How to fix unterminated short string error in Varnish VCL while configuring Fastly CDN

Question

I am trying to setup token based authentication on Fastly CDN with Varnish VCL and using this sample VCL snippet to generate and validate JWT tokens here -

sub vcl_recv {
    #FASTLY recv

    if (req.request != "HEAD" && req.request != "GET" && req.request != "FASTLYPURGE") {
        return(pass);
    }

    // Generate synth
    if(req.url ~ "generate") {
        error 901;
    }

    // Validate token
    if(req.url ~ "validate") {
        // Ensure token exists and parse into regex
        if (req.http.X-JWT !~ "^([a-zA-Z0-9\-_]+)?\.([a-zA-Z0-9\-_]+)?\.([a-zA-Z0-9\-_]+)?$") {
            // Forbidden
            error 403 "Forbidden";
        }

        // Extract token header, payload and signature
        set req.http.X-JWT-Header = re.group.1;
        set req.http.X-JWT-Payload = re.group.2;
        set req.http.X-JWT-Signature = digest.base64url_nopad_decode(re.group.3);
        set req.http.X-JWT-Valid-Signature = digest.hmac_sha256("SupSecretStr", 
        req.http.X-JWT-Header "." req.http.X-JWT-Payload);
        // Validate signature
        if(digest.secure_is_equal(req.http.X-JWT-Signature, req.http.X-JWT-Valid-Signature)) {
            // Decode payload
            set req.http.X-JWT-Payload = digest.base64url_nopad_decode(req.http.X-JWT-Payload);
            set req.http.X-JWT-Expires = regsub(req.http.X-JWT-Payload, {"^.*?"exp"\s*?:\s*?([0-9]+).*?$"}, "\1");

            // Validate expiration
            if (time.is_after(now, std.integer2time(std.atoi(req.http.X-JWT-Expires)))) {
               // Unauthorized
               synthetic {"{"sign":""} req.http.X-JWT-Signature {"","header":""} req.http.X-JWT-Header {"","payload":""} req.http.X-JWT-Payload {"","valid": ""} req.http.X-JWT-Valid-Signature {""}"};
               return(deliver);
            }

            // OK
            synthetic {"{"header2":""} req.http.X-JWT-Header {"","payload":""} req.http.X-JWT-Payload {"","sign":""} req.http.X-JWT-Signature {"","valid": ""} req.http.X-JWT-Valid-Signature {""}"};
           return(deliver);
        } else {
            // Forbidden
            synthetic {"{"header3":""} req.http.X-JWT-Header {"","payload":""} req.http.X-JWT-Payload {"","sign":""} req.http.X-JWT-Signature {"","valid": ""} req.http.X-JWT-Valid-Signature {""}"};
             return(deliver);
        }
    }

    return(lookup);
}

sub vcl_error {
    #FASTLY error

    // Generate JWT token
    if (obj.status == 901) {
        set obj.status = 200;
        set obj.response = "OK";
        set obj.http.Content-Type = "application/json";

        set obj.http.X-UUID = randomstr(8, "0123456789abcdef") "-" randomstr(4, "0123456789abcdef") "-4" randomstr(3, "0123456789abcdef") "-" randomstr(1, "89ab") randomstr(3, "0123456789abcdef") "-" randomstr(12, "0123456789abcdef");

        set obj.http.X-JWT-Issued = now.sec;
        set obj.http.X-JWT-Expires = strftime({"%s"}, time.add(now, 3600s));

        set obj.http.X-JWT-Header = digest.base64url_nopad({"{"alg":"HS256","typ":"JWT""}{"}"});
        set obj.http.X-JWT-Payload = digest.base64url_nopad({"{"sub":""} obj.http.X-UUID {"","exp":"} obj.http.X-JWT-Expires {","iat":"} obj.http.X-JWT-Issued {","iss":"Fastly""}{"}"});
        set obj.http.X-JWT-Signature = digest.base64url_nopad(digest.hmac_sha256("SupSecretStr", obj.http.X-JWT-Header "." obj.http.X-JWT-Payload));

        set obj.http.X-JWT = obj.http.X-JWT-Header "." obj.http.X-JWT-Payload "." obj.http.X-JWT-Signature;
        unset obj.http.X-UUID;
        unset obj.http.X-JWT-Issued;
        unset obj.http.X-JWT-Expires;
        unset obj.http.X-JWT-Header;
        unset obj.http.X-JWT-payload;
        unset obj.http.X-JWT-Signature;
        synthetic {"{"payload":""} obj.http.X-JWT-Payload {"","header":""} obj.http.X-JWT-Header {"","sign":""} obj.http.X-JWT-Signatre {"","token": ""} obj.http.X-JWT {""}"};
        return(deliver);
    }

    // Valid token
    if (obj.status == 902) {
        set obj.status = 200;
        set obj.response = "OK";
        set obj.http.Content-Type = "application/json";

        synthetic {"{ "token": ""} req.http.X-JWT {"" }"};
        return(deliver);
    }

}

Now, when I am trying to compile this it returns -

Syntax error: Unterminated _short-string_
at: (input Line 106 Pos 197)
               synthetic {"{"sign":""} req.http.X-JWT-Signature {"","header":""} req.http.X-JWT-Header {"","payload":""} req.http.X-JWT-Payload {"","valid": ""} req.http.X-JWT-Valid-Signature {""}"};

Looks like i am not somehow escaping the values correctly here during the synthetic block.

The only reason I am trying to do add this synthetic block in the vcl_recv subroutine is because I want to test how the digest is generating the JWT token and validating it and with that I wanted to create similar JWT tokens on server side in Node.Js so i was trying to output the different intermediate parts of the token for debugging.

I am not quite familiar with the Varnish syntax and semantics but still I looked for help finding any docs regarding this schedule subroutine but found none so far.

So, can anyone help out in how to fix this and have the vcl_recv, vcl_error interpolate different intermediate values in the json response.

I tried to use some of the Node.Js based base64 url decode libraries to decode the returned token parts and was able to decode the header and payload part but the signature part I am not able to generate from the Node.Js. So, can anyone suggest what is the equivalent of base64url_nopad() in node.js or any javascript libraries ?

For the hmac_256 encryption part we are trying to use the crypto library and creating an hmac like crypto.createHmac('sha256', 'SupSecretStr').update().digest('hex'); but all base64 encode url libraries in js i think return padded urls which is why the base64 encoded part of this hmac 256 digest doesn't match the one generated with varnish

Answer 1

My syntax coloring tool is telling me pretty much the same thing as the error message: you screwed up your quotes :-)

Your last block {""}"}; is opening quotes ({"), immediately closing them ("}), then you are opening simple quotes " and the newline arrives before you close them.

To fix, just put a space between after the final quote of the json: {"" }"};