This blog contains experience gained over the years of implementing (and de-implementing) large scale IT applications/software.

Tag: CORS

Cookies, SAP Analytics Cloud and CORS in Netweaver & HANA

Back in 2019 (now designated as 2019AC – Anno-Covid19), I wrote a post explaining in simple terms what CORS is and how it can affect a SAP landscape.
In that post I showed a simple “on-premise” setup using Fiori, a back-end system and how a Web Dispatcher can help alleviate CORS issues without needing too much complexity.
This post is about a recent CORS related issue that impacts access to back-end SAP data repositories.

Back To The Future

If we hit the “Fast-Forward” button to 2020MC (Mid-Covid19), CORS is now an extremely important technical setup to enable Web Browser based user interfaces to be served from Internet based SAP SaaS services (like SAP Analytics Cloud) and communicate with back-end on-premise/private data sources such as SAP BW systems or SAP HANA databases.

We see that CORS is going to become ever more important going forward, since Web Browser based user interfaces will become more abundant (due to the increase of SaaS products) for the types of back-end data access. The old world of installing a software application on-premise takes too much time and effort to keep up with changing technology.
Using SaaS applications as user interfaces to on-premise data allows a far more agile delivery of user functionality.

The next generation of Web Interfaces will be capable of processing ever larger data sets, with richer capabilities and more in-built intelligence. We’re talking about the Web Browser being a central hub of cross-connected Web Based services.
Imagine, one “web application” that needs a connection to a SaaS product that provides the analytical interface and version management, a connection to one or more back-end data repositories, a connection to a separate SaaS product for AI data analysis and pattern matching (deep insights), a connection to a separate SaaS product for content management (publishing), a connection to a separate SaaS product for marketing and customer engagement.

All of that, from one central web “origin” will mean CORS will become critical to prevent unwanted connections and data leaks. The Web Browser is already the target of many cyber security exploits, therefore staying secure is extremely important, but security is always at the expense of functionality.

IETF Is On It

The Internet Engineering Task Force already have this in hand. That’s how we have CORS in the first place (tools.ietf.org/html/rfc6454).
The Web Origin Concept is constantly evolving to provide features for useability and also security. Way back in 2016 an update to RFC 6265 was proposed, to enhance the HTTP state management mechanism, which is commonly known to you and I as “cookies”.

This amendment (the RFC details are here: tools.ietf.org/html/draft-ietf-httpbis-cookie-same-site-00) was the SameSite attribute that can be set for cookies.
Even in this RFC, you can see that it actually attributes the idea of “samedomain-cookies” back to Mozilla, in 2011. So this is not really a “new” security feature, it’s a long time coming!

The Deal With SAC

The “problem” that has brought me back around to CORS, is recent experience with a CORS issue and SAP Analytics Cloud (SAC).
The issue led me to a blog post by Dong Pan of SAP Canada in Feb 2020 and a recent blog post by Ian Henry, also of SAP in Aug 2020.

Dong Pan wrote quite a long technical blog post on how to fix or work-around the full introduction of the SameSite cookie attribute in Google Chrome version 80 when using SAP Analytics Cloud (SAC).

Ian Henry’s post is also based on the same set of solutions that Dong Pan wrote about, but his issue was accessing a backend HANA XS Engine via Web Dispatcher.

The problem in both cases is that SAP Analytics Cloud (SAC) uses the Web Browser as a middleman to create a “Live Connection” back to an “on-premise” data repository (such as SAP BW or SAP S/4HANA), but the back-end SAP Netweaver/SAP ABAP Platform stack/HANA XS engine, that hosts the “on-premise” data repository does not apply the “SameSite” attribute to cookies that it creates.

You can read Dong Pan’s blog post here: www.sapanalytics.cloud/direct-live-connections-in-sap-analytics-cloud-and-samesite-cookies/
You can read Ian Henry’s blog post here: https://blogs.sap.com/2020/08/26/how-to-fix-google-chrome-samesite-cookie-issue-with-sac-and-hana/

By not applying the “SameSite” attribute to the cookie, Google Chrome browsers of version 80+ will not allow SAC to establish a full session to the back-end system.
You will see an HTTP 400 “session expired” error when viewing the HTTP browser traffic, because SAC tries to establish the connection to the back-end, but no back-end system cookies are allowed to be visible to SAC. Therefore SAC thinks you have no session to the back-end.

How to See the Problem

You will need to be proficient at tracing HTTP requests to be able to capture the problem, but it looks like the following in the HTTP response from the back-end system:

You will see (in Google Chrome) two yellow warning triangles on the “set-cookie” headers in the response from the back-end during the call to “GetServerInfo” to establish the actual connection.
The call is the GET for URL “/sap/bw/ina/GetServerInfo?sap-client=xxx&sap-language=EN&sap-sessionviaurl=X“, with the sap-sessionviaurl in the query-string being the key part.
The text when you hover over the yellow triangle is: “This Set-Cookie didn’t specify a “SameSite” attribute and was defaulted to “SameSite=Lax,” and was blocked because it came from a cross-site response which was not the response to a top-level navigation. The Set-Cookie had to have been set with “SameSite=None” to enable cross-site usage.“.

The Fix(es)

SAP Netweaver (or SAP ABAP Platform) needs some code fixes to add the required cookie attribute “SameSite”.

A workaround (it is a workaround) is possible by using the rewrite module capability of the Internet Communication Management (ICM) or using a rewrite rule in a Web Dispatcher, to re-write the responses and include a generic “SameSite” attribute on each cookie.
This is a workaround for a reason, because using the rewrite method causes unnecessary extra work in the ICM (or Web Dispatcher) for every request (matched or not matched) by the rewrite engine.

It’s always better (more secure, more efficient) to apply the code fix to Netweaver (or ABAP Platform) so the “SameSite” attribute is added at the point of the cookie creation.
For HANA XS, it will need a patch to be applied (if it ever gets fixed in the XS since it is soon deprecated).
With the workaround, we are forcing a setting onto cookies outside of the creation process of those cookies.

Don’t get me wrong, I’m not saying that the workaround should not be used. In some cases it will be the only way to fix this problem in some older SAP systems. I’m just pointing out that there are consequences and it’s not ideal.

Dong Pan and Ian Henry have done a good job of providing options for fixing this in a way that should work for 99% of cases.

Is There a Pretty Picture?

This is something I always find useful when I try and work something through in my mind.
I’ve adjusted my original CORS diagram to include an overview of how I think this “SameSite” attribute issue can be imagined.
Hopefully it will help.

We see the following architecture setup with SAC and it’s domain “sapanalytics.cloud”, issuing CORS requests to back-end system BE2, which sits in domain “corp.net”:

Using the above picture for reference, we can now show where the “SameSite” issue occurs in the processing of the “Resource Response” when it comes back to the browser from the BE2 back-end system:

The blocking, by the Chrome Web browser, of the cookies set by the back-end system in domain “corp.net”, means that from the point of view of SAC, no session was established.
There are a couple more “Request”, “Response” exchanges, before the usual HTTP Authorization header is sent from SAC, but at that point it’s really too late as the returned SAP SSO cookie will also be blocked.

At this point you could see a number of different error messages in SAC, but in the Chrome debugging you will see no HTTP errors because the actual HTTP request/response mechanism is working and HTTP content is being returned. It’s just that SAC will know it does not have a session established, because it will not be finding the usual cookies that it would expect from a successfully established session.

Hopefully I’ve helped explain what was already a highly technical topic, in a more visual way and helped convey the problem and the solution.


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CORS in a SAP Netweaver Landscape

In this brief article I’m going to try to simplify and articulate what Cross-Origin Resource Sharing (CORS) is, how it works and how in an SAP environment (we use Fiori in our example) we can get around CORS without the complexity of rigidly defining the resource associations in the landscape.

Let’s Look At What CORS Is:

Fundamentally CORS is a protection measure introduced in around 2014 inside Web browsers, to try and prevent in-browser content manipulation issues associated with JavaScipt accessing resources from other websites without the knowledge/consent of the Web browser user.

You may be thinking “Why is this a problem?”, well, it’s complex, but a simple example is that you access content on one Web server, which uses JavaScript to access content on another Web server.  You have no control over where the JavaScript is going and what it is doing.
It doesn’t mean the other Web server in our example, is malicious, it could actually be the intended victim of malicious JavaScript being executed in the context of the source Web server.

What Does “Consent” Mean?

There is no actual consent given by the Web browser user (you). You do not get asked.

It is more of an understanding, built into the Web browser which means the Web browser knows where a piece of JavaScript has been downloaded from (its origin), versus where it is trying to access content from (its target), and causes the Web browser to seek consent from the target Web server before allowing the JavaScript to make its resource request to the target.

A simple analogy:
Your parents are the Web browser.
You (the child) are the untrusted JavaScript downloaded from the source Web server.
You want to go and play at your friend’s house (the target Web server).
Your parents contact your friend’s parents to confirm it’s OK.
Your parents obtain consent for you to go and play and the type of play you will be allowed to perform, before they let you go and play at your friend’s house.

Based on the simple analogy, you can see that the Web browser is not verifying the content on the target, neither is it validating the authenticity of the target (apart from the TLS level verification if using HTTPS).
All the Web browser is doing, is recognising that the origin of the JavaScript is different to its target, and requesting consent from the target, before it lets the JavaScript make it’s resource request.

If the target Web server does not allow the request, then the Web browser will reject the JavaScript request and an error is seen in the Web browser JavaScript debugger/console.

What Does “Accessing” Mean?

When we talk about JavaScript accessing resources on the target Web server, we are saying that it is performing an HTTP call (XML HTTP), usually via the AJAX libraries using one of a range of allowed methods. These methods are the usual HTTP methods such as GET, PUT, POST, HEAD etc.

What is the flow of communication between origin Web server, Web browser and target Web server?

Below I have included a diagram that depicts the flow of communication from a user’s Web browser, between a Fiori Front-End Server (FE1) and a Back-End SAP system (BE2).

In the example, pay attention to the fact that the domain (the DNS domain) of the FE1 and BE2 SAP systems, are different.

So, for example the FE1 server could be fe1.group.corp.net and the BE2 server could be be2.sub.corp.net.

1, The user of the Web browser navigates within Fiori to a tile which will load and execute a JavaScript script from FE1.

2, The JavaScript contains a call to obtain (HTTP PUT) a piece of information into the BE2 system via an XML HTTP Request (XHR) call inside the JavaScript.

3, The user’s Web browser detects the JavaScript’s intention and sends a pre-flight HTTP request to the BE2 system, including the details about the origin of the JavaScript and the HTTP method it would like to perform.

4, The BE2 system responds with an “allow” response (if it wishes to allow the JavaScript’s request).

5, The Web browser permits the JavaScript to make its request and it sends it’s HTTP request to BE2.

What Needs to Be Configured in BE2?

For the above situation to work, the BE2 system needs to be configured to permit the required HTTP methods from JavaScript on the origin FE1.

This means that a light level of trust needs to be added to configuration of BE2. This is documented in SAP notes and help.sap.com for NW 7.40 onwards.

Is There a Simpler Way?

An alternative method to configuring Netweaver itself, is to adjust the ICM on the target (BE2) to rewrite the inbound HTTP request to add a generic “origin” request. This means you can have many domains making the access request, without needing to maintain too much configuration at the cost of security.
I’m thinking more about what needs to be done, not just in production, but it in all DEV, TST and PrePRD systems, plus config re-work after system copies.
Not only this, but it would be difficult for your URL rewrite to be accurate, so it may end up being applied to all URL accesses, no matter where they come from.  This will impact performance of the Web Dispatcher.
You could solve the performance issue by using a different front-end IP address (service name) for your Web Dispatcher, which is used specifically for requests from your origin system (FE1).  Another option could be (if it’s your own code being called in BE2) to apply a URL path designation e.g. “/mystuff/therealstuff”, whereby the ICM on BE2 can match based on “/mystuff” and rewrite the URL to be “/therealstuff”.

How About an Even Simpler Way?

A much better way, which solves the CORS problem altogether and removes the need to place config on individual systems, is to front both the origin and the target behind the same Web Dispatcher.

This way, CORS becomes irrelevant as the domain of the Web Dispatcher is seen by the Web browser, as both the origin and the target.

To enable the above configuration, we need to ensure that we align the Web Dispatcher DNS domain to either the origin or the target.
It has to be aligned to whichever system we use the message server to load balance the HTTP call. This is a SAP requirement.

For the other back-end server (behind the Web Dispatcher), we use the EXTSRV option of the Web Dispatcher to allow it to talk to the BE2 system.
This has the capability of supplying multiple servers for HA and load balancing (round-robin).   It also means the DNS domain of that system can be different to that of the Web Dispatcher’s.

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