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Content Security Policy – How Hard Can It Be?

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Content Security Policy – How Hard Can It Be? Content Security Policy – How hard can it be? Michael Schneider Marc Ruef (Editor) Offense Department, scip AG Research Department, scip AG [email protected] [email protected] https://www.scip.ch https://www.scip.ch Abstract: The Content Security Policy (CSP) is an important method for protection of web applications. Correct implementation is highly challenging. Many developers and administrators therefore omit it entirely. However, smart implementation is possible – and advisable. Keywords: Area41, Block, Browser, Checkpoint, Firefox, Google, HTML, HTTP, Internet Explorer, Javascript 1. Preface browser is informed which web application resources will be loaded and processed, such as images, style sheets (CSS) This paper was written in 2016 as part of a research project or scripts. Here, CSP should be considered a component of at scip AG, Switzerland. It was initially published online at a defense-in-depth strategy rather than a replacement for https://www.scip.ch/en/?labs.20160818 and is available in input validation or coding of output. English and German. Providing our clients with innovative research for the information technology of the future is an The X-Content-Security-Policy header is supported by essential part of our company culture. Mozilla Firefox (since Version 4) and Microsoft Internet Explorer 10, but is already considered obsolete and should 2. Introduction no longer be used. The Content-Security-Policy header stands for CSP Level 1 and all further levels. It has been The Content Security Policy [1] (CSP) has been a separate supported since browser versions Google Chrome 25, checkpoint for our Web Application Penetration Tests [2] Mozilla Firefox 23, Safari 7 and Internet Explorer Edge. since September 2015. One of the first projects so tested is emblematic of the acceptance and implementation of CSP Through directives, the policy can be tailored to the in general. In that project, multiple cross-site scripting intended use of the web application. A directive is an (XSS) vulnerabilities were found. Along with strict input instruction to a web browser how to handle several resource validation, we also recommended introduction of a CSP types. The default-src directive is considered the standard header as an additional measure. The developers for all other directives where they are not specified. implemented this and defined a policy. But the script-src Directives exist for management of JavaScript, images, directive continued to permit execution of inline objects and style sheets (CSS). Each directive is defined by JavaScript. This effectively meant installation of a policy a source expression. Valid values include none, self, data: that offered no real protection against XSS attacks. In the and one or more hosts/domains. The Content Security end, the CSP header was taken out again. No additional Policy Reference [5] offers a good overview. resources were invested in a stricter policy definition or an amendment to the web application. In the case of the directive for JavaScript named script- src, it became apparent that the existing source expressions Introduction of an effective Content Security Policy is a were insufficient for real-life scenarios and thus inhibited demanding task, one that should be carried out in implementation of CSP. With Level 2 [6], published on 21 accordance with web development. This article describes July 2015, the attribute nonce was introduced to ensure how CSP came about and how it is used in practice. additional flexibility. The following section discusses its use – and shortcomings. 3. Emergence and development The frame-ancestors directive was also introduced with The first draft of the Content Security Policy Level 1 [3] the aim of replacing the HTTP header X-Frame-Options. was published by the W3C Consortium [4] on 15 November Here, the expression none equates to the value DENY in the 2012. Browser vendors initially implemented it as an HTTP header. Veit Hailperin already explained this header experimental function with the header X-Content- in his article Inglorious Headers [7]. For now we Security-Policy or X-WebKit-CSP. recommend a dual use strategy, set the policy and the The goal of CSP is to provide both web developers and web header for it self because not all browsers support this CSP level. server administrators with a tool for mitigation of injection vulnerabilities. When this kind of policy is defined, the web The nonce attribute has not caught on in practice. It was the as dependencies in the library itself or a script element is inclusion of JavaScript libraries from third-party sites that used, this leads to an error since the value for nonce is not proved particularly problematic. The introduction of the set there. The second case relates to parser-inserted [11] source expression strict-dynamic in the Working Draft [8] script elements. A simple example of a parser-inserted for the CSP Level 3 dated 21 June 2016 attempts to solve element looks like this: this. As a further change, the directive report-uri was renamed report-to. var scriptPath = “https://othercdn.not- example.net/dependency.js” document.write(“<script src=” + scriptPath + 4. Use in conjunction with JavaScript “></script>”); Where CSP has not previously been used, we recommend With the expression strict-dynamic, CSP Level 3 offers a starting with the following directive: solution for this problem and simplifies the use of external libraries. Using strict-dynamic in conjunction with nonce default-src “none”; script-src “self”; connect-src “self”; img-src “self”; style-src results in two changes, or effects: “self”; 1. The whitelists defined in the directives default- This will allow the use of images, styles (CSS) and src or script-src, as well as the use of unsafe- JavaScript from your own domain. When using JavaScript, inline, are rejected. however, this also means that only integrated script files are 2. Reloading of dependencies in integrated libraries executed. Any in-line instructions in script tags are not and script elements that are not parser-inserted are permitted. For web applications developed in this way, this now permitted. leads to the problem that the script element is processed directly in the HTML code. For practical purposes, this offers two benefits. The first effect permits a backwards-compatible rollout of strict- To avoid recoding the entire website, the directive is dynamic. The policy usually expanded to include the expression unsafe-inline. This means that the web application continues to function Content-Security-Policy: “unsafe-inline” https: “nonce-abcdefg” “strict-dynamic” as usual, but is once again vulnerable to reflected or stored XSS attacks. This removes the benefit, or protective effect, becomes "unsafe-inline" https: in browsers that support of the policy. CSP Level 1. The https: "nonce-abcdefg" part applies in the case of Level 2 support and "nonce-abcdefg" "strict- In many web applications, the use of JavaScript is not dynamic" applies in browsers that have implemented CSP restricted to the owner“s domain. External libraries are also Level 3. The second effect entrusts the integrated script integrated into the context of the owner“s page. This means with the integration of its own dependencies without the that a whitelist of such domains has to be maintained in the need to list them specifically in the whitelist. script-src directive. Maintenance is no easy matter and in a worst-case scenario can even lead to a CSP bypass. This One worthwhile presentation on CSP bypasses and the is vividly illustrated in solutions for the mini-challenge by introduction of strict-dynamic is the talk by Lukas Cure53 [9] entitled “H5SC Minichallenge 3: Sh*t, its CSP! Weichselbaum [12] and Michele Spagnuolo [13], both [10]. information security engineers at Google, entitled Breaking Bad CSP! [14] at this year“s Area41 2016 [15]. The slides With the introduction of the attribute nonce in CSP Level 2, [16] from the talk are also available. there is now the option of use of embedded script elements. The principle behind nonce is that a random nonce value is 5. Reporting recorded in the script-src directive. Ideally, this occurs through the web application itself. Every script element that The Content-Security-Policy header instructs the web sets this value in the nonce attribute is then authorized. This browser to enforce the defined directives. This can, and means that legitimate script elements are executed, but XSS experience shows that it does, lead to problems with the attacks are stopped. introduction of CSP or in the development phase of web applications. Therefore, a second CSP header called The use of libraries, including JavaScript CDNs like Content-Security-Policy-Report-Only reports jQuery, React.js and AngularJS, in conjunction with nonce infringements of the directive without blocking them. The leads to a new problem as described below. The nonce web browser sends a report to the URL defined in the attribute is defined as: directive report-uri or, in future, report-to. Content-Security-Policy: script-src “nonce- The report contains the following elements: i7bGtfs” document-uri – the page where the infringement A library is then integrated into the website and correctly occurred provided with the matching value in the nonce attribute: referrer – the referrer of the page <script blocked-uri – the base address of the blocked URI src=“https://cdn.example.com/script.js” violated_directive – the specific directive that nonce=“i7bGtfs”></script> was infringed original-policy – the complete CSP Because the correct value for nonce is set, the integration of the library is permitted. But if further libraries are reloaded These reports can be evaluated in respect to overly strict another layer in a secure web application. We would directives, infringements of the directive caused by certainly prefer to be able to analyze more CSP headers in programming errors or flaws in the web application, as well the future, instead of simply entering the CSP header is not as potential XSS attacks.
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