CVE-2019-9901 in Envoy
Summary
by MITRE
Envoy 1.9.0 and before does not normalize HTTP URL paths. A remote attacker may craft a relative path, e.g., something/../admin, to bypass access control, e.g., a block on /admin. A backend server could then interpret the non-normalized path and provide an attacker access beyond the scope provided for by the access control policy.
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Analysis
by VulDB Data Team • 09/07/2023
The vulnerability described in CVE-2019-9901 represents a critical path traversal issue within the Envoy proxy software version 1.9.0 and earlier. This flaw stems from the proxy's failure to properly normalize HTTP URL paths during request processing, creating a significant security gap that can be exploited by remote attackers to bypass access control mechanisms. The issue specifically affects how Envoy handles relative path references in HTTP requests, allowing malicious actors to craft URLs containing sequences like "../" that should normally be resolved to prevent directory traversal attacks.
The technical implementation of this vulnerability lies in the absence of proper path normalization within Envoy's HTTP request handling pipeline. When a client sends a request containing relative path components such as "something/../admin", the proxy fails to canonicalize this path before forwarding it to backend servers. This normalization process is essential for security as it ensures that path references are resolved to their absolute form, eliminating any potential for malicious path manipulation. Without this critical step, the proxy acts as a conduit for malformed paths that can bypass security policies configured at the proxy level.
The operational impact of CVE-2019-9901 extends beyond simple access control bypass to potentially enable unauthorized access to sensitive backend resources. When an attacker crafts a request with a non-normalized path, the backend server receives the malformed URL and may interpret it according to its own path resolution rules, which could differ from the proxy's security policies. This creates a dangerous scenario where an attacker can access administrative interfaces or restricted resources that should be blocked by the proxy's access control lists. The vulnerability essentially undermines the security boundary that the proxy is designed to maintain between clients and backend services.
This vulnerability aligns with CWE-22 Path Traversal and maps to attack patterns in the MITRE ATT&CK framework under the T1059.007 technique for command and scripting interpreter. The issue demonstrates a classic security misconfiguration where the proxy fails to implement proper input validation and sanitization for URL paths. Organizations using Envoy versions prior to 1.9.1 should prioritize immediate patching as this vulnerability can be exploited remotely without authentication, making it particularly dangerous in environments where the proxy serves as a primary security control. The fix implemented in version 1.9.1 involved adding proper path normalization to ensure that relative path components are resolved appropriately before requests are forwarded to backend services, thereby closing the security gap that allowed attackers to bypass access controls through crafted HTTP URLs.
The broader implications of this vulnerability highlight the critical importance of proper input sanitization in API gateways and reverse proxies. Security professionals should implement comprehensive testing procedures that include path traversal scenarios when evaluating proxy configurations. Additionally, organizations should consider implementing additional monitoring and logging of URL path patterns to detect potential exploitation attempts. The vulnerability serves as a reminder that even well-established security tools can contain critical flaws that require careful attention to path handling and normalization processes.