API Platform Core: Relation IRIs are not type-checked: a related resource can be denormalised as the wrong resource type (type confusion)

Summary

The API Platform serializer's AbstractItemNormalizer does not validate the resource type returned when resolving relation IRIs, allowing type confusion where a resource of an unintended type can be silently assigned to a relation property.

Impact

An attacker who can submit write requests (POST/PUT/PATCH) to an API Platform endpoint with writable relations can supply a relation IRI pointing to a resource of a different type than the relation's declared class. Because getResourceFromIri() does not pass an $operation to IriConverter::getResourceFromIri(), the is_a type guard at IriConverter.php:86 is skipped. For untyped relation properties (legacy @var-only style), the wrong-typed object is silently assigned, corrupting invariants and potentially feeding downstream logic that assumes the declared type (CWE-843). For typed properties (modern PHP 8.x), the substitution is blocked by Symfony's PropertyAccessor with an InvalidTypeException.

Affected versions

• api-platform/core < 4.1.30
• api-platform/core >= 4.2.0, < 4.2.26
• api-platform/core >= 4.3.0, < 4.3.12

Older major series (2.x, 3.x) ship the same vulnerable code path and are end-of-life; no fix is planned.

Patched versions

• 4.1.30
• 4.2.26
• 4.3.12

Fix

An is_a guard is added inside AbstractItemNormalizer::getResourceFromIri() (and the equivalent inline call sites on 4.1) so that a mismatched IRI throws InvalidArgumentException, mirroring the operation-aware check the IriConverter already performs when an operation is supplied. This forces a 400 Bad Request response for cross-type IRIs instead of a silent assignment.

Workarounds

Declare a PHP type on every writable relation property (e.g. public ?Foo $relation = null; instead of @var Foo $relation). Symfony's PropertyAccessor will then reject a mismatched object with InvalidTypeException. This does not cover collections of mixed-type interfaces; upgrading to a patched version is the only complete fix.

Proof of concept

A functional test posts a Bar IRI to a Foo-declared relation on an untyped property. Without the fix the server responds with HTTP 201 and the Bar IRI appears in the response payload. With the fix the server responds with HTTP 400 (Invalid IRI "/bars/1").

Full PoC: tests/Functional/Security/TypeConfusionRelationIriTest.php in the patched branches.

References

• src/Serializer/AbstractItemNormalizer.php — vulnerable relation IRI load
• src/Symfony/Routing/IriConverter.php — conditional is_a guard (operation-aware path)

Credit

Reported by @alexandre-daubois.