ESA-2012-032: RSA BSAFE(r) Micro Edition Suite Security Update for BEAST (Browser Exploit Against SSL/TLS) attacks
- From: Security Alert <Security_Alert@xxxxxxx>
- Date: Fri, 7 Sep 2012 14:18:01 -0400
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ESA-2012-032: RSA BSAFE® Micro Edition Suite Security Update for BEAST (Browser Exploit Against SSL/TLS) attacks
EMC Identifier: ESA-2012-032
CVE Identifier: CVE-2011-3389
Severity Rating: CVSS v2 Base Score: 4.3 (AV:N/AC:M/Au:N/C:P/I:N/A:N)
All versions of RSA BSAFE Micro Edition Suite prior to 4.0, all platforms
RSA BSAFE Micro Edition Suite 4.0 and higher
RSA BSAFE Micro Edition Suite contains updates designed to prevent BEAST attacks (CVE-2011-3389)
There is a known vulnerability in SSLv3 and TLS v1.0 to do with how the Initialization Vector (IV) is generated. For symmetric key algorithms in CBC mode, the IV for the first record is generated using keys and secrets set during the SSL or TLS handshake. All subsequent records are encrypted using the ciphertext block from the previous record as the IV. With symmetric key encryption in CBC mode, plain text encrypted with the same IV and key generates the same cipher text, which is why having a variable IV is important.
The BEAST exploit uses this SSLv3 and TLS v1.0 vulnerability by allowing an attacker to observe the last ciphertext block, which is the IV, then replace this with an IV of their choice, inject some of their own plain text data, and when this new IV is used to encrypt the data, the attacker can guess the plain text data one byte at a time.
The best way to help prevent this attack is to use TLS v1.1 or higher. The vulnerability to do with IV generation was fixed in TLS v1.1 (released in 2006) so implementations using only TLS v1.1 or v1.2 are engineered to be secure against the BEAST exploit. However, support for these higher level protocols is limited to a smaller number of applications, so supporting only TLS v1.1 or v1.2 might cause interoperability issues.
A second solution is to limit the negotiated cipher suites to exclude those that do not require symmetric key algorithms in CBC mode. However, this substantially restricts the number of cipher suites that can be negotiated. That is, only cipher suites with NULL encryption or cipher suites with streaming encryption algorithms (the RC4 algorithm) could be negotiated, which might result in reduced security.
For customers who cannot or should not implement either of these two methods, RSA BSAFE Micro Edition Suite 4.0 introduces a new feature called first block splitting. First block splitting prevents the BEAST exploit by introducing unknown data into the encryption scheme prior to the attackers inserted plain text data. This is done as follows:
?1. The first plain text block to be encrypted is split into two blocks. The first block contains the first byte of the data, the second block contains the rest.
?2. A MAC is generated from the one byte of data, the MAC key, and an increasing counter. This MAC is included in the first block.
?3. The one byte of data, along with the MAC, is encrypted and becomes the IV for the next block. Because the IV is now essentially random data, it is impossible for an attacker to predict it and replace it with one of their own.
To implement first block splitting in RSA BSAFE Micro Edition Suite 4.0, either for an SSL context or SSL object, call R_SSL_CTX_set_options_by_type() or R_SSL_set_options_by_type() respectively, with the SSL_OP_TYPE_SECURITY option type and the SSL_OP_SPLIT_FIRST_FRAGMENT identifier.
For more information about these functions and identifiers, see the RSA BSAFE Micro Edition Suite API Reference Guide.
For an explanation of Severity Ratings, refer to the Knowledge Base Article, ?Security Advisories Severity Rating? at https://knowledge.rsasecurity.com/scolcms/knowledge.aspx?solution=a46604. RSA recommends all customers take into account both the base score and any relevant temporal and environmental scores which may impact the potential severity associated with particular security vulnerability.
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