October 1, 2020
Cross-site search (XS-Search) is an important attack principle in the family of XS-Leaks. This type of attack abuses Query-Based Search Systems to leak user information from an attacker origin 1 2. The original attack uses timing measurements to detect whether or not a search system returns results and works as follows:
Establish a baseline of the time needed for a request to return results (hit), and a baseline for the time needed by a request with no results (miss).
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October 8, 2020
If a page sets its opener property to null or is using COOP protection depending on the users’ state, it becomes possible to infer cross-site information about that state. For example, attackers can detect whether a user is logged in by opening an endpoint in an iframe (or a new window) which only authenticated users have access to, simply by checking its window reference. Run demo
Code Snippet # The below snippet demonstrates how to detect whether the opener property was set to null, or whether the COOP header is present with a value other than unsafe-none.
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October 1, 2020
Cross-Origin Read Blocking (CORB) is a web platform security feature aimed at reducing the impact of speculative side-channel attacks such as Spectre. Unfortunately, blocking certain types of requests introduced a new type of XS-Leaks 1 that allows attackers to detect if CORB was enforced on one request, but wasn’t on another. Nevertheless, the introduced XS-Leaks are much less problematic than the issues actively protected by CORB (e.g. Spectre).
info
This is a known issue in Chromium, and while it might remain unfixed, its impact is greatly reduced by the rollout of SameSite Cookies by default in Chromium-based browsers.
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October 1, 2020
Explanation # Cross-Origin Resource Policy (CORP) is a web platform security feature that allows websites to prevent certain resources from being loaded by other origins. This protection complements CORB since it is an opt-in defense, whereas CORB blocks some cross-origin reads by default. Unfortunately, similar to CORB, applications can introduce a new XS-Leak if they misconfigure the use of this protection.
A webpage will introduce an XS-Leak if CORP is enforced based on user data.
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October 1, 2020
When a webpage issues a request to a server (e.g. fetch, HTML tags), the server receives and processes this request. When received, the server decides whether the request should succeed (e.g. 200) or fail (e.g. 404) based on the provided context. When a response has an error status, an error event is fired by the browser for the page to handle. These errors also cover situations where the parser fails, for example when trying to embed HTML content as an image.
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October 1, 2020
Window references allow cross-origin pages to get access to some of the attributes of other pages. These references become available when using or allowing iframe and window.open. The references provide (limited) information about the window as they still respect the same-origin policy.
One of the accessible attributes is window.length which provides the number of frames in the window. This attribute can provide valuable information about a page to an attacker.
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October 1, 2020
Detecting if a cross-site page triggered a navigation (or didn’t) can be useful to an attacker. For example, a website may trigger a navigation in a certain endpoint depending on the status of the user.
To detect if any kind of navigation occurred, an attacker can:
Use an iframe and count the number of times the onload event is triggered. Check the value of history.length, which is accessible through any window reference.
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October 1, 2020
Network Timing side-channels have been present on the web since its inception 1 2. These attacks have had different levels of impact over time, gaining new attention when browsers started shipping high-precision timers like performance.now().
To obtain timing measurements, attackers must use a clock, either an implicit or an explicit one. These clocks are usually interchangeable for the purposes of XS-Leaks and only vary in accuracy and availability. For simplicity, this article assumes the use of the performance.
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October 1, 2020
Performance API # The Performance API provides access to performance-related information enhanced by the data from the Resource Timing API which provides the timings of network requests such as the duration but when there’s a Timing-Allow-Origin: * header sent by the server the transfer size and domain lookup time is also provided.
This data can be accessed by using performance.getEntries or performance.getEntriesByName It can also be used to get the execution time using the difference of performance.
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The principle of Cache Probing consists of detecting whether a resource was cached by the browser. The concept has been known since the beginning of the web 1 and initially relied on detecting timing differences.
When a user visits a website, some resources such as images, scripts, and HTML content are fetched and later cached by the browser (under certain conditions). This optimization makes future navigations faster as the browser serves those resources from disk instead of requesting them again.
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