CVE-2021-22945: Haxx Libcurl Use-After-Free Vulnerability

CVE-2021-22945 Overview

CVE-2021-22945 is a critical memory corruption vulnerability in libcurl affecting versions up to 7.73.0 and 7.78.0. When sending data to an MQTT server, libcurl could erroneously retain a pointer to already freed memory, subsequently using that pointer in later operations and potentially freeing it again. This double free and use-after-free condition can lead to memory corruption, information disclosure, and denial of service.

Critical Impact

This vulnerability enables attackers to exploit memory corruption via network-accessible MQTT communication, potentially leading to information disclosure and service crashes. The network attack vector with no authentication requirements makes this particularly dangerous for exposed systems.

Affected Products

• Haxx libcurl (versions up to 7.73.0 and 7.78.0)
• Fedora Project Fedora (versions 33 and 35)
• NetApp Cloud Backup and Clustered Data ONTAP
• Oracle MySQL Server
• Apple macOS
• Siemens SINEC INS
• Debian Linux 11.0
• Splunk Universal Forwarder
• NetApp Hardware (H300S, H500S, H700S, H300E, H500E, H700E, H410S series)
• NetApp SolidFire Baseboard Management Controller

Discovery Timeline

• 2021-09-23 - CVE-2021-22945 published to NVD
• 2025-06-09 - Last updated in NVD database

Technical Details for CVE-2021-22945

Vulnerability Analysis

This vulnerability represents a classic double free (CWE-415) condition occurring within libcurl's MQTT protocol implementation. The flaw manifests when libcurl processes data transmissions to MQTT servers, where memory management logic fails to properly track pointer state after initial deallocation.

The vulnerability occurs in a specific code path where libcurl sends data to an MQTT broker. Under certain circumstances, the library incorrectly retains a pointer reference to memory that has already been deallocated. This dangling pointer is then used in subsequent send operations, leading to a use-after-free condition. Additionally, the same memory region may be freed again, resulting in a double free vulnerability.

The impact of this vulnerability is significant because it can be triggered remotely through network communication with an MQTT server. An attacker who can influence the MQTT communication flow could potentially exploit this condition to cause memory corruption, crash the application, or potentially leak sensitive information from the process memory space.

Root Cause

The root cause is improper memory lifecycle management in libcurl's MQTT send functionality. When transmitting data to an MQTT server, the code fails to properly nullify or update pointer references after freeing memory, leaving dangling pointers that can be dereferenced or freed again in subsequent operations. This represents a failure in the memory ownership model within the MQTT protocol handler code.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability through:

1. Malicious MQTT Broker: Operating a rogue MQTT server that triggers the vulnerable code path when libcurl-based clients connect
2. Man-in-the-Middle: Intercepting MQTT communications and manipulating responses to trigger the memory corruption
3. Controlled Network Environment: In scenarios where attackers control network infrastructure between a victim application and MQTT services

The vulnerability is particularly concerning for applications that connect to untrusted or external MQTT servers, as the attack requires no special privileges and can be triggered entirely over the network.

Detection Methods for CVE-2021-22945

Indicators of Compromise

• Unexpected application crashes or segmentation faults in processes using libcurl for MQTT communication
• Memory corruption errors or heap-related warnings in system logs from curl-dependent applications
• Abnormal MQTT traffic patterns or connections to unusual MQTT broker endpoints
• Core dumps indicating double-free or use-after-free conditions in curl library functions

Detection Strategies

• Monitor for crash reports and core dumps from applications utilizing libcurl, particularly those implementing MQTT functionality
• Implement runtime memory sanitizers (AddressSanitizer, Valgrind) in development and testing environments to detect memory corruption issues
• Deploy network monitoring to identify anomalous MQTT connection patterns or connections to unexpected broker addresses
• Use SentinelOne's behavioral analysis capabilities to detect exploitation attempts targeting memory corruption vulnerabilities

Monitoring Recommendations

• Enable detailed logging for MQTT-enabled applications to capture connection and data transfer events
• Monitor system memory utilization and process health for applications using vulnerable libcurl versions
• Implement network intrusion detection rules to identify potential exploitation attempts targeting MQTT protocol handlers
• Regularly scan software inventory to identify systems running vulnerable libcurl versions

How to Mitigate CVE-2021-22945

Immediate Actions Required

• Update libcurl to version 7.79.0 or later which contains the fix for this vulnerability
• For applications bundling libcurl, update to vendor-patched versions as soon as available
• Review and restrict MQTT broker connections to trusted endpoints only
• Implement network segmentation to isolate MQTT-enabled systems from untrusted networks
• Apply vendor-specific patches from Oracle, Apple, NetApp, Siemens, Debian, and other affected vendors

Patch Information

Multiple vendors have released patches addressing this vulnerability. Key resources include:

• HackerOne Report #1269242 - Original vulnerability report
• Oracle CPU October 2021 - Oracle MySQL Server patches
• Siemens Product Security Advisory - SINEC INS patches
• Debian Security Advisory DSA-5197 - Debian Linux patches
• Apple Support Document - macOS security updates
• NetApp Security Advisory NTAP-20211029-0003 - NetApp product patches
• Gentoo GLSA 202212-01 - Gentoo Linux patches

Workarounds

• Disable MQTT functionality in libcurl if not required by compiling without MQTT support
• Implement strict network access controls to prevent connections to untrusted MQTT brokers
• Use application-level firewalls to filter and inspect MQTT traffic
• Consider using alternative MQTT client libraries until patching is complete

# Check installed libcurl version
curl --version

# On Debian/Ubuntu systems, update libcurl
sudo apt-get update && sudo apt-get upgrade libcurl4

# On RHEL/CentOS/Fedora systems
sudo dnf update curl libcurl

# Verify the updated version
curl --version | head -1