In this excerpt of a Trend Micro Vulnerability Research Service vulnerability report, Guy Lederfein and Yazhi Wang of the Trend Micro Research Team detail a recent code injection bug in the Atlassian Confluence server. Since the publication of the vendor advisory, U.S. Cybercom has reported that mass exploitation of this bug for crypto mining is ongoing and expected to accelerate. The following is a portion of their write-up covering the root cause CVE-2021-26084, with a few minimal modifications.

An Object-Graph Navigation Language (OGNL) injection has been reported in the Webwork module of Atlassian Confluence Server and Data Center. The vulnerability is due to insufficient input validation leading to OGNL evaluation of user-supplied input.

A remote, unauthenticated attacker could exploit this vulnerability by sending a crafted request to the target server. Successful exploitation can result in arbitrary code execution in the security context of the affected server.

The Vulnerability

Atlassian Confluence is a collaboration platform written in Java. Users can create content using spaces, pages, and blogs that other users can comment on and edit. It is written primarily in Java and runs on a bundled Apache Tomcat application server. By default, Confluence is accessible via HTTP on port 8090/TCP.

HTTP is a request/response protocol described in RFCs 7230 - 7237 and other RFCs. A request is sent by a client to a server, which in turn sends a response back to the client. An HTTP request consists of a request line, various headers, an empty line, and an optional message body:

where CRLF represents the new line sequence Carriage Return (CR) followed by Line Feed (LF). SP represents a space character. Parameters can be passed from the client to the server as name-value pairs in either the Request-URI or in the message-body, depending on the Method used and the Content-Type header. For example, a simple HTTP request passing a parameter named “param” with value “1”, using the GET method might look like:

A corresponding HTTP request using the POST method might look as follows:

Confluence uses the Webwork web application framework to map URLs to Java classes, creating what is known as an “action”. Action URLs end with the “.action” suffix and are defined in the xwork.xml file in confluence- <version>.jar (where <version> is the Confluence version number) and in the atlassian-plugin.xml file in JAR files of included plugins. Each action entry contains at least a name attribute, defining the action name, a class attribute, defining the Java class implementing the action, and at least one result element which decides the Velocity template to render after the action is invoked based on the result of the action. Common return values from actions are “error”, “input”, and “success”, but any value may be used if there is a matching result element in the associated XWork XML. Action entries can contain a method attribute, which allows invocation of a specific method of the specified Java class. When no command is specified, the doDefault() method of the action class is called. The following is a sample action entry for the doenterpagevariables action:

In the above example, the doEnter() method of the com.atlassian.confluence.pages.actions.PageVariablesAction class handles requests to “doenterpagevariables.action” and will return values such as “success”, “input”, or “error”. This results in the appropriate Velocity template being rendered.

Confluence supports the use of Object Graph Navigational Language (OGNL) expressions to dynamically generate web page content from Velocity templates using the Webwork library. OGNL is a dynamic Expression Language (EL) with terse syntax for getting and setting properties of Java objects, list projections, lambda expressions, etc. OGNL expressions contain strings combined to form a navigation chain. The strings can be property names, method calls, array indices, and so on. OGNL expressions are evaluated against the initial, or root context object supplied to the evaluator in the form of OGNL Context.

Confluence uses a container object of class com.opensymphony.webwork.views.jsp.ui.template.TemplateRenderingContext to store objects needed to execute an Action. These objects include session identifiers, request parameters, spaceKey, etc. TemplateRenderingContext also contains a com.opensymphony.xwork.util.OgnlValueStack object to push and store objects against which dynamic Expression Languages (EL) are evaluated. When the EL compiler needs to resolve an expression, it searches down the stack starting with the latest object pushed into it. OGNL is the EL used by the Webwork library to render Velocity templates defined in Confluence, allowing access to Confluence objects exposed via the current context. For example, the $action variable returns the current Webwork action object.

OGNL expressions in Velocity templates are parsed using the ognl.OgnlParser.expression() method. The expression is parsed into a series of tokens based on the input string. The ognl.JavaCharStream.readChar() method, called by the OGNL parser, evaluates Unicode escape characters in the form of “\uXXXX” where “XXXX” is the hexadecimal code of the Unicode character represented. Therefore, if an expression includes the character “\u0027”, the character is evaluated as a closing quote character ('), escaping the context of evaluation as a string literal, allowing to append an arbitrary OGNL expression. If an OGNL expression is parsed in a Velocity template within single quotes and the expression’s value is obtained from user input without any sanitization, an arbitrary OGNL expression can be injected.

An OGNL injection vulnerability exists in Atlassian Confluence. The vulnerability is due to insufficient validation of user input used to set variables evaluated in Velocity templates within single quotes. By including the “\u0027” character in user input, an attacker can escape the string literal and append an arbitrary OGNL expression.

Before OGNL expressions are evaluated by Webwork, they are compared against a list of unsafe node types, property names, method names, and variables names in the com.opensymphony.webwork.util.SafeExpressionUtil.containsUnsafeExpression() method. However, this list is not exhaustive, and arbitrary Java objects can be instantiated without using any of the unsafe elements listed. For example, the following expression, executing an OS command, would be accepted as a safe expression by this method:

A remote attacker can exploit this vulnerability by sending a crafted HTTP request containing a malicious parameter to a vulnerable server. Successful exploitation can result in the execution of arbitrary code with the privileges of the server.

Remote Detection of Generic Attacks

To detect this attack, you should monitor all HTTP traffic requests where the path component of the request-URI contains one of the strings in the “URI path” column of the following table:

If such a request is found, you should inspect the HTTP request method. If the request method is POST, look for the respective vulnerable parameters from the table above in the body of the HTTP request, and if the request method is GET, you should look for the parameters in the request-URI of the HTTP request. Check to see if the value of any of the vulnerable parameters contains the string "\u0027" or its URL-encoded form. If so, the traffic should be considered malicious and an attack exploiting this vulnerability is likely underway.

Conclusion

Atlassian patched this vulnerability on August 25, 2021. Since the initial publication of their advisory, they have updated it multiple times to note the active attacks and to add clarification on how customers can identify if they are using Confluence Cloud. They do list some mitigations, but considering how widespread this bug is currently being exploited, the best course of action is to apply the patch and enhance system monitoring.

Special thanks to Guy Lederfein and Yazhi Wang of the Trend Micro Research Team for providing such a thorough analysis of this vulnerability. For an overview of Trend Micro Research services please visit http://go.trendmicro.com/tis/.

The threat research team will be back with other great vulnerability analysis reports in the future. Until then, follow the ZDI team for the latest in exploit techniques and security patches.