolve the vulnerability.

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.