CVE-2021-33574: GNU Glibc Use-After-Free Vulnerability

CVE-2021-33574 Overview

CVE-2021-33574 is a use-after-free vulnerability affecting the mq_notify function in the GNU C Library (glibc) versions 2.32 and 2.33. This memory corruption flaw occurs when the function improperly references the notification thread attributes object after the caller has freed it. The vulnerability can be exploited to cause denial of service through application crashes, with potential for additional unspecified impacts including possible remote code execution.

Critical Impact

This use-after-free vulnerability in a fundamental system library affects numerous Linux distributions and embedded systems, potentially enabling attackers to crash applications or achieve arbitrary code execution through memory corruption.

Affected Products

• GNU glibc versions 2.32 and 2.33
• Fedora 33 and 34
• Debian Linux 10.0
• NetApp Cloud Backup
• NetApp E-Series SANtricity OS Controller
• NetApp SolidFire Baseboard Management Controller Firmware
• NetApp H-Series Storage Systems (H300S, H500S, H700S, H300E, H500E, H700E, H410S)

Discovery Timeline

• 2021-05-25 - CVE-2021-33574 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-33574

Vulnerability Analysis

The vulnerability resides in the mq_notify function within glibc, which is responsible for registering for notification when a message is available on a POSIX message queue. The function accepts a struct sigevent parameter that may contain thread attributes for the notification mechanism.

The core issue involves improper lifecycle management of the thread attributes object. When a caller passes thread attributes through the sigevent structure, the mq_notify implementation may continue to reference this memory after the calling function has freed it. This creates a classic use-after-free condition where the library accesses deallocated memory.

Exploitation of this vulnerability could result in application crashes due to accessing invalid memory, memory corruption that could be leveraged for code execution, or unpredictable application behavior affecting system stability.

Root Cause

The root cause is a temporal memory safety violation in the mq_notify implementation. The function fails to properly copy or retain the thread attributes object before the caller's scope ends. Specifically, the notification thread may attempt to use the pthread_attr_t structure after the caller has deallocated it, violating memory safety guarantees.

This represents CWE-416 (Use After Free), where dynamically allocated memory is referenced after being freed, leading to undefined behavior. The glibc implementation should have either copied the attributes internally or ensured synchronization to prevent access after deallocation.

Attack Vector

The attack vector is network-accessible with no authentication required. An attacker can trigger this vulnerability by:

1. Crafting an application or exploiting an existing application that uses POSIX message queues with notification
2. Manipulating the timing of thread attribute allocation and deallocation
3. Causing the mq_notify function to access freed memory at a controlled time

The vulnerability requires the attacker to influence an application using glibc's message queue notification functionality. In scenarios where applications expose message queue operations to external input, remote exploitation becomes feasible.

The exploitation mechanism relies on manipulating memory allocation patterns to control the contents of freed memory when mq_notify accesses it. Successful exploitation could overwrite critical data structures or function pointers, potentially leading to arbitrary code execution.

Detection Methods for CVE-2021-33574

Indicators of Compromise

• Unexpected application crashes with memory-related errors in applications using POSIX message queues
• Core dumps indicating access violations in glibc's mq_notify or related notification thread code
• Anomalous process behavior following message queue operations
• Memory corruption signatures in application logs

Detection Strategies

• Monitor for segmentation faults and memory access violations in applications utilizing mq_notify functionality
• Implement memory sanitizers (AddressSanitizer) in development and testing environments to detect use-after-free conditions
• Deploy endpoint detection solutions that can identify exploitation attempts targeting glibc functions
• Audit application code for POSIX message queue usage patterns that may trigger the vulnerability

Monitoring Recommendations

• Enable system-level crash reporting and analyze core dumps for glibc-related memory violations
• Monitor process stability metrics for applications known to use message queue notifications
• Implement runtime application self-protection (RASP) to detect memory corruption attacks
• Track glibc version deployment across infrastructure to identify vulnerable systems

How to Mitigate CVE-2021-33574

Immediate Actions Required

• Upgrade GNU glibc to version 2.34 or later where the vulnerability has been addressed
• Apply distribution-specific patches from Fedora, Debian, Gentoo, or your Linux distribution vendor
• Prioritize patching systems running network-exposed applications that utilize POSIX message queues
• Review and update NetApp appliance firmware for affected H-Series and storage systems

Patch Information

Security patches are available from multiple sources:

• Sourceware Bug Report - Original bug report with fix details
• Fedora Package Announcement - Fedora security update
• Debian LTS Security Announcement - Debian security advisory
• Gentoo GLSA 2021-07-07 - Gentoo Linux security advisory
• NetApp Security Advisory - NetApp product updates

Workarounds

• If immediate patching is not feasible, restrict or disable functionality that relies on mq_notify with thread attributes
• Implement application-level workarounds to avoid passing thread attributes through sigevent structures
• Use network segmentation to limit exposure of vulnerable systems to untrusted networks
• Consider deploying containerized applications with updated glibc versions as an interim measure

# Check current glibc version
ldd --version

# Verify if vulnerable version is installed
rpm -q glibc  # For RHEL/Fedora/CentOS
dpkg -l libc6  # For Debian/Ubuntu

# Update glibc on Fedora
sudo dnf update glibc

# Update glibc on Debian/Ubuntu
sudo apt update && sudo apt upgrade libc6