External risk intelligence

PHP mail() Double-Free Vulnerability on Windows

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2019-11049

The vulnerability affects the PHP mail() function on Windows. While the network attack vector allows remote exploitation, triggering the flaw requires specific application logic to pass user-controlled input as lowercase headers. Public reachability is therefore contingent upon whether the application implements this vulnerable pattern, rather than the service being inherently exposed by default.

Php

7.3.0 to 7.3.137.4.0303110.0before 5.19.0

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

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in PHP's mail function on Windows could allow attackers to cause a denial of service. This issue arises when specific conditions related to custom headers are met, potentially impacting systems that rely on this function for sending emails. The primary concern is confirming if your environment utilizes this function in a way that could be exploited.

PHP mail function has a memory error.
It allows remote attackers to disrupt services.
Confirm relevance and exposure in your environment.

Attack Path

How an attacker could exploit the issue

An attacker could reach this vulnerability by sending specially crafted, lowercase headers to the PHP `mail()` function on Windows. This could occur if an application passes user-supplied data directly into these headers without proper sanitization or case checking. If successful, the vulnerability could lead to a crash or potentially allow an attacker to execute arbitrary code.

Network access to a vulnerable PHP application.
Sending custom, lowercase mail headers.
Memory corruption leading to code execution.

Live Threat

Current exploitation, exposure, and threat context

When custom, lowercase headers are supplied to PHP's `mail()` function on Windows, a memory corruption vulnerability could allow an attacker to cause a double-free condition. This could potentially lead to crashes or other unpredictable behavior in affected PHP applications.

PHP application memory could be corrupted.
Exploited via specific `mail()` function usage.
Application instability or denial of service.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability, affecting PHP's mail() function on Windows when custom headers are supplied in lowercase, is likely the responsibility of application owners and platform teams. The first practical step is to identify all instances of PHP on Windows systems, determine if the mail() function is used with custom headers, assess business criticality and external reachability, and then coordinate remediation with the responsible parties.

Application and platform owners should investigate.
Verify PHP usage with custom mail headers.
Plan remediation based on identified risk.

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 PHP and why does it have a mail function?

PHP is a widely used server-side scripting language that powers dynamic websites and applications. The mail() function is a built-in feature that allows these applications to send emails directly from the server, such as contact form submissions, account notifications, or password resets, by interfacing with the underlying operating system's mail transfer capabilities.

What is the memory vulnerability in CVE-2019-11049?

This vulnerability is classified as a double-free, which is a type of memory corruption error. It occurs when a program attempts to release the same area of memory twice, confusing the computer's memory management system. In this specific CVE, the mistake happens within the PHP mail() function on Windows, potentially leading to system instability or crashes.

How can an attacker trigger this memory error?

An attacker needs to interact with an application that passes user-controlled input into the PHP mail() function as custom email headers. The flaw is triggered specifically when those headers are provided in lowercase. If the application does not sanitize this input or enforce case sensitivity, it may inadvertently pass these lowercase headers to the underlying function, causing the double-free condition.

How does Halo Surface Signal assess this risk?

Halo Surface Signal notes that while the vulnerability is network-accessible, the risk is contingent upon application logic. It is not triggered by default service exposure alone; rather, it requires that the application specifically uses the vulnerable mail() pattern with user-supplied input. You should evaluate whether your public-facing or internal applications perform this specific function.

What is the first step to address this CVE?

Begin by auditing your environment to identify systems running PHP on Windows. Once identified, determine if these applications utilize the mail() function and whether they allow for the submission of custom headers. If your applications use this function, prioritize checking for updates to your PHP version or modifying your code to sanitize header inputs while you coordinate a patch plan.

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.