Vulnerabilities > CVE-2021-1293 - External Control of Assumed-Immutable Web Parameter vulnerability in Cisco products

An attacker corrupts or modifies the content of XML schema information passing between client and server for the purpose of undermining the security of the target. XML Schemas provide the structure and content definitions for XML documents. Schema poisoning is the ability to manipulate a schema either by replacing or modifying it to compromise the programs that process documents that use this schema. Possible attacks are denial of service attacks by modifying the Schema so that it does not contain required information for subsequent processing. For example, the unaltered schema may require a @name attribute in all submitted documents. If the attacker removes this attribute from the schema then documents create using the new grammar will lack this field, which may cause the processing application to enter an unexpected state or record incomplete data. In addition, manipulation of the data types described in the schema may affect the results of calculations taken by the document reader. For example, a float field could be changed to an int field. Finally, the attacker may change the encoding defined in the schema for certain fields allowing the contents to bypass filters that scan for dangerous strings. For example, the modified schema might us a URL encoding instead of ASCII, and a filter that catches a semicolon (;) might fail to detect its URL encoding (%3B).

This attack relies on the use of HTTP Cookies to store credentials, state information and other critical data on client systems. The first form of this attack involves accessing HTTP Cookies to mine for potentially sensitive data contained therein. The second form of this attack involves intercepting this data as it is transmitted from client to server. This intercepted information is then used by the attacker to impersonate the remote user/session. The third form is when the cookie's content is modified by the attacker before it is sent back to the server. Here the attacker seeks to convince the target server to operate on this falsified information.

In circumstances where an application holds important data client-side in tokens (cookies, URLs, data files, and so forth) that data can be manipulated. If client or server-side application components reinterpret that data as authentication tokens or data (such as store item pricing or wallet information) then even opaquely manipulating that data may bear fruit for an Attacker. In this pattern an attacker undermines the assumption that client side tokens have been adequately protected from tampering through use of encryption or obfuscation.