GHSA-9rjg-x2p2-h68h

## 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.