External risk intelligence

HTML Gumbo Heap Disclosure via Template Element Type Confusion

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2025-15646

This is a Perl library used for parsing HTML. While it can be integrated into web applications that process untrusted user-supplied HTML, it is a backend library/module rather than a standalone internet-facing service or appliance. Its exposure depends entirely on how a developer implements it within an application, making public internet reachability possible but not inherently common by design.

Out-of-bounds Read

Halo Surface Signal: 3 out of 5 — possibly public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This vulnerability involves a Perl library used for parsing HTML, which could potentially disclose memory contents when processing specific input. The primary concern is to determine if this library is used within your environment and if it handles untrusted data.

Memory disclosure in HTML processing library.
Key issue is confirming relevance to your systems.
Focus on confirming if library is in use.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted HTML containing a `<template>` element to an application that uses the affected HTML parsing library. When the library attempts to process this input, a flaw in handling the `<template>` element can lead to an over-read of heap memory. This disclosed information could then be used in further attacks.

No authentication or privileges needed.
Processing malformed HTML with a `<template>` tag.
Disclosure of sensitive heap memory.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could impact applications that use HTML::Gumbo to parse HTML content, specifically when processing the `<template>` element. When this element is encountered, the parser may over-read heap memory and include these bytes in the returned output, potentially disclosing sensitive information from the application's memory. This disclosure is limited to callers using the 'string' or 'tree' output formats and does not affect the 'callback' format.

Heap memory contents.
HTML parsing with specific elements.
Disclosure of sensitive memory.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in HTML::Gumbo affects applications parsing untrusted HTML, potentially exposing heap memory. Responsibility likely falls to the development team that integrated the library, with support from platform or infrastructure teams for deployment. The immediate first step is to identify all instances of the affected library, determine their reachability and business criticality, and then engage the accountable owner to plan remediation, prioritizing environments processing user-supplied HTML.

Application owners should prioritize remediation.
Verify affected library usage and exposure.
Plan risk-based remediation and vendor coordination.

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.

Free EASM Trial See How Signal Works

Frequently asked questions

What is HTML::Gumbo used for?

HTML::Gumbo is a Perl library designed to parse and structure HTML content. Developers integrate it into applications to process, clean, or analyze web markup. Because it is a backend module rather than a standalone service, its function is to provide the underlying parsing logic for custom software that needs to understand HTML structures programmatically.

What happens during CVE-2025-15646?

This issue involves CWE-843 (type confusion) and CWE-125 (out-of-bounds read). When the parser encounters a <template> element, it mistakes it for a simple text node. Because the internal logic doesn't correctly handle the <template> structure, the parser reads past the intended memory boundaries, accidentally grabbing and returning private data stored in the application's heap memory along with the parsed output.

How is this vulnerability triggered?

An attacker triggers the bug by providing specially crafted HTML containing a <template> element to an application using an outdated version of the library. This only occurs when the code calls the parse() function using the 'string' or 'tree' output formats. If your application specifically uses the 'callback' format, the library terminates safely before the over-read can occur, making that implementation path immune.

Is my system at risk?

According to Halo Surface Signal, this is a library, not a public-facing appliance, so risk depends on how your developers use it. If your application takes untrusted HTML from the internet and parses it using this library, the risk is higher. You should prioritize checking any internet-facing components that process user-submitted HTML content, as these are the most likely paths for an external trigger.

What steps should I take to fix this?

First, conduct an inventory to identify which applications include HTML::Gumbo in their code. Once identified, work with the development teams to verify if they are using the affected 'string' or 'tree' output formats. The primary response is to update the library to version 0.19 or later, which resolves the handling of the <template> element. Prioritize systems that handle untrusted input from external users.

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.