Remove Loader alias to UnsafeLoader and enhance security #851
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* Remove Loader class as an alias to UnsafeLoader * Add warning messages when using UnsafeLoader, CUnsafeLoader, unsafe_load(), and unsafe_load_all() * Change default loader in scan(), parse(), compose(), and compose_all() from UnsafeLoader to SafeLoader * Improve documentation with clear security warnings These changes help prevent accidental RCE vulnerabilities when processing untrusted YAML data. 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>
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Thank you! |
* Replace yaml.Loader with yaml.UnsafeLoader in test_structure.py * Update test_yaml_ext.py to use UnsafeLoader/CUnsafeLoader instead of removed Loader class * Ensures tests still work with the removal of the Loader alias 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>
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Good job |
lucadonnoh
approved these changes
Mar 7, 2025
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Why not alias |
dOrgJelli
approved these changes
Mar 7, 2025
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Background
The Lazarus Group, a North Korean state-sponsored hacking group, has been actively exploiting PyYAML's unsafe loader functionality to conduct advanced persistent threat (APT) attacks. Most recently, they were responsible for the $1.5 billion Bybit cryptocurrency exchange hack in February 2025 - the largest heist in history.
The attackers specifically used PyYAML's
yaml.Loadervulnerability to execute remote code execution (RCE) attacks by tricking exchange employees into running seemingly legitimate Python code that contained:Changes in this PR
This PR significantly enhances PyYAML's security posture through several critical changes:
Loaderalias toUnsafeLoader: This prevents accidental use of the unsafe loaderUnsafeLoader,CUnsafeLoader, orunsafe_load()functions are usedUnsafeLoadertoSafeLoaderin various core functionsSecurity Impact
These changes provide multiple layers of protection:
Loaderalias prevents the specific attack pattern used in the Bybit hackBackward Compatibility
While this PR makes security-focused breaking changes, the actual impact should be minimal:
yaml.SafeLoaderoryaml.safe_load()will continue to work without changesyaml.FullLoaderoryaml.full_load()will continue to work without changesyaml.UnsafeLoaderoryaml.unsafe_load()will continue to work but will now generate runtime warningsyaml.Loaderalias will need to be updated to either useyaml.UnsafeLoader(not recommended) or preferably migrate toyaml.SafeLoaderoryaml.FullLoaderReferences
This PR helps eliminate a serious security vulnerability that has been exploited by nation-state actors to steal billions in cryptocurrency assets. By removing the unsafe loader alias and promoting safer defaults, we can help prevent future attacks.