CVE-2022-20729 describes a serious security flaw in the command-line interface (CLI) of Cisco’s Firepower Threat Defense (FTD) software. This vulnerability allows authenticated, local users to inject malicious XML into command parsing, leading to unanticipated command execution or output. Since Cisco FTD protects some of the most secure and high-traffic networks, it’s important to understand how this vulnerability works, the possible risks, and how you can protect your systems.

Background

This vulnerability is due to insufficient input validation in FTD’s command parser. Instead of properly sanitizing user-supplied data, the CLI allows pieces of XML to slip through as part of its input. If an attacker with CLI access knows the right structure, they can input specially crafted XML, potentially affecting how the device interprets commands.

According to the original Cisco advisory, this issue is tracked as CSCwa17147 and affects several versions of FTD software prior to the patched releases.

How the Exploit Works

Attackers must first authenticate locally on the device; this is not a remote exploit and cannot be run without at least basic CLI access. After login, instead of issuing simple commands, the attacker inputs a command with embedded XML fragments. If the command parser is vulnerable, this injected XML is processed as if it was trusted input.

Let’s break down how a simplified version of this exploit might look.

Suppose the FTD CLI command expects an input value like a name or identifier

ftd> set service name USER_INPUT

A normal administrator might type

ftd> set service name MyService

A malicious user, knowing about CVE-2022-20729, might try

ftd> set service name MyService<foo>malicious</foo>

Pseudo-code snippet illustrating what happens inside the parser

def handle_set_service_name(input_string):
    # unsafe: directly sends input to an XML parser
    # (the bug!) no validation or sanitization
    xml = "<command><name>" + input_string + "</name></command>"
    process_xml(xml)

With a malicious input, the resulting XML becomes

<command>
  <name>MyService<foo>malicious</foo></name>
</command>

If the parser does not escape, reject, or check this, the embedded <foo>malicious</foo> could unexpectedly alter how the command is processed. This might cause extra actions, change configuration, or produce weird output—potentially giving the attacker greater insight into the device’s internals, or access to unintended functions.

Cisco confirmed that this type of injection could result in

* Unexpected processing of the command: The injected XML could be interpreted by backend routines to perform extra steps.
* Unexpected command output: Information not normally accessible might be displayed, or nonstandard errors could reveal further weakness.

A simple proof-of-concept exploit (for education and legal research only!) would just try to inject markup and observe whether the system accepts and interprets it.

Proof-of-Concept (PoC) Command

ftd> set service name vulnerableTest<test>owned</test>

If the resulting output from FTD displays the nested XML or behaves strangely, the device is likely still at risk.

Cisco provided patches for all supported versions. All users should immediately

* Update to the latest FTD software version.
- Find patched releases and advisories at: Cisco Security Advisory for CVE-2022-20729
* Limit CLI access to the smallest number of trusted operators.
* Monitor logs for suspicious CLI input or XML error messages.
* Use network segmentation to restrict local CLI to secure terminals only.

References

- Cisco Security Advisory: FTD XML Injection (CVE-2022-20729)
- NIST NVD Entry for CVE-2022-20729
- Cisco Bug Tracker: CSCwa17147

Conclusion

*CVE-2022-20729* is a reminder that even powerful security appliances can be taken down by insecure input validation. If you use Cisco FTD, update your system now! Keep your team informed—this kind of bug is precisely what attackers look for after they get an initial foothold.

By understanding how these bugs work, network admins and security professionals can better safeguard their organizations from simple but impactful configuration and input errors.

Timeline

Published on: 05/03/2022 04:15:00 UTC
Last modified on: 05/12/2022 18:12:00 UTC