This describes network services provided by OpenBMC-based systems, some threats the BMC faces from its network interfaces, and steps OpenBMC takes to address these threats.
This is only intended to be a guide; security is ultimately the responsibility of projects which choose to incorporate OpenBMC into their project. If you find a security vulnerability, please consider how to report a security vulnerability.
Threats to the BMC are classified using the CIA triad. All threat types are significant; here is an example of each:
This document is organized by how OpenBMC services connect to the network. The general flow is:
OpenBMC provides services on TCP and UDP ports. For example, the HTTPS protocol on port 443 is used to provide REST APIs and serve Web applications. These services are detailed below. Implicit is that all other ports are inactive.
OpenBMC also initiates network communications, for example, NTP, LDAP, etc. These are covered with their associated functions.
Network traffic is handled by the Linux kernel. The exact kernel and device driver have security considerations which are important to BMC security, but are better addressed by the Linux kernel community. You can learn which kernel and patches are used from the kernel recipes typically found in the board support packages for the BMC referenced by your machine's configuration. For example, in the https://github.com/openbmc/meta-aspeed repository under recipes-kernel/linux/linux-aspeed_git.bb.
Per CVE 1999-0524, responding to certain ICMP packets can give an attacker more information about the BMC's clock or subnet, which can help with subsequent attacks. OpenBMC responds to all ICMP requests.
General considerations for ICMP messages apply. For example, packet fragmentation and packet flooding vulnerabilities.
It is sometimes useful to filter and log network messages for debug and other diagnostic purposes. OpenBMC provides no support for this.
Several services perform user identification and authentication:
OpenBMC's phosphor-user-manager provides the underlying authentication and authorization functions and ties into IPMI, Linux PAM, LDAP, and logging. Some of OpenBMC services use phosphor-user-manager.
Transport layer security (TLS) protocols are configured for each service at compile time, become part of the image, and cannot be changed dynamically. The protocols which use TLS include:
Automated network agents (such as hardware management consoles) may malfunction in a way that the BMC continuously gets authentication failures, which may lead to denial of service. For example, a brief delay before reporting the failure, for example, of one second, may help prevent this problem or lessen its severity. See OWASP Blocking Brute Force Attacks.
Network agents may fail to end a session properly, which causes the service to use resources to keep track of orphaned sessions. To help prevent this, services may limit the maximum number of concurrent sessions, or have a session inactivity timeout.
Services which are not required should be disabled to limit the BMC's attack surface. For example, a large scale data center may not need a Web interface. Services can be disabled in several ways:
Network services should log all authentication attempts with their outcomes to satisfy basic monitoring and forensic analysis requirements. For example, as part of a real-time monitoring service, or to answer who accessed which services at what times.
OpenBMC does not have a firewall.
Laws may require products built on OpenBMC to have reasonable security built into them, for example, by not having a default password. See, for example, CA Law SB-327.
The Secure Shell (SSH) protocol is provided, including secure shell (ssh command) access to the BMC's shell, and secure copy (scp command) to the BMC's file system.
The default SSH server implementation is provided by Dropbear. All configuration is at compile-time with defaults for:
ssh-keygen command).SSH access to the BMC's shell is not the intended way to operate the BMC, gives the operator more privilege than is needed, and may not be allowed on BMCs which service hosts that process sensitive data. However, BMC shell access may be needed to provision the BMC or to help diagnose problems during its operation.
BMCWeb is the Web server for:
The BMCWeb configuration controls which services are provided.
General security considerations for HTTP servers apply such as given by OWASP Application Security.
BMCWeb controls which HTTPS transport layer security (TLS) ciphers it offers via compile-time header file include/ssl_key_handler.hpp in the https://github.com/openbmc/bmcweb repository. The implementation is provided by OpenSSL.
BMCWeb provides appropriate HTTP response headers, for example, in header file include/security_headers_middleware.hpp and crow/include/crow/websocket.h in the https://github.com/openbmc/bmcweb repository.
BMCWeb offers three authentication methods:
The username and password are presented to phosphor-user-manager for authentication.
The first two methods create the same kind of session but return different credentials. For example, you can create a Redfish session, and use your credentials to invoke Phosphor D-Bus REST APIs. Note, however, that the X-Auth tokens are required to use POST, PUT, PATCH, or DELETE methods.
General security considerations for REST APIs apply: https://github.com/OWASP/CheatSheetSeries/blob/master/cheatsheets/REST_Security_Cheat_Sheet.md
Redfish provides security considerations in the "Security Detail" section of the "Redfish Specification" (document ID DSP0266) available from https://www.dmtf.org/standards/redfish.
General considerations for Web applications such as given by OWASP Web Application Security Guidance apply to OpenBMC. The phosphor-webui uses username and password-based authentication, and REST APIs for subsequent access.
The web app also provides interfaces to use the host serial console, virtual media, and host KVM.
Access to the BMC's host serial console is provided via the SSH protocol on port 2200.
This uses the same server implementation as port 22, including the same TLS mechanisms.
How the host secures its console (for example, username and password prompts) is outside the scope of this document.
General security considerations for service discovery apply. For example, described here: https://attack.mitre.org/techniques/T1046/
General security considerations for service discovery apply.
The IPMI network-facing design is described here: https://github.com/openbmc/docs/blob/master/ipmi-architecture.md and the implementation is described here: https://github.com/openbmc/phosphor-net-ipmid. Note that host IPMI is outside the scope of this document.
General security considerations for IPMI apply. For example, described here: https://www.us-cert.gov/ncas/alerts/TA13-207A
OpenBMC implements RCMP+ and IPMI 2.0. The phosphor-user-manager provides the underlying authentication mechanism.
Supported IPMI ciphers can be found in the code, for example, by searching for function isAlgorithmSupported, or from the ipmitool command such as ipmitool channel getciphers ipmi.
OpenBMC supports IPMI "serial over LAN" (SOL) connections (via impitool sol activate) which shares the host serial console socket.