CVE-2023-41337 in h2o
Summary
by MITRE • 12/12/2023
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. In version 2.3.0-beta2 and prior, when h2o is configured to listen to multiple addresses or ports with each of them using different backend servers managed by multiple entities, a malicious backend entity that also has the opportunity to observe or inject packets exchanged between the client and h2o may misdirect HTTPS requests going to other backends and observe the contents of that HTTPS request being sent.
The attack involves a victim client trying to resume a TLS connection and an attacker redirecting the packets to a different address or port than that intended by the client. The attacker must already have been configured by the administrator of h2o to act as a backend to one of the addresses or ports that the h2o instance listens to. Session IDs and tickets generated by h2o are not bound to information specific to the server address, port, or the X.509 certificate, and therefore it is possible for an attacker to force the victim connection to wrongfully resume against a different server address or port on which the same h2o instance is listening.
Once a TLS session is misdirected to resume to a server address / port that is configured to use an attacker-controlled server as the backend, depending on the configuration, HTTPS requests from the victim client may be forwarded to the attacker's server.
An H2O instance is vulnerable to this attack only if the instance is configured to listen to different addresses or ports using the listen directive at the host level and the instance is configured to connect to backend servers managed by multiple entities.
A patch is available at commit 35760540337a47e5150da0f4a66a609fad2ef0ab. As a workaround, one may stop using using host-level listen directives in favor of global-level ones.
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Analysis
by VulDB Data Team • 01/04/2024
The vulnerability described in CVE-2023-41337 affects the h2o HTTP server implementation, specifically targeting configurations that utilize multiple listening addresses or ports with distinct backend server configurations managed by separate entities. This security flaw represents a sophisticated session hijacking attack vector that exploits the absence of address-specific binding in TLS session resumption mechanisms. The vulnerability stems from h2o's failure to properly associate session identifiers with specific server contexts, creating opportunities for malicious actors to manipulate connection routing and potentially intercept sensitive data. This issue manifests when a client attempts to resume a TLS session and an attacker redirects packets to an alternative address or port within the same h2o instance, effectively bypassing intended security boundaries.
The technical exploitation of this vulnerability requires an attacker to first gain legitimate backend access to one of the configured listening endpoints within the h2o instance. This attacker can then manipulate TLS session resumption by leveraging the fact that h2o's session identifiers are not bound to specific server addresses, ports, or X.509 certificates. The attack exploits fundamental weaknesses in session management where session IDs and tickets generated by h2o lack contextual binding to the specific server configuration that originally established the session. This architectural flaw enables attackers to force clients into misdirected TLS resumption attempts against different backend servers, potentially leading to data interception and unauthorized access to sensitive information. The vulnerability is particularly concerning because it operates at the TLS layer, making it difficult to detect through traditional network monitoring approaches.
The operational impact of CVE-2023-41337 extends beyond simple data interception to potentially enable complete man-in-the-middle attacks against specific client-server communications. When a victim client attempts to resume a TLS session, an attacker can redirect the connection to a different backend server that they control, potentially exposing sensitive HTTPS requests to unauthorized parties. This vulnerability affects organizations that rely on h2o's multi-tenant capabilities where different teams or entities manage separate backend services, creating legitimate attack vectors for malicious actors who have been granted backend access to one of the configured endpoints. The attack scenario becomes particularly dangerous in environments where sensitive data flows through the h2o proxy, as attackers could potentially access confidential information from other tenants within the same h2o instance.
This vulnerability aligns with CWE-310, which addresses cryptographic issues related to session management and key handling, and represents a specific implementation weakness in TLS session resumption protocols. From an ATT&CK framework perspective, this vulnerability maps to techniques involving session hijacking and credential exposure, with potential for lateral movement and privilege escalation. The attack requires initial access to one of the backend configurations but does not require additional authentication for the target systems, making it particularly dangerous in multi-tenant environments. The fix implemented in commit 35760540337a47e5150da0f4a66a609fad2ef0ab addresses the core issue by ensuring that session identifiers are properly bound to specific server contexts, preventing the cross-tenant session resumption attacks that were previously possible.
Organizations affected by this vulnerability should immediately implement the available patch to ensure proper binding of TLS session identifiers to specific server addresses and ports. As a temporary mitigation strategy, administrators should consider migrating from host-level listen directives to global-level configurations, which eliminates the specific conditions that enable this attack vector. The workaround approach effectively reduces the attack surface by preventing the multi-address listening configurations that make this vulnerability exploitable. Security teams should also conduct comprehensive audits of their h2o configurations to identify any instances that may be vulnerable to similar session management issues, particularly in environments where multiple backend entities are managed within a single h2o instance. Continuous monitoring for unauthorized backend access and proper access control enforcement will help prevent exploitation of this vulnerability in environments where patching may take time.