Application Links Cross-site Scripting

Application Links before version 5.0.11, from version 5.1.0 before 5.2.10, from version 5.3.0 before 5.3.6, from version 5.4.0 before 5.4.12, and from version 6.0.0 before 6.0.4 allows remote attackers to inject arbitrary HTML or JavaScript via a cross site scripting (XSS) vulnerability in the applinkStartingUrl parameter. The product is used as a plugin in various Atlassian products where the following are affected: Confluence before version 6.15.2, Crucible before version 4.7.0, Crowd before version 3.4.3, Fisheye before version 4.7.0, Jira before version 7.13.3 and 8.x before 8.1.0. (2019-04-30, CVE-2018-20239)

Various administrative application link resources in Atlassian Application Links before version 5.4.4 allow remote attackers with administration rights to inject arbitrary HTML or JavaScript via a cross site scripting (XSS) vulnerability in the display url of a configured application link. (2018-04-10, CVE-2018-5227)

Cross site scripting is an attack where a web page executes code that is injected by an adversary. It usually appears, when users input is presented. This attack can be used to impersonate a user, take over control of the session, or even steal API keys.

The attack can be executed e.g. when you application injects the request parameter directly into the HTML code of the page returned to the user:

https://server.com/confirmation?message=Transaction+Complete

what results in:

<span>Confirmation: Transaction Complete</span>

In that case the message can be modified to become a valid Javascript code, e.g.:

https://server.com/confirmation?message=<script>dangerous javascript code here</script>

and it will be executed locally by the user's browser with full access to the user's personal application/browser data:

<span>Confirmation: <script>dangerous javascript code here</script></span>