As pointed out by the Ars Technica article, XML Encryption is used widely as part of server-to-server Web services connections to transmit secure information mixed with non-sensitive data, based on cipher-block chaining. But it is apparently too weak, as demonstrated by Juraj Somorovsky and Tibor Jager. They were able to decrypt data by sending modified ciphertexts to the serve by gathering information from the received error messages. The attack was tested against a popular open source implementation of XML Encrytion, and against the implementations of companies that responded to the responsible disclosure — in all cases the result was the same: the attack worked.
Fixing the vulnerability will require a revision of the W3C XML encryption standard, Somorovsky said. The researchers informed all possibly affected companies through the mailing list of W3C, following a clear responsible disclosure process.