CVE-2021-39156 in Istio
Summary
by MITRE • 08/25/2021
Istio is an open source platform for providing a uniform way to integrate microservices, manage traffic flow across microservices, enforce policies and aggregate telemetry data. Istio 1.11.0, 1.10.3 and below, and 1.9.7 and below contain a remotely exploitable vulnerability where an HTTP request with `#fragment` in the path may bypass Istio’s URI path based authorization policies. Patches are available in Istio 1.11.1, Istio 1.10.4 and Istio 1.9.8. As a work around a Lua filter may be written to normalize the path.
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Analysis
by VulDB Data Team • 08/27/2021
The vulnerability identified as CVE-2021-39156 affects Istio service mesh platforms version 1.11.0 and below, 1.10.3 and below, as well as 1.9.7 and below, representing a critical authorization bypass flaw that undermines the security controls designed to protect microservices environments. This vulnerability specifically targets the URI path-based authorization policies that Istio implements to control access to services within the mesh, creating a potential entry point for malicious actors to gain unauthorized access to protected resources.
The technical flaw stems from how Istio processes HTTP requests containing URL fragments or hash components in the path portion of the URI. When an HTTP request includes a fragment identifier preceded by a hash symbol, the system fails to properly normalize or sanitize the path before applying authorization policies. This allows attackers to craft requests where the fragment portion of the URL bypasses the intended access controls, effectively rendering the path-based authorization mechanisms ineffective. The vulnerability operates at the application layer and can be exploited remotely without requiring authentication, making it particularly dangerous in production environments.
The operational impact of this vulnerability is significant for organizations relying on Istio for microservices security, as it enables attackers to bypass access controls that are fundamental to maintaining service isolation and data protection. Attackers can exploit this flaw to gain unauthorized access to services that should be restricted based on path patterns, potentially leading to data breaches, service disruption, or lateral movement within the microservices architecture. The vulnerability affects the core authorization functionality of Istio, which is essential for implementing zero-trust security models in distributed applications.
Organizations can address this vulnerability through the available patches in Istio versions 1.11.1, 1.10.4, and 1.9.8, which implement proper path normalization to prevent fragment-based bypasses. As a temporary mitigation, administrators can implement a Lua filter to normalize the request paths before they are processed by Istio's authorization policies. This workaround ensures that fragment components are properly handled and do not interfere with access control decisions. The vulnerability aligns with CWE-20 Improper Input Validation and maps to ATT&CK technique T1566.001 Phishing as it represents an authorization bypass that could enable further attacks within the compromised environment. Organizations should prioritize patching this vulnerability and review their existing authorization policies to ensure proper implementation of path-based access controls in their service mesh deployments.