CVE-2020-21913: ICU Use After Free Vulnerability

CVE-2020-21913 Overview

CVE-2020-21913 is a Use After Free vulnerability discovered in International Components for Unicode (ICU) v66.1. The vulnerability exists in the pkg_createWithAssemblyCode function within the tools/pkgdata/pkgdata.cpp file. This memory corruption flaw can be triggered when processing specially crafted input, potentially leading to application crashes and denial of service conditions.

Critical Impact

Successful exploitation of this use after free vulnerability can cause application denial of service through memory corruption, affecting systems that rely on ICU for internationalization support.

Affected Products

• Unicode International Components for Unicode (ICU) v66.1 and earlier versions
• Debian Linux 9.0 (Stretch)
• Debian Linux 10.0 (Buster)

Discovery Timeline

• 2021-09-20 - CVE-2020-21913 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-21913

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to use a memory pointer after the memory it references has been freed. In the context of ICU's pkgdata tool, the pkg_createWithAssemblyCode function improperly manages memory lifecycle, allowing access to deallocated memory regions.

The attack requires local access and user interaction, meaning an attacker would need to convince a user to process a malicious file or input with the vulnerable ICU pkgdata utility. While the vulnerability does not allow for data exfiltration or integrity violations, it can reliably cause application crashes, resulting in denial of service.

Root Cause

The root cause lies in improper memory management within the pkg_createWithAssemblyCode function in tools/pkgdata/pkgdata.cpp. The function deallocates memory but retains a pointer that is subsequently dereferenced, triggering the use after free condition. This type of vulnerability typically occurs when object lifecycle management is not properly synchronized across different code paths.

Attack Vector

The vulnerability requires local access to the system and user interaction to exploit. An attacker would need to craft malicious input that triggers the vulnerable code path in the pkgdata tool. When the pkg_createWithAssemblyCode function processes this input, it incorrectly accesses freed memory, causing unpredictable behavior including application crashes. The attack primarily results in availability impact through denial of service.

The vulnerability manifests in the pkgdata utility's assembly code generation functionality. When processing certain inputs, the function fails to properly track memory allocations, leading to dangling pointer access after deallocation. Technical details can be found in the ICU Jira Issue ICU-20850 and the corresponding GitHub Pull Request #886.

Detection Methods for CVE-2020-21913

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using ICU libraries
• Core dumps showing memory access violations in pkgdata or ICU-related processes
• Application log entries indicating memory corruption or invalid pointer access in ICU components

Detection Strategies

• Monitor for abnormal termination of processes utilizing ICU libraries, particularly the pkgdata tool
• Implement memory sanitizers (AddressSanitizer, Valgrind) in development and testing environments to detect use after free conditions
• Deploy application-level monitoring to detect repeated crashes in internationalization-dependent services

Monitoring Recommendations

• Enable crash reporting and analysis for applications that depend on ICU libraries
• Audit systems for vulnerable ICU versions using software composition analysis tools
• Monitor system logs for repeated application failures that may indicate exploitation attempts

How to Mitigate CVE-2020-21913

Immediate Actions Required

• Update ICU to a patched version that addresses the use after free vulnerability
• Apply security updates from Debian for affected systems (DSA-5014 for stable, DLA-2780 for LTS)
• Inventory all systems and applications that use ICU libraries to identify vulnerable deployments
• Restrict access to the pkgdata utility to trusted users only

Patch Information

Unicode has addressed this vulnerability through GitHub Pull Request #886. The fix corrects the memory management issue in the pkg_createWithAssemblyCode function.

For Debian users, security patches are available:

• Debian Security Advisory DSA-5014 for Debian stable releases
• Debian LTS Announcement DLA-2780 for Debian 9 LTS

Workarounds

• Limit use of the pkgdata tool to trusted inputs and controlled environments
• Implement process isolation for applications using ICU to contain potential crashes
• Consider sandboxing ICU-dependent applications to minimize impact of exploitation

# Verify ICU version on Debian systems
dpkg -l | grep libicu

# Update ICU packages on Debian
sudo apt-get update && sudo apt-get upgrade libicu63

# For Debian 10 (Buster)
sudo apt-get install libicu63=63.1-6+deb10u2