CVE-2021-23839

OpenSSL 1.0.2 supports SSLv2. If a client attempts to negotiate SSLv2 with
a server that is configured to support both SSLv2 and more recent SSL and
TLS versions then a check is made for a version rollback attack when
unpadding an RSA signature. Clients that support SSL or TLS versions
greater than SSLv2 are supposed to use a special form of padding. A server
that supports greater than SSLv2 is supposed to reject connection attempts
from a client where this special form of padding is present, because this
indicates that a version rollback has occurred (i.e. both client and server
support greater than SSLv2, and yet this is the version that is being
requested). The implementation of this padding check inverted the logic so
that the connection attempt is accepted if the padding is present, and
rejected if it is absent. This means that such as server will accept a
connection if a version rollback attack has occurred. Further the server
will erroneously reject a connection if a normal SSLv2 connection attempt
is made. Only OpenSSL 1.0.2 servers from version 1.0.2s to 1.0.2x are
affected by this issue. In order to be vulnerable a 1.0.2 server must: 1)
have configured SSLv2 support at compile time (this is off by default), 2)
have configured SSLv2 support at runtime (this is off by default), 3) have
configured SSLv2 ciphersuites (these are not in the default ciphersuite
list) OpenSSL 1.1.1 does not have SSLv2 support and therefore is not
vulnerable to this issue. The underlying error is in the implementation of
the RSA_padding_check_SSLv23() function. This also affects the
RSA_SSLV23_PADDING padding mode used by various other functions. Although
1.1.1 does not support SSLv2 the RSA_padding_check_SSLv23() function still
exists, as does the RSA_SSLV23_PADDING padding mode. Applications that
directly call that function or use that padding mode will encounter this
issue. However since there is no support for the SSLv2 protocol in 1.1.1
this is considered a bug and not a security issue in that version. 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 1.0.2y. Other users
should upgrade to 1.1.1j. Fixed in OpenSSL 1.0.2y (Affected 1.0.2s-1.0.2x).

Notes