SSL/TLS Protocol Initialization Vector Implementation Information Disclosure Vulnerability (BEAST)
Medium Nessus Plugin ID 58751
SynopsisIt may be possible to obtain sensitive information from the remote host with SSL/TLS-enabled services.
DescriptionA vulnerability exists in SSL 3.0 and TLS 1.0 that could allow information disclosure if an attacker intercepts encrypted traffic served from an affected system.
TLS 1.1, TLS 1.2, and all cipher suites that do not use CBC mode are not affected.
This plugin tries to establish an SSL/TLS remote connection using an affected SSL version and cipher suite and then solicits return data.
If returned application data is not fragmented with an empty or one-byte record, it is likely vulnerable.
OpenSSL uses empty fragments as a countermeasure unless the 'SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS' option is specified when OpenSSL is initialized.
Microsoft implemented one-byte fragments as a countermeasure, and the setting can be controlled via the registry key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SecurityProviders\SCHANNEL\SendExtraRecord.
Therefore, if multiple applications use the same SSL/TLS implementation, some may be vulnerable while others may not be, depending on whether or not a countermeasure has been enabled.
Note that this plugin detects the vulnerability in the SSLv3/TLSv1 protocol implemented in the server. It does not detect the BEAST attack where it exploits the vulnerability at HTTPS client-side (i.e., Internet browser). The detection at server-side does not necessarily mean your server is vulnerable to the BEAST attack, because the attack exploits the vulnerability at the client-side, and both SSL/TLS clients and servers can independently employ the split record countermeasure.
SolutionConfigure SSL/TLS servers to only use TLS 1.1 or TLS 1.2 if supported.
Configure SSL/TLS servers to only support cipher suites that do not use block ciphers. Apply patches if available.
Note that additional configuration may be required after the installation of the MS12-006 security update in order to enable the split-record countermeasure. See Microsoft KB2643584 for details.