CVE-2020-15889: Lua Buffer Overflow Vulnerability

CVE-2020-15889 Overview

CVE-2020-15889 is a heap-based buffer over-read vulnerability affecting Lua 5.4.0. The flaw exists in the getobjname function due to improper handling by the youngcollection function in lgc.c, which uses markold for an insufficient number of list members. This memory corruption vulnerability can allow attackers to read beyond allocated buffer boundaries, potentially leading to information disclosure, denial of service, or arbitrary code execution.

Critical Impact

This heap-based buffer over-read vulnerability in Lua 5.4.0's garbage collection mechanism can be exploited remotely without authentication, potentially allowing attackers to compromise systems embedding the Lua interpreter.

Affected Products

• Lua 5.4.0

Discovery Timeline

• 2020-07-21 - CVE-2020-15889 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15889

Vulnerability Analysis

This vulnerability stems from a memory safety issue in Lua's garbage collection subsystem. The youngcollection function within lgc.c is responsible for handling young generation garbage collection. During this process, the markold function is called to mark objects that should not be collected. However, the implementation incorrectly marks an insufficient number of list members, creating a condition where the getobjname function can read beyond the bounds of allocated heap memory.

When exploited, this out-of-bounds read condition (CWE-125) can expose sensitive data from adjacent memory regions, cause application crashes through memory access violations, or potentially be chained with other vulnerabilities to achieve more severe impacts such as code execution.

Root Cause

The root cause of CVE-2020-15889 lies in the youngcollection function's incorrect calculation of list member counts when invoking markold. This results in certain list elements not being properly marked during the generational garbage collection cycle. When subsequent operations like getobjname attempt to access these unmarked elements, they may read beyond the allocated buffer boundaries, triggering the heap-based buffer over-read condition.

Attack Vector

This vulnerability can be exploited remotely through network-accessible services that embed the Lua interpreter. An attacker can craft malicious Lua scripts or input data that triggers the vulnerable garbage collection code path. The attack requires no prior authentication or user interaction, making it particularly dangerous for applications that process untrusted Lua code or allow user-supplied scripts.

The exploitation involves triggering specific garbage collection scenarios that expose the insufficient marking behavior, followed by operations that access the improperly managed objects through the getobjname function.

Detection Methods for CVE-2020-15889

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using Lua 5.4.0, particularly during garbage collection cycles
• Memory access violation errors in system logs referencing lgc.c, getobjname, or related Lua functions
• Anomalous memory read patterns detected by memory protection tools in Lua-embedded applications

Detection Strategies

• Deploy memory safety monitoring tools (AddressSanitizer, Valgrind) to detect out-of-bounds read operations in Lua runtime environments
• Implement application-level logging to capture garbage collection errors and memory-related exceptions
• Use binary composition analysis tools to identify applications containing the vulnerable Lua 5.4.0 library

Monitoring Recommendations

• Monitor system logs for repeated crashes or memory errors in services embedding Lua interpreters
• Implement runtime application self-protection (RASP) solutions to detect memory corruption attempts
• Track Lua interpreter version usage across the infrastructure to identify vulnerable deployments

How to Mitigate CVE-2020-15889

Immediate Actions Required

• Upgrade Lua to a patched version that includes the fix from commit 127e7a6c8942b362aa3c6627f44d660a4fb75312
• Audit all applications and services that embed or depend on Lua 5.4.0 and prioritize patching
• Consider restricting untrusted Lua script execution until patches are applied

Patch Information

The Lua development team has addressed this vulnerability through a commit to the official Lua repository. This patch corrects the youngcollection function to properly mark all necessary list members when calling markold, preventing the heap-based buffer over-read condition.

Additional technical discussions about this vulnerability can be found on the Lua Users Mailing List.

Workarounds

• Disable or restrict execution of untrusted Lua scripts in affected applications until patching is complete
• Implement input validation and sandboxing for Lua script execution environments
• Deploy web application firewalls or intrusion prevention systems to filter potentially malicious requests targeting Lua-based services
• Consider running Lua-embedded applications with reduced privileges to limit impact of potential exploitation

# Verify Lua version and check for vulnerability
lua -v
# If output shows Lua 5.4.0, upgrade immediately

# Update Lua to latest patched version (example for building from source)
git clone https://github.com/lua/lua.git
cd lua
# Ensure commit 127e7a6c8942b362aa3c6627f44d660a4fb75312 is included
make linux
make install