CVE-2019-18802 in Envoy
Summary
by MITRE
An issue was discovered in Envoy 1.12.0. An untrusted remote client may send an HTTP header (such as Host) with whitespace after the header content. Envoy will treat "header-value " as a different string from "header-value" so for example with the Host header "example.com " one could bypass "example.com" matchers.
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Analysis
by VulDB Data Team • 03/12/2024
The vulnerability identified as CVE-2019-18802 resides within the Envoy proxy software version 1.12.0, representing a critical security flaw that undermines the integrity of HTTP header validation mechanisms. This issue stems from Envoy's improper handling of whitespace characters within HTTP header values, specifically when such whitespace appears after legitimate header content. The flaw allows malicious actors to exploit the proxy's header parsing logic by crafting HTTP requests containing trailing whitespace in header fields such as Host, thereby enabling bypass of security controls that rely on exact string matching.
The technical root cause of this vulnerability aligns with CWE-184, which addresses incomplete input validation, and more specifically with CWE-20, incomplete input sanitization, as Envoy fails to properly normalize or strip whitespace characters from HTTP headers during processing. When an untrusted remote client sends an HTTP header with trailing whitespace, the proxy treats the header value as distinct from its sanitized counterpart, creating a vector for bypassing access control lists and other security mechanisms that depend on precise header value matching. This behavior violates the principle of least privilege and can be exploited to circumvent security policies configured within the proxy's configuration files.
The operational impact of this vulnerability extends beyond simple header parsing errors and represents a significant threat to network security infrastructure that relies on Envoy as a reverse proxy or service mesh component. Attackers can leverage this weakness to bypass host-based access controls, potentially gaining unauthorized access to backend services that should be restricted to specific domains or hosts. The vulnerability is particularly concerning in environments where Envoy is used as a gateway for microservices communication or as an edge proxy for public-facing applications, as it could allow attackers to enumerate or access services that are not intended to be publicly accessible.
The security implications of CVE-2019-18802 are further amplified when considering the ATT&CK framework's techniques for command and control communications and privilege escalation. This vulnerability enables adversaries to perform host header injection attacks, potentially allowing them to bypass authentication mechanisms or redirect traffic to malicious endpoints. The flaw can also be combined with other techniques such as HTTP request smuggling or cache poisoning, creating more sophisticated attack vectors. Organizations using Envoy in production environments should consider this vulnerability as part of their threat modeling, particularly in scenarios where the proxy is used to enforce security policies based on specific host header values or domain restrictions.
Mitigation strategies for this vulnerability require immediate patching of affected Envoy instances to versions that properly normalize whitespace characters in HTTP headers. Organizations should also implement additional security controls such as HTTP header sanitization at the network level, monitoring for anomalous header patterns, and regular security audits of proxy configurations. Configuration management practices should be enhanced to ensure that header validation rules are robust against whitespace variations, and security teams should monitor for potential exploitation attempts through log analysis and intrusion detection systems. The fix implemented in subsequent versions typically involves normalization of header values to remove trailing whitespace before processing, ensuring that "header-value " and "header-value" are treated as equivalent for security policy enforcement purposes.