Summary

• On December 12, 2017, a research paper with the title Return of Bleichenbacher's Oracle Threat was made publicly available. This paper describes how some Transport Layer Security (TLS) stacks are vulnerable to variations of the classic Bleichenbacher attack on RSA key exchange. Multiple vulnerabilities were identified based on this research.
  
  An attacker could iteratively query a server running a vulnerable TLS stack implementation to perform cryptanalytic operations that may allow decryption of previously captured TLS sessions.
  
  To exploit these vulnerabilities, an attacker must be able to perform both of the following actions:
  

  • Capture traffic between clients and the affected TLS server.
  • Actively establish a considerable number of TLS connections to the vulnerable server. The actual number of connections required varies with the implementation-specific vulnerabilities, and could range from hundreds of thousands to millions of connections.

  
  Multiple Cisco products are affected by these vulnerabilities.
  
  Cisco has released software updates that address some of these vulnerabilities.
  
  There may be workarounds available for selected products.
  
  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20171212-bleichenbacher

Affected Products

• Cisco investigated its product line to determine which products may be affected by these vulnerabilities and the impact on each affected product. Refer to the "Vulnerable Products" and "Products Confirmed Not Vulnerable" sections of this advisory for information about whether a product is affected.
  
  The "Vulnerable Products" section includes Cisco bug IDs for each affected product. The bugs are accessible through the Cisco Bug Search Tool and contain additional platform-specific information, including workarounds (if available) and fixed software releases.
  
  

  Vulnerable Products

  The following table lists Cisco products that are affected by one or more of the vulnerabilities that are described in this advisory:
  
  

  Product	Cisco Bug ID	Fixed Release Availability
  Cisco Cloud Hosted Services
  Cisco WebEx Business Suite (including Meeting Center, Training Center, Event Center, Support Center)	CSCvh10981	Remediation has been completed. (15-Jan-2018)
  Cisco WebEx Meetings	CSCvh10981	Remediation has been completed. (15-Jan-2018)
  Cisco WebEx Messenger	CSCvh25064	Remediation has been completed. (15-Jan-2018)
  Network Application, Service, and Acceleration
  Cisco ACE 4710 Application Control Engine Appliance	CSCvg74693	No fix will be provided for this product. 1
  Cisco ACE30 Application Control Engine Module	CSCvg74693	No fix will be provided for this product. 2
  Network and Content Security Devices
  Cisco Adaptive Security Appliance (ASA) - ASA 5505, 5510, 5520, 5540, and 5550	CSCvg97652	9.1.7.21 (5-Jan-2018) 9.2.4.25 (8-Jan-2018)
  Cisco SSL Appliance	CSCvh00296	3.12.2.1 (18-May-2018)

  
  ¹ Cisco ACE 4710 Application Control Engine Appliance entered the end-of-life cycle on July 26, 2013. For reference, see the End-of-Sale and End-of-Life Announcement.
  ² Cisco ACE30 Application Control Engine Module entered the end-of-life cycle on July 26, 2013. For reference, see the End-of-Sale and End-of-Life Announcement.
  
  

  Products Confirmed Not Vulnerable

  No other Cisco products are currently known to be affected by these vulnerabilities.
  
  Cisco has confirmed that these vulnerabilities do not affect the following products:
  
  Cisco Cloud Hosted Services

  • Cisco Cloud Web Security (CWS)
  • Cisco Spark
  • Cisco Umbrella

  
  Network Application, Service, and Acceleration
  

  • Cisco Wide Area Application Services (WAAS)

  
  Network and Content Security Devices

  • Cisco Adaptive Security Appliance (ASA) - All models not listed in the Vulnerable Products section
  • Cisco ASA Next-Generation Firewall Services
  • Cisco Content Security Management Appliance (SMA)
  • Cisco Email Security Appliance (ESA)
  • Cisco Firepower 2100 Series
  • Cisco Firepower 4100 Series
  • Cisco Firepower 7000 Series
  • Cisco Firepower 8000 Series
  • Cisco Firepower 9300 Security Appliance
  • Cisco Identity Services Engine (ISE)
  • Cisco Web Security Appliance (WSA)

  
  Routing and Switching - Enterprise and Service Provider

  • Cisco VPN Internal Service Module for ISR (VPN ISM)

Details

• Bleichenbacher's attack is an adaptive chosen-ciphertext attack against RSA PKCS#1 v1.5 encryption block formatting. Daniel Bleichenbacher originally performed this attack against the RSA key exchange on Secure Sockets Layer (SSL) Version 3.0.
  
  The attack relies on the presence of a side channel indicating, for any chosen ciphertext, whether the corresponding plaintext has the correct format according to the RSA PKCS#1 v1.5 standard. An attacker could exploit this side channel as an oracle, iteratively constructing crafted TLS messages. Eventually the attacker might be able to recover the plaintext for a given TLS session.
  
  The new research focused on identifying current TLS implementations affected by the classic Bleichenbacher attack or those affected by newly introduced variations. These new variations use the overall TCP connection state as a possible side channel and use a nonstandard, abbreviated TLS message flow.
  
  The following vulnerabilities were identified based on this research:
  
  Cavium SSL SDK Bleichenbacher Attack Information Disclosure Vulnerability
  
  A vulnerability in the TLS protocol implementation of Cavium SSL Software Development Kits (SDKs) could allow an unauthenticated, remote attacker to access sensitive information.
  
  The vulnerability is due to improper implementation of countermeasures against the Bleichenbacher attack for cipher suites that rely on RSA for key exchange. An attacker could exploit this vulnerability by sending crafted TLS messages to the device, which would act as an oracle and allow the attacker to carry out a chosen-ciphertext attack.
  
  This vulnerability has been assigned the following CVE ID: CVE-2017-17428.

  Legacy Cisco ASA 5500 Series Bleichenbacher Attack Information Disclosure Vulnerability
  
  A vulnerability in the TLS protocol implementation of legacy Cisco ASA 5500 Series (ASA 5505, 5510, 5520, 5540, and 5550) devices could allow an unauthenticated, remote attacker to access sensitive information.
  
  The vulnerability is due to improper implementation of countermeasures against the Bleichenbacher attack for cipher suites that rely on RSA for key exchange. An attacker could exploit this vulnerability by sending crafted TLS messages to the device, which would act as an oracle and allow the attacker to carry out a chosen-ciphertext attack.
  
  This vulnerability affects the listed ASA models when they are using a 2048-bit RSA key.
  
  This vulnerability has been assigned the following CVE ID: CVE-2017-12373.

  SSL Visibility Bleichenbacher Attack Information Disclosure Vulnerability

  A vulnerability in the TLS protocol implementation of Cisco SSL Appliance (Bluecoat SSL Visibility OEM appliance) could allow an unauthenticated, remote attacker to access sensitive information.
  
  The vulnerability is due to improper implementation of countermeasures against the Bleichenbacher attack for cipher suites that rely on RSA for key exchange. An attacker could exploit this vulnerability by sending crafted TLS messages to the device, which would act as an oracle and allow the attacker to carry out a chosen-ciphertext attack.

  This vulnerability has been assigned the following CVE ID: CVE-2017-15533.

Workarounds

• Disabling use of TLS ciphers that rely on RSA for key exchange is a possible mitigation for these vulnerabilities. It is applicable only to products that support other types of ciphers, such as those based on Diffie-Hellman key exchange. This mitigation may have an impact on interoperability with legacy clients that do not support these ciphers. This mitigation is not applicable to Cisco Application Control Engine (ACE).
  
  For potential workarounds on a specific Cisco product, refer to the relevant Cisco bug, which is available from the Cisco Bug Search Tool.

Fixed Software

• For information about fixed software releases, consult the Cisco bug ID(s) at the top of this advisory. Questions concerning the WebEx environment can be directed to the Cisco Technical Assistance Center (TAC).

  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories and Alerts page, to determine exposure and a complete upgrade solution.

  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco TAC or their contracted maintenance providers.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any malicious use of the vulnerabilities that are described in this advisory.

Source

• Cisco would like to thank Hanno Böck, Juraj Somorovsky of Ruhr-Universität Bochum/Hackmanit GmbH, Craig Young of Tripwire VERT, and Dirk Wetter of testssl.sh/Dr. Wetter IT-Consulting for reporting these vulnerabilities.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20171212-bleichenbacher

Revision History

• Version	Description	Section	Status	Date
  1.6	Updated information for vulnerable products. Added CVE-2017-15533. Modified CVE-2017-17428 description. Changed Document Status to Final.	Vulnerable Products, Details	Final	2018-May-18
  1.5	Updated information about fixed release availability for Cisco WebEx products. Updated information for nonvulnerable products.	Vulnerable Products, Products Confirmed Not Vulnerable	Interim	2018-January-16
  1.4	Updated information about fixed release availability for Cisco ASA devices.	Vulnerable Products	Interim	2018-January-08
  1.3	Updated information for vulnerable and nonvulnerable products and products under investigation. Modified CVE-2017-12373 description.	Affected Products, Vulnerable Products, Products Confirmed Not Vulnerable, Details	Interim	2017-December-21
  1.2	Updated information for nonvulnerable products.	Products Confirmed Not Vulnerable	Interim	2017-December-15
  1.1	Updated information for vulnerable and nonvulnerable products and products under investigation. Added CVE-2017-12373.	Vulnerable Products, Products Confirmed Not Vulnerable, Products Under Investigation, Details	Interim	2017-December-13
  1.0	Initial public release.	?	Interim	2017-December-12

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