CVE-2020-6096: GNU glibc ARMv7 memcpy() RCE Vulnerability

CVE-2020-6096 Overview

An exploitable signed comparison vulnerability exists in the ARMv7 memcpy() implementation of GNU glibc 2.30.9000. When memcpy() is called on ARMv7 targets utilizing the GNU glibc implementation with a negative value for the 'num' parameter, a signed comparison vulnerability is triggered. This flaw allows an attacker who can underflow the 'num' parameter to cause undefined behavior, including out-of-bounds memory writes and potentially remote code execution. The vulnerability is particularly dangerous because the memcpy() implementation allows program execution to continue in scenarios where a segmentation fault or crash should have occurred, leading to subsequent code iterations executing with corrupted data.

Critical Impact

This vulnerability can lead to remote code execution through out-of-bounds memory writes on ARMv7 systems running GNU glibc. The network-based attack vector combined with the potential for arbitrary code execution makes this a significant threat to embedded systems and IoT devices using ARM architectures.

Affected Products

• GNU glibc (versions prior to patch)
• Fedora 31 and 32
• Debian Linux 10.0

Discovery Timeline

• April 1, 2020 - CVE-2020-6096 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-6096

Vulnerability Analysis

This vulnerability stems from an integer underflow combined with a signed comparison flaw in the ARMv7-optimized memcpy() function within GNU glibc. The vulnerability specifically affects ARMv7 architecture targets where the GNU glibc implementation is used. When an attacker can control or influence the size parameter passed to memcpy(), they can trigger an integer underflow that results in the size being interpreted as a large positive number due to the signed-to-unsigned conversion in the comparison logic.

The consequences extend beyond a simple crash—the implementation allows execution to continue even when memory corruption has occurred, meaning subsequent operations work with corrupted data, potentially leading to more severe exploitation scenarios including arbitrary code execution.

Root Cause

The root cause is a signed comparison vulnerability in the ARMv7 assembly implementation of memcpy() within GNU glibc. The vulnerability arises from improper handling of the size parameter when a negative value is provided. The ARMv7-specific code uses signed comparison instructions where unsigned comparisons should be used, allowing negative values to pass boundary checks that should otherwise fail. This represents a classic integer underflow leading to out-of-bounds write scenario (CWE-191: Integer Underflow and CWE-195: Signed to Unsigned Conversion Error).

Attack Vector

The attack vector is network-based, requiring an attacker to manipulate input that eventually reaches a memcpy() call with a controlled or underflowed size parameter. This can occur through various application-level vectors where user-controlled data influences memory copy operations. The attack complexity is considered high due to the specific conditions required—the attacker must be able to cause an integer underflow in the 'num' parameter passed to memcpy() on an ARMv7 target running vulnerable versions of glibc.

The exploitation mechanism involves providing crafted input that causes the size calculation to underflow, resulting in a negative signed integer. When this negative value is used in the signed comparison within the ARMv7 memcpy() implementation, it bypasses length checks and triggers out-of-bounds memory operations. Technical details are available in the Talos Vulnerability Report TALOS-2020-1019 and the Sourceware Bug Report #25620.

Detection Methods for CVE-2020-6096

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using memcpy() on ARMv7 systems
• Memory corruption artifacts in process memory that persist after operations that should have failed
• Anomalous network traffic patterns targeting services on ARMv7-based embedded systems
• Process behavior continuing after memory access violations that would normally terminate execution

Detection Strategies

• Monitor for applications exhibiting memory corruption symptoms on ARMv7 architecture systems
• Implement runtime memory safety tools like AddressSanitizer (ASan) on development and test environments to detect out-of-bounds writes
• Deploy intrusion detection rules for network traffic anomalies targeting services on ARM-based embedded devices
• Audit application code for user-controllable size parameters passed to memory operations

Monitoring Recommendations

• Enable detailed logging for applications running on vulnerable ARMv7 systems to capture memory-related errors
• Implement memory integrity monitoring on critical ARMv7-based embedded systems and IoT devices
• Monitor system logs for signs of exploitation attempts including unusual process terminations and memory allocation failures
• Use endpoint detection and response (EDR) solutions to monitor for suspicious memory access patterns

How to Mitigate CVE-2020-6096

Immediate Actions Required

• Update GNU glibc to the latest patched version that addresses the signed comparison vulnerability
• Apply security updates from your Linux distribution (Fedora, Debian, Gentoo, etc.)
• Audit applications running on ARMv7 targets for potential exposure to this vulnerability
• Implement input validation to prevent integer underflows in size parameters passed to memory functions

Patch Information

Patches have been released by various Linux distributions to address this vulnerability. Refer to the following security advisories for distribution-specific update instructions:

• Debian LTS Security Announcement - Debian 10 (Buster) security update
• Fedora Package Announcements - Fedora 31/32 updates
• Gentoo GLSA 202101-20 - Gentoo security advisory

Update your glibc package using your distribution's package manager to receive the security fix.

Workarounds

• Implement application-level input validation to ensure size parameters cannot underflow to negative values
• Use compiler flags and runtime checks to detect integer overflows/underflows in size calculations
• Consider deploying application firewalls or input sanitization layers in front of vulnerable services
• Isolate vulnerable ARMv7 systems on network segments with restricted access until patches can be applied

# Check current glibc version on affected systems
ldd --version

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

# Update glibc on Fedora systems
sudo dnf update glibc

# Update glibc on Gentoo systems
sudo emerge --sync && sudo emerge -u sys-libs/glibc