A critical security vulnerability (CVE-2022-36788) has been discovered in the TriangleMesh cloning functionality of the popular slicer software Slic3r's libslic3r 1.3. and Master Commit b1a550. This heap-based buffer overflow vulnerability can be exploited by an attacker by providing a specially crafted STL (STereoLithography) file, which can lead to a heap buffer overflow, potentially resulting in arbitrary code execution on the victim's system. This long-read post will delve into the details of the vulnerability, provide code snippets to illustrate the exploited functionality, and link to original references for further investigation.

Code Snippet

The vulnerability lies within the TriangleMesh class's clone functionality, where a block of memory is allocated and a copy of the current mesh data is performed. The code snippet responsible for the vulnerability is as follows:

TriangleMesh* TriangleMesh::clone() const {
  TriangleMesh* other = new TriangleMesh;
  stl_file* stl = &other->stl;
  const stl_file* stl_this = &this->stl;
  
  *stl = *stl_this;
  memcpy(&stl->facet_start, stl_this->facet_start,
         stl->stats.number_of_facets * sizeof(stl_facet));
  return other;
}

The deepcopy process is performed using the memcpy function. The buffer overflow occurs when the size parameter of memcpy (i.e., stl->stats.number_of_facets * sizeof(stl_facet)) is larger than the allocated buffer's size, causing the adjacent memory to be overwritten and leading to the heap buffer overflow.

Exploit Details

To exploit this vulnerability, an attacker must create a specially crafted STL file and provide it to the unsuspecting user. When the user opens the malicious STL file in the Slic3r software, the TriangleMesh clone functionality would be called during the slicing process, ultimately triggering the heap buffer overflow. Due to the nature of the heap memory, the attacker may achieve arbitrary code execution, potentially compromising the victim's system.

The crafted STL file must follow the binary STL file format, with an altered number of facets field in the STL file header. By modifying the number_of_facets field to a larger value than the actual amount of facet data present, an exploitable buffer overflow can be created.

Recommendations and Mitigations

To mitigate the risk of exploitation, it is essential to fix the TriangleMesh clone functionality to avoid the heap-based buffer overflow vulnerability. One way to do this is by ensuring that the appropriate size of memory is allocated and checked before copying the data using memcpy. Here is an example of how the code could be modified:

TriangleMesh* TriangleMesh::clone() const {
  TriangleMesh* other = new TriangleMesh;
  stl_file* stl = &other->stl;
  const stl_file* stl_this = &this->stl;
  
  if(stl_this->stats.number_of_facets > ) {
    stl->facet_start = (stl_facet*)malloc(stl_this->stats.number_of_facets *
                                           sizeof(stl_facet));
    if(stl->facet_start != nullptr) {
      memcpy(&stl->facet_start, stl_this->facet_start,
             stl->stats.number_of_facets * sizeof(stl_facet));
    }
  }
  
  return other;
}

Additionally, users of Slic3r should avoid opening untrusted STL files, especially from unknown sources. The Slic3r development team is advised to release an updated version of the software that addresses this critical security vulnerability.

References

1. Slic3r Github Repository - Master Commit b1a550
2. CVE-2022-36788 - Heap-based Buffer Overflow Vulnerability
3. STL File Format - Binary STL#Binary_STL)

Conclusion

CVE-2022-36788 is a critical heap-based buffer overflow vulnerability that exists within the TriangleMesh clone functionality of Slic3r's libslic3r 1.3. and Master Commit b1a550. By providing a specially crafted malicious STL file, an attacker could potentially trigger the vulnerability and achieve arbitrary code execution on the victim’s system. This post has provided insights into the exploit details, code snippets illustrating the issue, and mitigation recommendations, as well as linking to original references for further investigation. To protect against such vulnerabilities, users must exercise caution when opening unknown files, and software developers should take the necessary steps to ensure secure handling of memory operations.

Timeline

Published on: 04/20/2023 16:15:00 UTC
Last modified on: 05/02/2023 15:05:00 UTC