TA14-290A: SSL 3.0 Protocol Vulnerability and POODLE Attack

["US-CERT" <US-CERT () ncas us-cert gov>]

NCCIC / US-CERT

National Cyber Awareness System:

TA14-290A: SSL 3.0 Protocol Vulnerability and POODLE Attack [ https://www.us-cert.gov/ncas/alerts/TA14-290A ] 
10/17/2014 12:27 PM EDT 
Original release date: October 17, 2014

Systems Affected

All systems and applications utilizing the Secure Socket Layer (SSL) 3.0 with cipher-block chaining (CBC) mode ciphers 
may be vulnerable. However, the POODLE (Padding Oracle On Downgraded Legacy Encryption) attack demonstrates this 
vulnerability using web browsers and web servers, which is one of the most likely exploitation scenarios.

Overview

US-CERT is aware of a design vulnerability found in the way SSL 3.0 handles block cipher mode padding. The POODLE 
attack demonstrates how an attacker can exploit this vulnerability to decrypt and extract information from inside an 
encrypted transaction.

Description

The SSL 3.0 vulnerability stems from the way blocks of data are encrypted under a specific type of encryption algorithm 
within the SSL protocol. The POODLE attack takes advantage of the protocol version negotiation feature built into 
SSL/TLS to force the use of SSL 3.0 and then leverages this new vulnerability to decrypt select content within the SSL 
session. The decryption is done byte by byte and will generate a large number of connections between the client and 
server.

While SSL 3.0 is an old encryption standard and has generally been replaced by Transport Layer Security (TLS) (which is 
not vulnerable in this way), most SSL/TLS implementations remain backwards compatible with SSL 3.0 to interoperate with 
legacy systems in the interest of a smooth user experience. Even if a client and server both support a version of TLS 
the SSL/TLS protocol suite allows for protocol version negotiation (being referred to as the “downgrade dance” in other 
reporting). The POODLE attack leverages the fact that when a secure connection attempt fails, servers will fall back to 
older protocols such as SSL 3.0. An attacker who can trigger a connection failure can then force the use of SSL 3.0 and 
attempt the new attack. [1 [ https://www.openssl.org/~bodo/ssl-poodle.pdf ]]

Two other conditions must be met to successfully execute the POODLE attack: 1) the attacker must be able to control 
portions of the client side of the SSL connection (varying the length of the input) and 2) the attacker must have 
visibility of the resulting ciphertext. The most common way to achieve these conditions would be to act as 
Man-in-the-Middle (MITM), requiring a whole separate form of attack to establish that level of access.

These conditions make successful exploitation somewhat difficult. Environments that are already at above-average risk 
for MITM attacks (such as public WiFi) remove some of those challenges.

Impact

The POODLE attack can be used against any system or application that supports SSL 3.0 with CBC mode ciphers. This 
affects most current browsers and websites, but also includes any software that either references a vulnerable SSL/TLS 
library (e.g. OpenSSL) or implements the SSL/TLS protocol suite itself. By exploiting this vulnerability in a likely 
web-based scenario, an attacker can gain access to sensitive data passed within the encrypted web session, such as 
passwords, cookies and other authentication tokens that can then be used to gain more complete access to a website 
(impersonating that user, accessing database content, etc.).

Solution

There is currently no fix for the vulnerability SSL 3.0 itself, as the issue is fundamental to the protocol; however, 
disabling SSL 3.0 support in system/application configurations is the most viable solution currently available.

Some of the same researchers that discovered the vulnerability also developed a fix for one of the prerequisite 
conditions; TLS_FALLBACK_SCSV is a protocol extension that prevents MITM attackers from being able to force a protocol 
downgrade. OpenSSL has added support for TLS_FALLBACK_SCSV to their latest versions and recommend the following 
upgrades: [2 [ https://www.openssl.org/news/secadv_20141015.txt ]]


  * OpenSSL 1.0.1 users should upgrade to 1.0.1j. 
  * OpenSSL 1.0.0 users should upgrade to 1.0.0o. 
  * OpenSSL 0.9.8 users should upgrade to 0.9.8zc. 

Both clients and servers need to support TLS_FALLBACK_SCSV to prevent downgrade attacks.

Other SSL 3.0 implementations are most likely also affected by POODLE. Contact your vendor for details. Additional 
vendor information may be available in the National Vulnerability Database (NVD) entry for CVE-2014-3566. [3 [ 
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-3566 ]]

References

  * [1] This Poodle Bites: Exploiting The SSL Fallback  [ https://www.openssl.org/~bodo/ssl-poodle.pdf ] 
  * [2] OpenSSL Security Advisory [15 Oct 2014] [ https://www.openssl.org/news/secadv_20141015.txt ] 
  * [3] Vulnerability Summary for CVE-2014-3566  [ http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-3566 ] 

Revision History

  * October 17, 2014 Initial Release 
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