@normanmaurer I can provide a PR for this. @hyperxpro sure lets do it :)
I think to be safe you want to use something like :

replaceAll("[^\\w.-]", "_"); With: `fqdn = fqdn.replaceFirst("\\*", "X");`, Output: `X.shieldblaze.com`
With: `fqdn = fqdn.replaceAll("[^\\w.-]", "_");`, Output: `_.shieldblaze.com`

Both are acceptable in terms of safety. However, we're already appending `_` (underscore) after `keyutil`. We'll end up with 2 underscores.
https://github.com/netty/netty/blob/86765c840cf85284780a0be90191fc530c73ce05/handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java#L368

Which one is good? @normanmaurer @chrisvest  I think we should use `fqdn.replaceAll("[^\\w.-]", "x")` Done, PTAL. why only `replaceFirst` and not `replaceAll` ?  Well, wildcards are expected to have an asterisk at first char. Like this: `*.netty.io`. However, invalid fqdn such as `*.me*ow.io` is an invalid hostname. And `SelfSignedCertificate` must throw an exception. true... that said I think it may be better safe then sorry and so I would just use `replaceAll` and call it a day  Alright, will change to `replaceAll`. :)
SelfSignedCertificate should use valid characters for file name
### Expected behavior
`SelfSignedCertificate` should filter out asterisk `*` while saving certificate and private key file.

### Actual behavior
`SelfSignedCertificate` creates a certificate and private key files and store them in a temporary directory. However, if the certificate uses a wildcard hostname that uses asterisk `*`, e.g. `*.shieldblaze.com`, it'll throw an error because `*` is not a valid character in the file system.

```
11 May 2021 14:34:22.457 [main] DEBUG i.n.h.s.u.SelfSignedCertificate - Failed to generate a self-signed X.509 certificate using sun.security.x509:
java.nio.file.InvalidPathException: Illegal char <*> at index 8: keyutil_*.shieldblaze.com_4376271060691248819.key
	at sun.nio.fs.WindowsPathParser.normalize(WindowsPathParser.java:182) ~[?:?]
	at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:153) ~[?:?]
	at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77) ~[?:?]
	at sun.nio.fs.WindowsPath.parse(WindowsPath.java:92) ~[?:?]
	at sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:229) ~[?:?]
	at java.nio.file.TempFileHelper.generatePath(TempFileHelper.java:59) ~[?:?]
	at java.nio.file.TempFileHelper.create(TempFileHelper.java:126) ~[?:?]
	at java.nio.file.TempFileHelper.createTempFile(TempFileHelper.java:160) ~[?:?]
	at java.nio.file.Files.createTempFile(Files.java:913) ~[?:?]
	at io.netty.util.internal.PlatformDependent.createTempFile(PlatformDependent.java:1423) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.newSelfSignedCertificate(SelfSignedCertificate.java:335) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.OpenJdkSelfSignedCertGenerator.generate(OpenJdkSelfSignedCertGenerator.java:82) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.<init>(SelfSignedCertificate.java:241) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.<init>(SelfSignedCertificate.java:167) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.<init>(SelfSignedCertificate.java:137) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at com.shieldblaze.expressgateway.configuration.tls.TLSConfigurationTest.addMappingWildcardTest(TLSConfigurationTest.java:84) ~[test-classes/:?]
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?]
	at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
	at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688) ~[junit-platform-commons-1.7.0.jar:1.7.0]
	at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:210) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:206) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:131) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:65) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at java.util.ArrayList.forEach(ArrayList.java:1541) ~[?:?]
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at java.util.ArrayList.forEach(ArrayList.java:1541) ~[?:?]
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:108) ~[junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88) ~[junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54) ~[junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:96) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:75) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:71) [junit5-rt.jar:?]
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33) [junit-rt.jar:?]
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:221) [junit-rt.jar:?]
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54) [junit-rt.jar:?]
11 May 2021 14:34:24.018 [main] DEBUG i.n.h.s.u.SelfSignedCertificate - Failed to generate a self-signed X.509 certificate using Bouncy Castle:
java.nio.file.InvalidPathException: Illegal char <*> at index 8: keyutil_*.shieldblaze.com_2205665342087087069.key
	at sun.nio.fs.WindowsPathParser.normalize(WindowsPathParser.java:182) ~[?:?]
	at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:153) ~[?:?]
	at sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77) ~[?:?]
	at sun.nio.fs.WindowsPath.parse(WindowsPath.java:92) ~[?:?]
	at sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:229) ~[?:?]
	at java.nio.file.TempFileHelper.generatePath(TempFileHelper.java:59) ~[?:?]
	at java.nio.file.TempFileHelper.create(TempFileHelper.java:126) ~[?:?]
	at java.nio.file.TempFileHelper.createTempFile(TempFileHelper.java:160) ~[?:?]
	at java.nio.file.Files.createTempFile(Files.java:913) ~[?:?]
	at io.netty.util.internal.PlatformDependent.createTempFile(PlatformDependent.java:1423) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.newSelfSignedCertificate(SelfSignedCertificate.java:335) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.BouncyCastleSelfSignedCertGenerator.generate(BouncyCastleSelfSignedCertGenerator.java:60) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.<init>(SelfSignedCertificate.java:246) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.<init>(SelfSignedCertificate.java:167) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at io.netty.handler.ssl.util.SelfSignedCertificate.<init>(SelfSignedCertificate.java:137) ~[netty-all-4.1.63.Final.jar:4.1.63.Final]
	at com.shieldblaze.expressgateway.configuration.tls.TLSConfigurationTest.addMappingWildcardTest(TLSConfigurationTest.java:84) ~[test-classes/:?]
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?]
	at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
	at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688) ~[junit-platform-commons-1.7.0.jar:1.7.0]
	at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:210) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:206) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:131) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:65) ~[junit-jupiter-engine-5.7.0.jar:5.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at java.util.ArrayList.forEach(ArrayList.java:1541) ~[?:?]
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at java.util.ArrayList.forEach(ArrayList.java:1541) ~[?:?]
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51) ~[junit-platform-engine-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:108) ~[junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88) ~[junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54) ~[junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:96) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:75) [junit-platform-launcher-1.7.0.jar:1.7.0]
	at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:71) [junit5-rt.jar:?]
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33) [junit-rt.jar:?]
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:221) [junit-rt.jar:?]
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54) [junit-rt.jar:?]
```

### Steps to reproduce
```java
SelfSignedCertificate ssc = new SelfSignedCertificate("*.shieldblaze.com", "EC", 256);
```

### Netty version
4.1.63.Final

### JVM version (e.g. `java -version`)
openjdk version "1.8.0_292"
OpenJDK Runtime Environment Corretto-8.292.10.1 (build 1.8.0_292-b10)
OpenJDK 64-Bit Server VM Corretto-8.292.10.1 (build 25.292-b10, mixed mode)

### OS version (e.g. `uname -a`)
Windows 10 Pro x64

handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java
handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java
handler/src/test/java/io/netty/handler/ssl/util/SelfSignedCertificateTest.java
diff --git a/handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java b/handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java
index 46a28d272f6..be57cd60caf 100644
--- a/handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java
+++ b/handler/src/main/java/io/netty/handler/ssl/util/SelfSignedCertificate.java
@@ -332,6 +332,9 @@ static String[] newSelfSignedCertificate(
             wrappedBuf.release();
         }
 
+        // Change all asterisk to 'x' for file name safety.
+        fqdn = fqdn.replaceAll("[^\\w.-]", "x");
+
         File keyFile = PlatformDependent.createTempFile("keyutil_" + fqdn + '_', ".key", null);
         keyFile.deleteOnExit();
 

diff --git a/handler/src/test/java/io/netty/handler/ssl/util/SelfSignedCertificateTest.java b/handler/src/test/java/io/netty/handler/ssl/util/SelfSignedCertificateTest.java
new file mode 100644
index 00000000000..59ca37635bc
--- /dev/null
+++ b/handler/src/test/java/io/netty/handler/ssl/util/SelfSignedCertificateTest.java
@@ -0,0 +1,50 @@
+/*
+ * Copyright 2021 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.handler.ssl.util;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.function.Executable;
+
+import java.security.cert.CertificateException;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SelfSignedCertificateTest {
+
+    @Test
+    void fqdnAsteriskDoesNotThrowTest() {
+        assertDoesNotThrow(new Executable() {
+            @Override
+            public void execute() throws Throwable {
+                new SelfSignedCertificate("*.netty.io", "EC", 256);
+            }
+        });
+
+        assertDoesNotThrow(new Executable() {
+            @Override
+            public void execute() throws Throwable {
+                new SelfSignedCertificate("*.netty.io", "RSA", 2048);
+            }
+        });
+    }
+
+    @Test
+    void fqdnAsteriskFileNameTest() throws CertificateException {
+        SelfSignedCertificate ssc = new SelfSignedCertificate("*.netty.io", "EC", 256);
+        assertFalse(ssc.certificate().getName().contains("*"));
+        assertFalse(ssc.privateKey().getName().contains("*"));
+    }
+}
