CVE-2026-44242

Micronaut Framework is a JVM-based full stack Java framework designed for building modular, easily testable JVM applications. Prior to 4.10.22, 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. This vulnerability is fixed in 4.10.22.

Micronaut Framework is a JVM-based full stack Java framework designed for building modular, easily testable JVM applications. Prior to 4.10.22, 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. This vulnerability is fixed in 4.10.22.

The Netty-based Micronaut HTTP Client does not impose a limit on HTTP redirections, potentially allowing an infinite redirect loop that could lead to a denial-of-service attack.

Patches

The following versions are patched:

• For Micronaut 5, versions equal or greater than 5.0.1 >=
• For Micronaut 4, versions equal or greater than 4.10.24 >=
• For Micronaut 3, versions equal or greater than 3.10.7 >=

Workarounds

No

Resources

Micronaut 5 Patch: https://github.com/micronaut-projects/micronaut-core/commit/6e88a972718d6e1521c5b3bb7766451798dba4e3 Micronaut 4 Patch: https://github.com/micronaut-projects/micronaut-core/commit/f1dffffec8fb5e3b7e94ae907ce0be3831e499d4 Micronaut 3 Patch: https://github.com/micronaut-projects/micronaut-core/commit/c06a2715ca7f78321bc3ca05f41cca78cd351320

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:

// 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:

// 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"); }):

# 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…

Impact

DefaultHttpClient follows redirects and forwards Authorization, Cookie, and Proxy-Authorization headers to redirect targets across domain boundaries. The blocklist only filters Host/Connection/TE/CT/CL. Additionally, no maximum redirect count exists, enabling infinite loop DoS. Affected: DefaultHttpClient.java lines 231-245, 1591, 2071

Suggested fix: Strip sensitive headers on cross-domain redirects

Patches

It has been patched for versions:

For Micronaut 5, versions equal or greater than 5.0.1 >= For Micronaut 4, versions equal or greater than 4.10.24 >= For Micronaut 3, versions equal or greater than 3.10.6 >=

Workarounds

No

References

Micronaut 5 Patch: https://github.com/micronaut-projects/micronaut-core/commit/9770328999f490bdfbb9e25addd45bf73d4a173a Micronaut 4 Patch: https://github.com/micronaut-projects/micronaut-core/commit/70cab4b44fbf985faba2846091f2356b5bd70719 Micronaut 3 Patch: https://github.com/micronaut-projects/micronaut-core/commit/64e539736b8168f201d868b02ace50fe14f57418

Micronaut Framework is a JVM-based full stack Java framework designed for building modular, easily testable JVM applications. Prior to 4.10.22, 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. This vulnerability is fixed in 4.10.22.