OpenSSL Security Advisory [8th September 2023]
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POLY1305 MAC implementation corrupts XMM registers on Windows (CVE-2023-4807)
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Severity: Low

Issue summary: The POLY1305 MAC (message authentication code) implementation
contains a bug that might corrupt the internal state of applications on the
Windows 64 platform when running on newer X86_64 processors supporting the
AVX512-IFMA instructions.

Impact summary: If in an application that uses the OpenSSL library an attacker
can influence whether the POLY1305 MAC algorithm is used, the application
state might be corrupted with various application dependent consequences.

The POLY1305 MAC (message authentication code) implementation in OpenSSL does
not save the contents of non-volatile XMM registers on Windows 64 platform
when calculating the MAC of data larger than 64 bytes. Before returning to
the caller all the XMM registers are set to zero rather than restoring their
previous content. The vulnerable code is used only on newer x86_64 processors
supporting the AVX512-IFMA instructions.

The consequences of this kind of internal application state corruption can
be various - from no consequences, if the calling application does not
depend on the contents of non-volatile XMM registers at all, to the worst
consequences, where the attacker could get complete control of the application
process. However given the contents of the registers are just zeroized so
the attacker cannot put arbitrary values inside, the most likely consequence,
if any, would be an incorrect result of some application dependent
calculations or a crash leading to a denial of service.

The POLY1305 MAC algorithm is most frequently used as part of the
CHACHA20-POLY1305 AEAD (authenticated encryption with associated data)
algorithm. The most common usage of this AEAD cipher is with TLS protocol
versions 1.2 and 1.3 and a malicious client can influence whether this AEAD
cipher is used by the server. This implies that server applications using
OpenSSL can be potentially impacted. However we are currently not aware of
any concrete application that would be affected by this issue therefore we
consider this a Low severity security issue.

As a workaround the AVX512-IFMA instructions support can be disabled at
runtime by setting the environment variable OPENSSL_ia32cap:

   OPENSSL_ia32cap=:~0x200000

OpenSSL versions 1.1.1 to 1.1.1v, 3.0.0 to 3.0.10, and 3.1.0 to 3.1.2 are
vulnerable to this issue. The FIPS provider is not affected because the
POLY1305 MAC algorithm is not FIPS approved and the FIPS provider does not
implement it.

OpenSSL version 1.0.2 is not affected by this issue.

Due to the low severity of this issue we are not issuing new releases of
OpenSSL at this time. The fix will be included in the next releases when they
become available. The fix is also available in commit 4bfac447 (for 3.1),
commit 6754de4a (for 3.0), and commit a632d534 (for 1.1.1) in the OpenSSL git
repository.

This issue was reported publicly on GitHub on 23rd July 2023 by Zach Wilson
(Nvidia) and subsequently to the OpenSSL security team on 28th August 2023
by Bernd Edlinger. The fix disabling the vulnerable codepath was developed
by Bernd Edlinger.

General Advisory Notes
======================

URL for this Security Advisory:
https://www.openssl.org/news/secadv/20230908.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

OpenSSL 1.1.1 will reach end-of-life on 2023-09-11. After that date security
fixes for 1.1.1 will only be available to premium support customers.