External risk intelligence

libcurl MQTT Double Free Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2021-22945

libcurl is a widely used library embedded within diverse applications, appliances, and operating systems rather than a standalone network service. While it can be involved in internet-facing communication, the specific vulnerability requires an application to use libcurl's MQTT functionality in a way that triggers the memory error, making direct internet exposure via this specific component non-guaranteed.

Haxx Libcurl

7.73.0 to 7.78.033355.7.0 to 5.7.358.0.0 to 8.0.2612.0.0 to before 12.3before 1.0.1.111.08.2.0 to before 8.2.129.0.0 to before 9.0.69.1.0

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 the libcurl data transfer library could allow an attacker to exploit a memory corruption issue when handling specific data transmission scenarios. This impacts a wide range of software and systems that rely on libcurl for network communication. The main concern is confirming relevance and exposure.

The issue involves memory errors during data transfer.
It affects software using the libcurl library.
Confirm relevance and exposure to understand potential impact.

Attack Path

How an attacker could exploit the issue

An attacker could target systems that use a vulnerable version of libcurl for MQTT communications. By sending specific data to the MQTT server through this library, an attacker could trigger a double-free memory error. This could potentially lead to a denial-of-service condition or allow for further exploitation.

External network access needed.
MQTT communication triggers memory error.
Data corruption or denial of service.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could impact applications that use libcurl to send data over MQTT. Under specific circumstances, a flaw in how libcurl handles memory after freeing it could lead to a crash or unexpected behavior during data transmission. This issue is not expected to expose Personally Identifiable Information (PII).

Application stability and availability.
Erroneous memory operations during data transfer.
Service disruption or crashes.

Operational Fix

Recommended remediation, mitigation, and detection steps

Application owners and platform teams are most likely responsible for addressing this vulnerability, as it affects the libcurl library, which is frequently embedded in various software and operating systems. The first practical step is to identify all systems where libcurl is used, particularly in applications that send data to an MQTT server. Once identified, confirm the reachability and criticality of these assets and their accountable owners to prioritize remediation efforts.

Identify affected applications and their owners.
Verify reachability and business criticality.
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 libcurl?

libcurl is a fundamental library used by countless software programs, operating systems, and appliances to transfer data over a network. Developers embed it into their applications to handle complex communication tasks—such as sending data to servers—without having to write the networking code from scratch. Because it is a utility library, it runs in the background of many different technologies rather than acting as a standalone, visible application.

What does CWE-415 mean for this CVE?

CWE-415 refers to a 'Double Free' vulnerability. In the context of CVE-2021-22945, this means the software attempts to clear the same block of computer memory twice. Because the system expects memory to be managed in a specific order, this error can confuse the application, potentially leading to a crash or creating instability when the program tries to use that memory area again after it has already been released.

How is this memory error triggered?

The error occurs specifically during MQTT data transfers. For the bug to trigger, an application must use the affected versions of libcurl to communicate with an MQTT server under precise, erroneous conditions. Simply using libcurl for other types of web traffic—such as standard HTTP requests—does not trigger this specific memory management flaw.

Do I need to worry about internet exposure?

While Halo Surface Signal identifies the potential for external network access, libcurl is typically embedded deep inside other software. Whether your systems are at risk depends on if your specific applications use libcurl's MQTT features. You should investigate which of your internal or internet-facing applications rely on this library for MQTT connectivity, as broad network exposure does not automatically mean this specific component is exploitable.

How should I start responding to this?

Your first step is to perform an inventory of software and systems in your environment that utilize libcurl. Focus your search on applications explicitly configured to send data via the MQTT protocol. Once you have identified these assets, check with the vendors or maintainers of those applications to see if they have provided updates that include a patched version of the libcurl library.

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.