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This article was originally published on the Red Hat Customer Portal. The information may no longer be current.

Today we see more and more attacks on operating systems taking advantage of various technologies, including obsolete cryptographic algorithms and protocols. As such, it is important for an operating system not only to carefully evaluate the new technologies that get introduced, but to also provide a process for phasing out technologies that are no longer relevant. Technologies with no practical use today increase the attack surface of the operating system and more specifically, in the cryptography field, introduce risks such as untrustworthy communication channels, when algorithms and protocols are being used after their useful lifetime.

That risk is not being confined to the users of the obsolete technologies; as the DROWN and other cross-protocol attacks have demonstrated, it is sufficient for a server to only enable a legacy protocol in parallel with the latest one, for all of its users to be vulnerable. Furthermore, the recent cryptographic advances against the SHA-1 algorithm used for digital signatures, demonstrate the need for algorithm agility in modern infrastructures. SHA-1 was an integral part of the Internet and private Public Key Infrastructures and despite that, we must envision a not so distant future with systems that no longer rely on SHA-1 for any cryptographic purpose.

To address the challenges above, with the release of Red Hat Enterprise Linux (RHEL) 7.4 beta, we are introducing several cryptographic updates to RHEL and we also are introducing a multitude of new features and enhancements. We continue and extend our protocol deprecation effort started in RHEL 6.9, improve access to kernel-provided PRNG with the getrandom system call, as well as ensure that HTTP/2 supporting technologies like ALPN in TLS, and DTLS 1.2, are supported universally in our operating system. Python applications are also made secure by default by enabling certificate verification by default in TLS sessions. We are also proud to bring the OpenSC smart card drivers into RHEL 7.4, incorporating our in-house developed drivers and merging our work with the OpenSC project community efforts. At the same time, the introduced crypto changes ensure that RHEL 7.4 meets the rigorous security certification requirements for FIPS140-2 cryptographic modules.

Removal of SSH 1.0, SSL 2.0 protocols and EXPORT cipher suites

For reasons underlined in our deprecation of insecure algorithms blog post, that is, to protect applications running in RHEL from severe attacks utilizing obsolete cryptographic protocols and algorithms, the SSH 1.0, SSL 2.0 protocols, as well as those marked as ‘export’ cipher suites will no longer be included in our supported cryptographic components. The SSH 1.0 protocol is removed from the server-side of the OpenSSH component, while support for it will remain on the client side (already disabled by default) to allow communications with legacy equipment. The SSL 2.0 protocol as well as the TLS ‘export’ cipher suites are removed unconditionally from the GnuTLS, NSS, and OpenSSL crypto components. Furthermore, the Diffie-Hellman key exchange is restricted only to values considered acceptable for today’s cryptographic protocols.

Note also, that through our review of accepted legacy hashes in the operating system we have discovered that the OpenSSL component enables obsolete hashes for digital signatures, such as SHA-0, MD5, and MD4. Since these hashes have no practical use today, and to reduce the risk of relying on legacy algorithms, we have decided to deviate from upstream OpenSSL settings and disable these hashes by default for all OpenSSL applications. That change is reversible (see release notes). Note that this issue was discussed with the upstream OpenSSL developers, and although that behavior is known to them, it is kept for backwards compatibility.

Below is a summary of the deprecated protocols and algorithms.

Introduced change Description Reversible
SSL 2.0 protocol The shipped TLS libraries will no longer include support for the SSL 2.0 protocol. No.
Export TLS ciphersuites The shipped TLS libraries will no longer include support for Export TLS ciphersuites. No.
SSH 1.0 protocol The shipped OpenSSH server will no longer include support for SSH 1.0 protocol. No.
TLS Diffie-Hellman key exchange Only parameters larger than 1024-bits will be accepted by default. Conditionally (information will be provided in the release notes).
Insecure hashes for digital signatures in OpenSSL The MD5, MD4 and SHA-0 algorithms will not be enabled by default for TLS sessions or certificate verification on OpenSSL. Yes. Administrators can revert this setting system-wide (information will be provided in the release notes).
RC4 algorithm The algorithm will no longer be enabled by default for OpenSSH sessions. Yes. Administrators can revert this setting system-wide (information will be provided in the release notes).

Phasing out of SHA-1

SHA-1 was published in 1993 and is still in use today in a variety of applications including but not limited to the web PKI. In particular, its primary use case is digital signatures on data, certificates, and OCSP responses. However, there are several known weaknesses on this hash and there was recently a demonstration of collision attack, something that, when combined with the experience of MD5 hash attacks, is an indication that a forged certificate attack may not be far in the future. For that reason, we have ensured that all our cryptographic tools1 which deal with digital signatures will no longer use SHA-1 as the default hash function, but instead switch to SHA2-256, which provides maximum compatibility with older clients.

In addition, to further strengthen the resistance of RHEL to such cryptographic attacks, we have lifted the OpenSSH server and client limitation to SHA-1 for digital signatures, enabling support for SHA2-256 digital signatures.

We do not yet plan to disable SHA-1 system-wide in RHEL 7 as a significant amount of infrastructure still depends on it, and disabling it would severely disrupt operations. Nonetheless, we would like to recommend software engineers working on RHEL to no longer rely on SHA-1 for cryptographic purposes, and system administrators to verify that they no longer use certificates, OCSP stapled responses, or any other cryptographic structure with SHA-1 based signatures.

To verify whether a certificate, OCSP response, or CRL utilize the SHA-1 hash algorithm for signature, you may use the following commands.

Test Command
Certificate in FILENAME uses SHA-1 openssl x509 -in FILENAME -text -noout | grep -i 'Signature.*sha1'
OCSP response in FILENAME uses SHA-1 openssl ocsp -resp_text -respin FILENAME -noverify | grep -i 'Signature.*sha1'
CRL in FILENAME uses SHA-1 openssl crl -in FILENAME -text -noout | grep -i 'Signature.*sha1'

HTTP/2 related updates

Modern internet applications strive for low-latency and good responsiveness, and the HTTP/2 protocol is a major driver to that effort. With the introduction of OpenSSL 1.0.2 into RHEL 7.4, we complete across our base crypto stack support for Application Layer Protocol Negotiation (ALPN). This TLS protocol extension enables applications to reduce TLS handshake round-trips by negotiating the application protocol and its version during the handshake process. In practice this is used for applications to signal their HTTP/2 support, eliminating the need for additional round-trips after the TLS connection is established. That, when combined with other kernel features such as TCP fast open, can further fine-tune the TLS session establishment.

DTLS 1.2 for all applications

Additionally, the introduction of OpenSSL 1.0.2 brings support for the Datagram TLS 1.2 (DTLS) protocol in OpenSSL applications, completing that support throughout the TLS stacks on the operating system. That support ensures that applications using the DTLS protocol can take advantage of the secure authenticated encryption (AEAD) cipher suites, eliminating the reliance on CBC cipher suites, which are known to be problematic for DTLS.

Crypto-related Python-language changes

The upstream version of Python 2.7.9 enabled SSL/TLS certificate verification in Python’s standard library modules that provide HTTP client functionality such as urllib, httplib or xmlrpclib. Prior to this change, no certificate verification was performed by default, making Python applications vulnerable to certain classes of attacks in SSL and TLS connections. It was a well known issue for a long time and several applications worked around the issue by implementing their own certificate checks. Despite these work-arounds, in order to ensure that all Python applications are secure by default, and follow a consistent certificate validation process, in Red Hat Enterprise Linux 7.4 we incorporate the upstream change and enable certificate verification by default in TLS sessions for all applications.

Crypto-related kernel changes

Despite the fact that the Linux kernel was one of the first to provide interfaces to access cryptographically-secure pseudo-random data via /dev/*random interfaces, these interfaces show their age today. The /dev/random is a system device which when read will provide random data, to the extent assessed by an internal estimator. That is, the device will block when not enough random events, related to internal kernel entropy sources, such as interrupts, CPU, and others, are accumulated. Due to that, the /dev/random interface may seem initially tempting to use. However, in practice /dev/random cannot be relied on; it requires a large amount of random events to be accumulated to provide few bytes of random data, and any use of it during boot time could risk blocking the system indefinitely.

On the other hand, the device /dev/urandom provides access to the same random generator, but will never block, nor apply any restrictions to the number of new random events that must be read in order to provide any output. That is expected, given that modern random generators when sufficiently seeded can provide an enormous amount of output prior to being considered broken in an informational-theoretic sense. Unfortunately, /dev/urandom suffers from a design flaw. When used early in the boot process prior to initialization of the kernel random number generator, it will still output data. How random is the output data is system-specific, but in modern platforms, which provide a CPU-based random generator, that is less of an issue.

To eliminate such risks, RHEL 7.4 introduces the getrandom() system call, which combines the best of the existing interfaces. It can provide non-blocking access to kernel CPRNG and it will block prior to the kernel random generator being initialized. Furthermore, it requires no file descriptor to be carried by application and libraries. The new system call is available through the syscall(2) interface.

Smart card related changes

For long time, RHEL provided the CoolKey smart-card driver for applications to access the PKCS#11 API on smart cards. This driver was limited to the smart cards that we focus on for our operating system, that is, the PIV, CAC, PKCS#15-compliant and CoolKey-applet cards. However, over the years, the community-based OpenSC project, which provided an alternative driver, has managed to provide support for the majority of available smart cards, provide solid code base with support for the industry standard PKCS#11 API, and is already included in our Fedora distribution for many years. Most importantly, however, the project also serves as a healthy example of a community-driven project, with collaborating engineers from diverse backgrounds and technologies.

With that in mind, we have proceeded by merging our projects, by introducing missing drivers and features to OpenSC, as well as by releasing our extensive test suite for smart card support. The outcome of this effort is made available on RHEL 7.4, where the OpenSC component will serve in parallel with the CoolKey component. The latter will remain supported for the lifetime of RHEL 7, however, new hardware enablement will only be available on the OpenSC component.

Future changes

In the lifetime of Red Hat Enterprise Linux 7 we plan to deprecate and disable by default the SSL 3.0 protocol and the RC4 cipher from TLS and Kerberos system-wide. Note that we do not plan to remove support for the above algorithms, but disable them for applications, which did not explicitly requested them.

Introduced change Description Reversible
SSL 3.0 protocol The shipped TLS libraries will no longer enable support for the SSL 3.0 protocol. This is a protocol which has been superseded by TLS 1.0 for almost two decades, and there is a multitude of attacks towards it. By explicitly enabling the protocol in applications that require it.
RC4 The shipped TLS libraries will no longer enable support for the RC4 based ciphersuites by default. Cryptographic advances have shown that accidental usage of that algorithm put applications at risk in relation with data secrecy. By explicitly enabling the ciphersuites in applications that require them.

Concluding remarks

All of the above changes ensure that Red Hat Enterprise 7 remains a leader in the adoption of new technologies with security built into the OS. Red Hat Enterprise Linux not only carefully evaluates and incorporates new technologies, but technologies that are no longer relevant and pose a security risk are regularly phased out. Specifically, HTTP/2-supporting protocols are made available in all of our cryptographic back-ends. We also align with the community on smart card development not only by bringing our improvements to the OpenSC project, but by introducing it as a fully supported component in Red Hat Enterprise Linux 7. Furthermore, with the introduction of Datagram TLS 1.2 in OpenSSL, and the updates in OpenSSH in order to support other than SHA-1 cryptographic hashes, we ensure that all Red Hat Enterprise Linux 7 applications utilize the right cryptographic algorithms which can resist today’s threats.

On the other hand, we reduce the attack surface of the operating system, by removing support for SSH 1.0, SSL 2.0, and the TLS ‘export’ cipher suites. That is a move which reduces the risk of successful future attacks, at the cost of removal of protocols which are today considered either too risky to use or plain insecure.

The removal of the previously mentioned protocols was an exceptional move because we are convinced that these are primarily used to attack misconfigured systems rather than for real-world use cases. Regarding any future changes, for example the disablement of SSL 3.0 or RC4, we would like to assure that it will be done in a way that systems can revert to the previous behavior when required by local policy.

All of these changes ensure Red Hat Enterprise Linux keeps pace with the reality of the security landscape around us, and improves its strong security foundation.

  1. The above applies to software from the GnuTLS, NSS, and OpenSSL crypto components. 


Nikos Mavrogiannopoulos has a background in mathematics and holds a Ph.D. in cryptography. Spent two decades in software engineering, mostly in security of back-end software; formed the RHEL cryptographic team. I'm now a manager in RHEL Security Engineering.

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