GHSA-crqm-m339-7m2p — pyzipper
MEDIUMGHSA-crqm-m339-7m2p is a medium-severity (CVSS 6.2) CWE-480 vulnerability in pyzipper. A fix is available for pyzipper — see the affected versions and patch details below.
pyzipper has an encryption bypass for small files encrypted using it
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-crqm-m339-7m2p.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
GHSA-crqm-m339-7m2p by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
pyzipperReal-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
Impact
A Python operator precedence bug in pyzipper/zipfile_aes.py caused the AE-2 format to never be automatically selected during encryption, regardless of file size or compression type. As a result, all encrypted entries are written in AE-1 format unless AE-2 is explicitly forced by the caller. AE-1 stores the plaintext CRC32 checksum unencrypted in the ZIP header. During investigation of this issue, it was also found that when writing to an unseekable zip archive, the CRC32 value was always written to the datadescripter section.
The AES encryption itself is not broken. An attacker who possesses the archive can read the CRC32 from the header without decrypting anything, then brute-force candidate plaintexts by computing CRC32(candidate) and comparing against the stored value. In practice, this attack is feasible today only against small or low-entropy files, as CRC32 exhaustion across a large plaintext space is computationally prohibitive on current hardware. Files with high-entropy or large content are not practically at risk under current computing constraints. Without this bug, pyzipper would have removed the CRC32 value for any file with content of less than 20 bytes uncompressed.
Patches
Upgrade to pyzipper 0.4.0 that changes the default behaviour of pyzipper to always use the AE-2 format and exclude the CRC32 values, unless instructed to do otherwise.
If rewriting the zip archive to remove the CRC values for small files, the entire zip archive should be recreated to avoid the original local file header with the CRC included remaining in the zip file in a detached state.
Credit
Thanks to Lucas Lavarello from Kulkan Security for identifying this issue.
References
https://www.winzip.com/en/support/aes-encryption/#CRC https://www.winzip.com/en/support/aes-encryption/#crc-faq
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pyzipper | all versions | 0.4.0pip install --upgrade 'pyzipper==0.4.0' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pyzipper, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pyzipper to 0.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-crqm-m339-7m2p is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is GHSA-crqm-m339-7m2p in your dependencies?
Find it across PyPI, including transitive dependencies.