Nginx policy writing practice: preventing XSS attacks

With the continuous development of Web technology, security issues have become more and more important, among which XSS attacks are extremely common. Attackers inject code into the website so that users will be attacked when browsing the website, thereby leaking personal privacy or conducting phishing scams. Therefore, in modern web development, preventing XSS attacks has become a basic requirement.

In order to prevent XSS attacks, we need to write some policies to guide the work of the Nginx server. These policies can include input inspection, output inspection, cookie filtering, etc. The following will be explained with examples.

1. Input check

Input check refers to checking the data input by the user, including whether the type of parameters is correct, filtering of special characters, etc. For Nginx, input checking can be implemented using a combination of Lua scripts and regular expressions. The following is an example:

location / {
    access_by_lua_block {
        local args = ngx.req.get_uri_args()
        for key, val in pairs(args) do
            if type(val) == "table" then
                for k,v in pairs(val) do
                    if string.match(v, "%W") then
                        ngx.exit(ngx.HTTP_FORBIDDEN)
                    end
                end
            else
                if string.match(val, "%W") then
                    ngx.exit(ngx.HTTP_FORBIDDEN)
                end
            end
        end
    }
}

In the above code, we obtain the parameters passed by the user through the GET method through the Lua script, and use regular expressions to determine whether the parameters contain special characters. If special characters are present, a 403 error page is returned.

2. Output check

Output check refers to checking the output content of the back-end program, including whether tags, attributes, Javascript, etc. are safe. For Nginx, the sub_filter directive in the NGX_LUA module can be used to implement output checking. Here is an example:

location / {
    proxy_pass http://backend;
    sub_filter_types application/json;
    sub_filter 'bad-word' 'good-word';
    sub_filter_last_modified on;
    sub_filter_once off;
}

In the above code, we enable the sub_filter directive to check the JSON data returned by the backend program. If the data returned by the backend program contains "bad-word", replace it with "good-word".

3. Cookie filtering

Cookie filtering refers to checking the cookies set by the user to ensure their security. For Nginx, Lua script can be used to implement cookie filtering. The following is an example:

location / {
    access_by_lua_block {
        local h = ngx.req.get_headers()
        local ck = h.cookie
        if ck ~= nil then
            if string.match(ck, "%W") then
                ngx.exit(ngx.HTTP_FORBIDDEN)
            end
        end
    }
}

In the above code, we use Lua script to obtain the Cookie set in the HTTP request header, and use regular expressions to determine whether the Cookie contains special characters. If special characters are present, a 403 error page is returned.

The above is the Nginx policy writing practice to prevent XSS attacks. Of course, there are many other aspects to consider in practical applications, such as HTTP header security, defense against SQL injection, etc. Therefore, during the development process, targeted configurations need to be made based on actual conditions to improve the security of web applications.

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