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Harden safe load - #799
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safe_load resolved !ruby/encoding directly via ::Encoding.find, bypassing the permitted_classes check that !ruby/object:Encoding already honors. Load it through the class loader so Encoding is only deserialized when permitted. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
!ruby/hash-with-ivars, !ruby/hash and !map are only emitted for Hash subclasses, but the loader allocated whatever class the tag named and populated its ivars directly. That let a permitted non-Hash class be instantiated with attacker-chosen ivars, bypassing its init_with validation. Verify the resolved class is a Hash subclass before use. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
!ruby/array, !seq and !ruby/string carry the same exposure just fixed for the hash tags: the loader allocated the named class and replaced its contents without checking the class was actually an Array or String subclass. Apply the same subclass check so a permitted unrelated class can no longer be allocated and populated through these tags. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Pull request overview
This PR hardens Psych’s YAML deserialization around several Ruby-specific tags to prevent safe/unsafe load bypasses (notably via choosing unrelated permitted classes and injecting ivars directly).
Changes:
- Route
!ruby/encodingthrough the class loader sopermitted_classesgoverns Encoding resolution. - Enforce that
!ruby/string,!ruby/array/!seq, and!ruby/hash*/!maptags only accept subclasses of their expected core types. - Add regression tests ensuring invalid (non-subclass) class names are rejected for these tags.
Reviewed changes
Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.
Show a summary per file
| File | Description |
|---|---|
lib/psych/visitors/to_ruby.rb | Adds resolve_subclass and applies it to string/array/hash tag handling; routes !ruby/encoding through class loader. |
lib/psych/class_loader.rb | Adds ENCODING constant so restricted loaders can gate Encoding via permitted_classes. |
test/psych/test_safe_load.rb | Adds safe-load tests verifying !ruby/encoding is disallowed unless Encoding is permitted. |
test/psych/test_string.rb | Adds tests ensuring !ruby/string:* rejects non-String classes. |
test/psych/test_array.rb | Adds tests ensuring !seq:* / !ruby/array:* reject non-Array classes. |
test/psych/test_hash.rb | Adds tests ensuring !ruby/hash*:* / !map:* reject non-Hash classes, covering ivar-injection bypass. |
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| def resolve_subclass klassname, parent | ||
| klass = resolve_class(klassname) | ||
| if klass && !(klass <= parent) | ||
| raise ArgumentError, "Invalid tag: expected a subclass of #{parent}, got #{klass}" | ||
| end | ||
| klass | ||
| end |
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!ruby/encodingnow resolves through the class loader, sopermitted_classesis honored just like!ruby/object:Encoding.!ruby/hash-with-ivars,!ruby/hash,!mapnow require a Hash subclass, preventinginit_withbypass via direct ivar injection.!ruby/array,!seq,!ruby/stringnow require an Array/String subclass for the same reason.