External risk intelligence

Use-after-free in Firefox and Thunderbird HTML Parser.

CVE advisorySeverity: CRITICAL (CVSS 9.6)

CVE-2026-117920

This vulnerability is located in the HTML Parser component of a web browser. As a client-side application feature, it requires a user to navigate to a malicious site or interact with specifically crafted content. It is not an internet-facing service, appliance, or gateway that is public-facing by design, and thus it lacks the characteristics of a directly reachable network-exposed attack surface.

Use After Free

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 in the HTML parser component of Firefox and Thunderbird could allow attackers to execute arbitrary code if a user visits a malicious website or opens a crafted document. This issue has been addressed in recent updates for these applications.

Use-after-free flaw in web browser component.
Affects user interaction with malicious web content.
Confirm relevance and exposure within your environment.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by tricking a user into visiting a malicious website or opening a crafted document that contains malicious HTML. When the browser's HTML parser processes this content, it can lead to a use-after-free condition, potentially allowing the attacker to crash the browser or execute arbitrary code.

No user authentication required.
Processing malicious HTML content.
Arbitrary code execution or crash.

Live Threat

Current exploitation, exposure, and threat context

A use-after-free vulnerability in the HTML parser component could allow an attacker to crash the application or potentially execute arbitrary code when a user visits a malicious website. This could affect the integrity and availability of the user's browsing session and the underlying system.

Browser process integrity.
User visits malicious site.
Application crash or code execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

This critical vulnerability in the HTML parser component of Firefox and Thunderbird browsers requires users to interact with malicious content. Ownership typically resides with the endpoint or client device management teams, potentially coordinated with application owners if custom browser configurations are in place. The immediate practical step is to identify all endpoints running the affected software, assess user exposure, and plan for a coordinated update or risk mitigation strategy.

Endpoint teams should own the issue.
Verify affected software on endpoints.
Plan coordinated software 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 HTML Parser component in Firefox and Thunderbird?

The HTML Parser is a foundational engine within these applications responsible for reading web page code and converting it into the visual interface you interact with. It processes incoming data from websites to render text, images, and layout elements securely. Because it must handle complex, untrusted content from the internet, it is a critical piece of code that defines how the browser understands and displays the web.

What does CWE-416 mean for CVE-2026-117920?

CWE-416 refers to a use-after-free vulnerability, a specific type of memory management error. It occurs when a program continues to use a pointer to a memory location after that memory has been cleared or reclaimed. In the context of this CVE, when the HTML parser incorrectly handles memory, it can create an opportunity for an attacker to manipulate that space to execute unauthorized code or disrupt the application's stability.

How is this vulnerability triggered by an attacker?

An attacker triggers this flaw by luring a user to view specifically crafted HTML content. This typically happens through visiting a malicious website or opening a document containing manipulated code. Crucially, the vulnerability does not trigger simply by having the software installed or connected to a network; it requires the active processing of the harmful content by the browser's parser to manifest.

Is CVE-2026-117920 an internet-facing threat?

According to Halo Surface Signal, this is not considered an internet-facing service or gateway. Because the vulnerability exists within a client-side application, it lacks the characteristics of a directly reachable, public-facing network service. The risk is tied to the user's interaction with the web rather than the application acting as a standalone, exposed server.

What steps should I take to address this flaw?

The primary response is to ensure your Firefox and Thunderbird installations are updated to the versions that contain the fix, specifically Firefox 159, Firefox ESR 153.6, or Thunderbird 159 and 153.6. Since this is a client-side issue, your IT or endpoint management team should verify that all managed devices have received these updates to prevent the software from being susceptible to malicious HTML content.

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.