Vulnerability Instance

Micronaut has Unbounded `bundleCache` in `ResourceBundleMessageSource` that Allows Memory Exhaustion via `Accept-Language` Header
## Summary

`ResourceBundleMessageSource` maintains two caches: `messageCache` (bounded at 100 entries via `ConcurrentLinkedHashMap`) and `bundleCache` (unbounded `ConcurrentHashMap`). The `bundleCache` is keyed by `(Locale, baseName)` where the locale originates from the HTTP `Accept-Language` header. In applications that explicitly register a `ResourceBundleMessageSource` bean and serve HTML error responses, an unauthenticated attacker can exhaust heap memory by sending requests with large numbers of unique `Accept-Language` values, each causing a new entry in the unbounded `bundleCache`. Unlike GHSA-2hcp-gjrf-7fhc and the sibling `messageCache` (both bounded), `bundleCache` was not updated to use a bounded cache implementation.

## Details

The `bundleCache` is initialized in `inject/src/main/java/io/micronaut/context/i18n/ResourceBundleMessageSource.java` at line 150:

```java
// ResourceBundleMessageSource.java:139-152
protected Map<MessageKey, Optional<String>> buildMessageCache() {
    return new ConcurrentLinkedHashMap.Builder<MessageKey, Optional<String>>()
            .maximumWeightedCapacity(100)    // ← BOUNDED ✓
            .build();
}

protected Map<MessageKey, Optional<ResourceBundle>> buildBundleCache() {
    return new ConcurrentHashMap<>(18);      // ← UNBOUNDED ✗
}
```

The `resolveBundle()` method at line 169 inserts into `bundleCache` with no eviction policy:

```java
// ResourceBundleMessageSource.java:169-185
private Optional<ResourceBundle> resolveBundle(Locale locale) {
    MessageKey key = new MessageKey(locale, baseName);
    final Optional<ResourceBundle> resourceBundle = bundleCache.get(key);
    if (resourceBundle != null) {
        return resourceBundle;
    } else {
        Optional<ResourceBundle> opt;
        try {
            opt = Optional.of(ResourceBundle.getBundle(baseName, locale, getClassLoader()));
        } catch (MissingResourceException e) {
            opt = Optional.empty();
        }
        bundleCache.put(key, opt);    // NO SIZE CHECK — unbounded growth
        return opt;
    }
}
```

The attack path requires:
1. The application registers a `ResourceBundleMessageSource` bean (non-default, requires explicit user configuration).
2. The attacker sends requests that trigger HTML error responses — i.e., requests with `Accept: text/html` to any URL that returns an error (e.g., 404 for any non-existent path).
3. Each request uses a unique `Accept-Language` value (e.g., `zz-AA`, `zz-AB`, …).
4. `DefaultHtmlErrorResponseBodyProvider.error()` calls `messageSource.getMessage(code, locale)` → `CompositeMessageSource` delegates to `ResourceBundleMessageSource` → `resolveBundle(locale)` inserts one entry per unique locale into `bundleCache`.

For locales that don't match any bundle file, `ResourceBundle.getBundle()` throws `MissingResourceException` and `Optional.empty()` is stored — a low-cost sentinel. For locales that DO match a bundle, a full `ResourceBundle` object is retained in memory. In either case, the map itself and the `MessageKey` objects grow without bound.

Note: the `messageCache` is bounded at 100 entries but does not prevent `bundleCache` growth, as `resolveBundle()` is called directly (bypassing `messageCache`) whenever a `messageCache` miss occurs.

## PoC

Against a Micronaut application with a `ResourceBundleMessageSource` bean registered (e.g., `@Bean ResourceBundleMessageSource messages() { return new ResourceBundleMessageSource("messages"); }`):

```bash
# Flood bundleCache with unique locales via HTML error path
for i in $(seq 1 100000); do
  curl -s -o /dev/null \
    -H "Accept: text/html" \
    -H "Accept-Language: zz-$(printf '%04d' $i)" \
    "http://localhost:8080/nonexistent-path-$(printf '%06d' $i)" &
  [ $((i % 200)) -eq 0 ] && wait
done
wait
```

Each unique `zz-XXXX` tag creates one new `bundleCache` entry. The `MessageKey` (Locale + baseName) and map overhead cost approximately 100-200 bytes per entry. At 100,000 entries, heap consumption from the cache alone reaches roughly 20 MB — significant in resource-constrained deployments. If a locale matches a bundle file, retained `ResourceBundle` objects cost substantially more per entry.

## Impact

- Only affects applications that explicitly register a `ResourceBundleMessageSource` bean (not the default configuration).
- Requires the ability to send HTTP requests with `Accept: text/html` headers and control over the `Accept-Language` value.
- Memory grows approximately 100-200 bytes per novel locale (for non-matching locales) up to several KB per locale if bundles are found. Sustained attack over time causes gradual heap exhaustion.
- Partial availability impact (A:L) under sustained attack in long-running services.

## Recommended Fix

Apply the same bounded-cache pattern used for the sibling `messageCache`:

```java
// In ResourceBundleMessageSource.java — replace buildBundleCache()
protected Map<MessageKey, Optional<ResourceBundle>> buildBundleCache() {
    return new ConcurrentLinkedHashMap.Builder<MessageKey, Optional<ResourceBundle>>()
            .maximumWeightedCapacity(50)    // small — one entry per (locale, baseName)
            .build();
}
```

The number of distinct resource bundle files is bounded at compile time; a limit of 50 entries is more than sufficient for any realistic i18n configuration while fully preventing unbounded growth.