CVE-2019-6706 in Lua
Summary
by MITRE
Lua 5.3.5 has a use-after-free in lua_upvaluejoin in lapi.c. For example, a crash outcome might be achieved by an attacker who is able to trigger a debug.upvaluejoin call in which the arguments have certain relationships.
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Analysis
by VulDB Data Team • 02/02/2025
The vulnerability identified as CVE-2019-6706 represents a critical use-after-free condition within the Lua 5.3.5 scripting engine, specifically within the lua_upvaluejoin function located in the lapi.c source file. This flaw arises from improper memory management during the execution of debug.upvaluejoin operations, creating a scenario where freed memory regions can be accessed or manipulated by subsequent operations. The vulnerability manifests when an attacker can control the parameters passed to the debug.upvaluejoin function, allowing them to establish specific relationships between arguments that trigger the use-after-free condition. The underlying issue stems from the Lua interpreter's failure to properly validate or sanitize the upvalue parameters during the join operation, leading to potential memory corruption that can be exploited for arbitrary code execution.
The technical exploitation of this vulnerability requires an attacker to craft specific input that can trigger the debug.upvaluejoin call with carefully constructed arguments that cause the interpreter to free memory structures while maintaining references to them. This use-after-free condition falls under the Common Weakness Enumeration category CWE-416, which specifically addresses the use of memory after it has been freed. The vulnerability demonstrates a classic memory safety issue where the Lua runtime fails to maintain proper reference counting or memory lifecycle management during debug operations. When the debug.upvaluejoin function executes with malicious parameters, it can cause the interpreter to free upvalue objects while other parts of the code still maintain references to them, creating opportunities for heap corruption and potential code execution.
The operational impact of this vulnerability extends beyond simple crash conditions to potentially enable remote code execution in environments where Lua scripts are executed with elevated privileges or when the scripting engine is embedded within applications that process untrusted input. Attackers can leverage this flaw to execute arbitrary code on systems running vulnerable Lua versions, particularly in web applications, game engines, or any software that incorporates Lua scripting capabilities. The vulnerability is particularly concerning because it can be triggered through debug operations that are commonly available in scripting environments, making it accessible to attackers who can inject malicious Lua code into applications that use the affected interpreter. The exploitation chain typically involves constructing a malicious script that calls debug.upvaluejoin with specific arguments, triggering the memory corruption that can then be leveraged for privilege escalation or system compromise.
Mitigation strategies for CVE-2019-6706 require immediate patching of affected Lua installations to version 5.3.6 or later, which includes the necessary memory management fixes for the lua_upvaluejoin function. Organizations should implement comprehensive input validation and sanitization for all Lua script execution environments, particularly when processing untrusted user input through scripting interfaces. Network segmentation and application whitelisting can help limit the attack surface by preventing unauthorized script execution. The vulnerability also highlights the importance of regular security audits of embedded scripting engines and proper memory safety practices in interpreted languages. Security teams should monitor for exploitation attempts through intrusion detection systems and implement proper logging of debug operations to identify potential abuse. Additionally, developers should avoid using vulnerable debug APIs in production environments and consider implementing sandboxing mechanisms that restrict access to potentially dangerous functions like debug.upvaluejoin. The ATT&CK framework categorizes this vulnerability under T1059.007 for scripting and T1068 for exploit development, emphasizing its role in automated exploitation and privilege escalation scenarios.