CVE-2021-25762 in Ktor
Summary
by MITRE • 02/03/2021
In JetBrains Ktor before 1.4.3, HTTP Request Smuggling was possible.
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Analysis
by VulDB Data Team • 02/23/2021
The vulnerability identified as CVE-2021-25762 represents a critical HTTP request smuggling issue affecting JetBrains Ktor versions prior to 1.4.3. This flaw enables attackers to manipulate HTTP requests in ways that can bypass security controls and potentially gain unauthorized access to sensitive resources. The vulnerability stems from improper handling of HTTP headers and request parsing within the Ktor framework, creating opportunities for malicious actors to exploit the application layer communication protocols. HTTP request smuggling occurs when an attacker can inject or manipulate HTTP headers in a way that causes the server to interpret the same request differently than intended, leading to potential security breaches and data exposure.
The technical root cause of this vulnerability lies in the framework's insufficient validation and sanitization of HTTP request headers, particularly those related to content length and transfer encoding. When Ktor processes incoming HTTP requests, it fails to properly validate the consistency between different header fields that control how requests should be interpreted and processed. This inconsistency creates a window where attackers can craft malicious requests that appear legitimate to the client but are interpreted differently by the server, potentially allowing them to smuggle requests through intermediaries or bypass authentication mechanisms. The vulnerability specifically affects the way the framework handles header normalization and request parsing, which falls under CWE-16 - Improper Handling of Abnormal or Exceptional Conditions and CWE-444 - Inconsistent Interpretation of HTTP Requests.
The operational impact of this vulnerability extends beyond simple request manipulation, as it can enable attackers to perform various malicious activities including session hijacking, cross-site scripting attacks, and unauthorized data access. An attacker could exploit this vulnerability to bypass security controls implemented at the application level, potentially gaining access to restricted resources or sensitive information. The vulnerability is particularly concerning because it affects the core HTTP processing capabilities of the Ktor framework, meaning any application built using this framework and running vulnerable versions could be compromised. The attack vector typically involves crafting HTTP requests with malformed or conflicting headers that cause the server to process requests in unintended ways, creating opportunities for data leakage and privilege escalation.
Mitigation strategies for CVE-2021-25762 focus primarily on upgrading to JetBrains Ktor version 1.4.3 or later, which includes proper header validation and request parsing mechanisms. Organizations should also implement additional security controls such as HTTP header sanitization, request validation at multiple layers, and comprehensive monitoring of HTTP traffic for suspicious patterns. Network-level protections including web application firewalls and proxy servers can help detect and block malicious requests attempting to exploit this vulnerability. Security teams should conduct thorough vulnerability assessments to identify all applications using affected Ktor versions and ensure proper patching across the entire infrastructure. The remediation process should also include reviewing and updating security configurations to prevent similar issues in other components of the application stack, aligning with ATT&CK technique T1190 - Exploit Public-Facing Application and T1212 - Exploitation for Credential Access. Regular security testing and vulnerability scanning should be implemented to identify potential HTTP-related vulnerabilities before they can be exploited by threat actors.