OpenSSL Security Advisory

[OpenSSL]

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OpenSSL Security Advisory [09 September 2020]
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Raccoon Attack (CVE-2020-1968)
==============================

Severity: Low

The Raccoon attack exploits a flaw in the TLS specification which can lead to
an attacker being able to compute the pre-master secret in connections which
have used a Diffie-Hellman (DH) based ciphersuite. In such a case this would
result in the attacker being able to eavesdrop on all encrypted communications
sent over that TLS connection. The attack can only be exploited if an
implementation re-uses a DH secret across multiple TLS connections. Note that
this issue only impacts DH ciphersuites and not ECDH ciphersuites.

OpenSSL 1.1.1 is not vulnerable to this issue: it never reuses a DH secret and
does not implement any "static" DH ciphersuites.

OpenSSL 1.0.2f and above will only reuse a DH secret if a "static" DH
ciphersuite is used. These static "DH" ciphersuites are ones that start with the
text "DH-" (for example "DH-RSA-AES256-SHA"). The standard IANA names for these
ciphersuites all start with "TLS_DH_" but excludes those that start with
"TLS_DH_anon_".

OpenSSL 1.0.2e and below would reuse the DH secret across multiple TLS
connections in server processes unless the SSL_OP_SINGLE_DH_USE option was
explicitly configured. Therefore all ciphersuites that use DH in servers
(including ephemeral DH) are vulnerable in these versions. In OpenSSL 1.0.2f
SSL_OP_SINGLE_DH_USE was made the default and it could not be turned off as a
response to CVE-2016-0701.

Since the vulnerability lies in the TLS specification, fixing the affected
ciphersuites is not viable. For this reason 1.0.2w moves the affected
ciphersuites into the "weak-ssl-ciphers" list. Support for the
"weak-ssl-ciphers" is not compiled in by default. This is unlikely to cause
interoperability problems in most cases since use of these ciphersuites is rare.
Support for the "weak-ssl-ciphers" can be added back by configuring OpenSSL at
compile time with the "enable-weak-ssl-ciphers" option. This is not recommended.

OpenSSL 1.0.2 is out of support and no longer receiving public updates.

Premium support customers of OpenSSL 1.0.2 should upgrade to 1.0.2w.  If
upgrading is not viable then users of OpenSSL 1.0.2v or below should ensure
that affected ciphersuites are disabled through runtime configuration. Also
note that the affected ciphersuites are only available on the server side if a
DH certificate has been configured. These certificates are very rarely used and
for this reason this issue has been classified as LOW severity.

This issue was found by Robert Merget, Marcus Brinkmann, Nimrod Aviram and Juraj
Somorovsky and reported to OpenSSL on 28th May 2020 under embargo in order to
allow co-ordinated disclosure with other implementations.

Note
====

OpenSSL 1.0.2 is out of support and no longer receiving public updates. Extended
support is available for premium support customers:
https://www.openssl.org/support/contracts.html

OpenSSL 1.1.0 is out of support and no longer receiving updates of any kind.
The impact of this issue on OpenSSL 1.1.0 has not been analysed.

Users of these versions should upgrade to OpenSSL 1.1.1.

References
==========

URL for this Security Advisory:
https://www.openssl.org/news/secadv/20200909.txt

Note: the online version of the advisory may be updated with additional details
over time.

For details of OpenSSL severity classifications please see:
https://www.openssl.org/policies/secpolicy.html
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