CVE-2020-13661 Overview
CVE-2020-13661 is an arbitrary program execution vulnerability in Telerik Fiddler through version 5.0.20202.18177. This vulnerability allows attackers to execute arbitrary programs on a victim's system by crafting a malicious hostname containing a trailing space character followed by specific command-line arguments. The attack requires user interaction, specifically that the victim must select the "Open On Browser" option when viewing the malicious hostname.
Critical Impact
Attackers can achieve arbitrary program execution on the victim's machine by exploiting improper input validation in hostname handling, potentially leading to full system compromise.
Affected Products
- Telerik Fiddler versions through 5.0.20202.18177
Discovery Timeline
- 2020-11-05 - CVE-2020-13661 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-13661
Vulnerability Analysis
This vulnerability exists due to improper input validation when Telerik Fiddler processes hostnames for the "Open On Browser" functionality. The application fails to properly sanitize hostname inputs before passing them to the underlying system shell for browser invocation. This allows an attacker to inject arbitrary command-line arguments that can be used to execute locally installed programs.
The attack exploits a parsing flaw where a trailing space in the hostname allows additional parameters to be appended. These parameters include --utility-and-browser and --utility-cmd-prefix= flags, which can specify the path to any locally installed executable program.
Root Cause
The root cause of this vulnerability is insufficient input validation and improper handling of whitespace characters in hostname strings. When the application constructs the command to open a URL in the browser, it does not properly escape or validate the hostname parameter, allowing command injection through carefully crafted input. The trailing space character acts as a delimiter that enables the injection of additional command-line arguments that are then processed by the system.
Attack Vector
The attack is network-based and requires user interaction to succeed. An attacker must craft a malicious URL or HTTP response containing a specially formatted hostname. The attack sequence involves:
The malicious hostname structure follows this pattern: a legitimate-looking hostname, followed by a trailing space character, then the --utility-and-browser flag, the --utility-cmd-prefix= parameter, and finally the full path to a locally installed program that the attacker wishes to execute.
When the victim intercepts this traffic in Fiddler and chooses to use the "Open On Browser" feature, the application passes the entire malformed string to the system, resulting in the execution of the specified program with the attacker's controlled parameters.
Detection Methods for CVE-2020-13661
Indicators of Compromise
- Unusual process spawning from Fiddler with unexpected command-line arguments
- Network traffic containing hostnames with trailing spaces followed by --utility-and-browser or --utility-cmd-prefix= parameters
- Unexpected program executions that can be traced back to Fiddler as the parent process
Detection Strategies
- Monitor for process creation events where Fiddler is the parent process and the child process is unexpected
- Implement network inspection rules to detect malformed hostnames containing command injection patterns
- Review Fiddler logs for suspicious hostname entries with trailing whitespace characters
Monitoring Recommendations
- Enable comprehensive endpoint detection and response (EDR) monitoring on systems where Fiddler is installed
- Configure SentinelOne behavioral AI to detect anomalous process execution chains originating from Fiddler
- Establish baseline behavior for Fiddler usage and alert on deviations, particularly unexpected child process spawning
How to Mitigate CVE-2020-13661
Immediate Actions Required
- Upgrade Telerik Fiddler to version 5.0.20204 or later immediately
- Restrict Fiddler usage to trusted network environments until the patch can be applied
- Educate users about the risks of using the "Open On Browser" feature with untrusted content
Patch Information
Telerik has addressed this vulnerability in Fiddler version 5.0.20204. Organizations should update to this version or later to remediate the vulnerability. Detailed release information is available from Telerik Fiddler Release History v5.0.
Workarounds
- Avoid using the "Open On Browser" functionality when inspecting traffic from untrusted sources until the update is applied
- Implement network-level filtering to detect and block requests containing suspicious hostname patterns with trailing spaces and command injection sequences
- Consider using application whitelisting to prevent unauthorized program execution even if the vulnerability is exploited
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
