SOAP Fault "MustUnderstand headers: [{http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd}Security] are not understood."

Issue

• We are using Jboss-cxf for publishing web service and wss4j for authentication and configured accordingly in cxf.xml.
• We pass the below as a part of soap header along with the request:

<soapenv:Header>
    <!--WSSE Security Header-->
    <wsse:Security soapenv:mustUnderstand="1" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
      <wsse:UsernameToken wsu:Id="UsernameToken-17644953" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
        <wsse:Username>xxxxxxx</wsse:Username>
        <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">yyyyyyyyy</wsse:Password>
      </wsse:UsernameToken>
    </wsse:Security>
  </soapenv:Header>

• when we test through soapUI we get sometimes the below error:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Body>
      <soap:Fault>
         <faultcode>soap:MustUnderstand</faultcode>
         <faultstring>MustUnderstand headers: [{http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd}Security] are not understood.</faultstring>
      </soap:Fault>
   </soap:Body>
</soap:Envelope>

• when we change the value of mustUnderstand as "0" in the SOAP request then it works, but many times it works with mustUnderstand as "1" as well.