External risk intelligence

glibc mq_notify Use-After-Free Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2021-33574

The vulnerability exists in the GNU C Library (glibc), a low-level foundational system library. It is not an internet-facing application or network service itself. While glibc is used by virtually all applications, the vulnerability is not exposed directly to the public internet by design, and its reachability is dependent on specific application code rather than common network deployment patterns.

Use After Free

Gnu Glibc

2.322.33333411.0 to 11.70.110.0

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 critical vulnerability was identified in the GNU C Library (glibc), a fundamental software component used across many systems. This issue could potentially lead to system instability or unintended consequences, necessitating an understanding of its relevance within our environment.

A software library flaw could cause system crashes.
It affects foundational system software.
Confirm relevance and potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could leverage this vulnerability by triggering a use-after-free condition within the `mq_notify` function. This occurs when a notification thread's attributes object is freed by the caller but then subsequently used by the function, potentially leading to application instability or other unspecified consequences.

Requires interaction with the function.
Triggers use-after-free in notification attributes.
Risk of denial of service or other impact.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, this vulnerability in the GNU C Library could impact applications by causing them to crash due to a use-after-free error in the `mq_notify` function. This occurs when the notification thread attributes object is freed by the caller and then subsequently used.

Application crashes.
Unspecified impact from freed memory use.
Potential for system instability.

Operational Fix

Recommended remediation, mitigation, and detection steps

This critical vulnerability in the GNU C Library (glibc) is likely to impact applications and systems managed by infrastructure, platform, and application teams. The first practical step is to identify all systems running glibc versions 2.32 or 2.33, determine their business criticality and network exposure, and then engage the accountable owners to plan remediation.

Identify affected systems and owners.
Verify asset criticality and exposure.
Plan remediation based on 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 the GNU C Library (glibc)?

The GNU C Library, or glibc, is a fundamental software component that serves as the core interface between applications and the Linux operating system. It provides essential building blocks—such as memory allocation, file access, and process control—that nearly all programs rely on to function correctly.

What is the vulnerability in CVE-2021-33574?

This vulnerability is a use-after-free, classified as CWE-416. It occurs when a program attempts to use a specific memory address—in this case, a notification thread attributes object—after that memory has already been released. This mistake in the mq_notify function can cause an application to crash or behave unexpectedly.

How is the mq_notify vulnerability triggered?

An attacker triggers this condition by interacting with the mq_notify function in a way that causes the system to access a notification thread attributes object after the caller has already freed it. Simply having glibc installed does not trigger the bug; the specific code path involving the attributes object must be executed.

Is my system at risk from this CVE?

While Halo Surface Signal notes this is an external-facing classification, the actual risk depends on how your applications use the glibc library. Because glibc is a low-level library, it is not an internet-facing service itself; its vulnerability is only reachable if your specific application code calls the affected function with certain parameters.

What should I do if I run affected glibc versions?

First, inventory your systems to identify those running glibc versions 2.32 or 2.33. Next, prioritize these systems based on their business importance and overall connectivity. Finally, coordinate with your system administrators to schedule and apply the necessary updates provided by your software vendors or Linux distribution maintainers.

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.