CVE-2020-15866 in mruby
Summary
by MITRE
mruby through 2.1.2-rc has a heap-based buffer overflow in the mrb_yield_with_class function in vm.c because of incorrect VM stack handling. It can be triggered via the stack_copy function.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 07/22/2020
The vulnerability CVE-2020-15866 represents a critical heap-based buffer overflow affecting mruby versions through 2.1.2-rc, specifically within the mrb_yield_with_class function located in vm.c. This flaw arises from improper handling of the virtual machine stack during execution, creating a condition where memory corruption can occur when the stack_copy function is invoked. The vulnerability demonstrates a classic stack manipulation error that can lead to arbitrary code execution or system instability.
The technical root cause of this vulnerability stems from inadequate bounds checking during stack operations within the mruby interpreter's virtual machine. When the mrb_yield_with_class function processes method calls and their associated stack frames, it fails to properly validate the size of data being copied to the virtual stack. The stack_copy function, which is responsible for transferring stack contents between different execution contexts, does not perform sufficient validation to prevent buffer overflows. This type of vulnerability falls under CWE-121, which categorizes buffer overflow conditions where insufficient boundary checks allow writing beyond allocated memory regions. The flaw specifically manifests when mruby processes nested method calls or blocks that require stack frame manipulation, creating a scenario where attacker-controlled data can overwrite adjacent memory locations.
The operational impact of CVE-2020-15866 extends beyond simple memory corruption, as it can potentially enable remote code execution when the vulnerable mruby interpreter is used in web applications or server environments. Attackers can craft malicious inputs that trigger the vulnerable code path through stack_copy operations, leading to exploitation of the heap-based buffer overflow. This vulnerability is particularly concerning in environments where mruby is embedded within web servers or application frameworks, as it can be leveraged for privilege escalation or denial of service attacks. The vulnerability aligns with ATT&CK technique T1059.007 for scripting languages and T1203 for exploitation for client execution, as it targets the interpreter's execution environment to achieve code execution. Systems running affected versions of mruby are at risk when processing untrusted input through the virtual machine's execution engine.
Mitigation strategies for CVE-2020-15866 primarily focus on updating to patched versions of mruby where the stack handling logic has been corrected. Organizations should prioritize upgrading to mruby 2.1.2 or later versions that contain the necessary fixes for the buffer overflow vulnerability. Additionally, implementing runtime protections such as stack canaries, address space layout randomization, and heap-based memory protection mechanisms can provide defense-in-depth against exploitation attempts. Input validation and sanitization should be enforced at all levels where mruby code execution occurs, particularly in web applications that process user-provided data. The vulnerability also highlights the importance of proper memory management practices and the need for comprehensive testing of stack operations in interpreted languages, as outlined in industry best practices for secure coding and vulnerability remediation.