Nacos is a platform designed for dynamic service discovery and configuration and service management. In Nacos before version 1.4.1, when configured to use authentication (-Dnacos.core.auth.enabled=true) Nacos uses the AuthFilter servlet filter to enforce authentication. This filter has a backdoor that enables Nacos servers to bypass this filter and therefore skip authentication checks. This mechanism relies on the user-agent HTTP header so it can be easily spoofed. This issue may allow any user to carry out any administrative tasks on the Nacos server.

Affected: Maven: com.alibaba.nacos:nacos-common

When configured to use authentication (`-Dnacos.core.auth.enabled=true`) Nacos uses the `AuthFilter` servlet filter to enforce authentication. This filter has a [backdoor](https://github.com/alibaba/nacos/blob/5fa05aef52f7432aeab19fe53035431b9d8c91d9/core/src/main/java/com/alibaba/nacos/core/auth/AuthFilter.java#L78-L81) that enables Nacos servers to bypass this filter and therefore skip authentication checks. This mechanism relies on the `user-agent` HTTP header so it can be easily spoofed.

The following request to the `configuration` endpoint gets rejected as we are not providing any credentials: ``` ❯ curl -X POST "http://127.0.0.1:8848/nacos/v1/cs/configs?dataId=nacos.cfg.dataIdfoo&group=foo&content=helloWorld" {"timestamp":"2020-12-02T14:33:57.154+0000","status":403,"error":"Forbidden","message":"unknown user!","path":"/nacos/v1/cs/configs"} ```

However the following one gets accepted by using the `Nacos-Server` user-agent header: ``` ❯ curl -X POST -A Nacos-Server "http://127.0.0.1:8848/nacos/v1/cs/configs?dataId=nacos.cfg.dataIdfoo&group=foo&content=helloWorld" true ```

#### Impact

This issue may allow any user to carry out any administrative tasks on the Nacos server.