CERTVU:958563SSH CBC vulnerability
Overview
A vulnerability exists in SSH messages that employ CBC mode that may allow an attacker to recover plaintext from a block of ciphertext.
Description
The Secure Shell (SSH) is a network protocol that creates a secure channel between two networked devices in order to allow data to be exchanged. SSH can create this secure channel by using Cipher Block Chaining (CBC) mode encryption. This mode adds a feedback mechanism to a block cipher that operates in a way that ensures that each block is used to modify the encryption of the next block.
SSH contains a vulnerability in the way certain types of errors are handled. Attacks leveraging this vulnerabilty would lead to the loss of the SSH session. According to CPNI Vulnerability Advisory SSH:
If exploited, this attack can potentially allow an attacker to recover up to 32 bits of plaintext from an arbitrary block of ciphertext from a connection secured using the SSH protocol in the standard configuration. If OpenSSH is used in the standard configuration, then the attacker’s success probability for recovering 32 bits of plaintext is 2^{-18}. A variant of the attack against OpenSSH in the standard configuration can verifiably recover 14 bits of plaintext with probability 2^{-14}. The success probability of the attack for other implementations of SSH is not known.
Impact
An attacker may be able to recover up to 32 bits of plaintext from an arbitrary block of ciphertext.
Solution
We are currently unaware of a practical solution to this problem.
Use CTR Mode
SSH can be done using Counter (CTR) mode encryption. This mode generates the keystream by encrypting successive values of a “counter” function. For more information see the Block Cipher Modes article on wikipedia.
In order to mitigate this vulnerabilty SSH can be setup to use CTR mode rather CBC mode. According to CPNI Vulnerability Advisory SSH:
_The most straightforward solution is to use CTR mode instead of CBC mode, since this renders SSH resistant to the attack. An RFC already exists to standardise counter mode for use in SSH (RFC 4344) … _
Vendor Information
958563
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Bitvise Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
FiSSH Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
Icon Labs Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
OSSH Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
OpenSSH Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
PuTTY __ Affected
Notified: November 07, 2008 Updated: January 05, 2009
Status
Affected
Vendor Statement
The latest release (0.60) of PuTTY will always preferentially select CTR-mode ciphers over CBC-mode, and cannot even be configured by the user to do otherwise. Therefore, it is immune to this vulnerability when talking to any server which supports CTR mode.
Development snapshots of PuTTY beginning with 2008-11-27 will contain a countermeasure which avoids leaking information through this attack even when operating in CBC mode. Future releases of PuTTY will also contain this countermeasure.
(That is, the countermeasures will prevent PuTTY from leaking information about data previously sent from the server to the client. Protecting data sent from client to server, such as passwords, must be done by the server.)
We are currently not treating this vulnerability as severe enough to warrant an emergency security release.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
Redback Networks, Inc. Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
SSH Communications Security Corp Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
TTSSH Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
VanDyke Software __ Affected
Notified: November 07, 2008 Updated: January 12, 2009
Status
Affected
Vendor Statement
VShell® version 3.5.1 and earlier, SecureCRT® version 6.1.2 and earlier, SecureFX® version 6.1.2 and earlier, and VanDyke ClientPack 6.1.2 and earlier are potentially vulnerable to this attack.
Vendor Information
The advisory recommends using the AES cipher in CTR mode rather than CBC mode. VShell for some platforms, SecureCRT, SecureFX, and the VanDyke ClientPack for some platforms now prefer the AES cipher in CTR mode by default. Please see the following web page for more information.
<http://www.vandyke.com/support/advisory/2008/12/cpni-957037.html>
Wind River Systems, Inc. Affected
Notified: November 07, 2008 Updated: November 24, 2008
Status
Affected
Vendor Statement
We have not received a statement from the vendor.
Vendor Information
The vendor has not provided us with any further information regarding this vulnerability.
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CVSS Metrics
| Group | Score | Vector |
|---|---|---|
| Base | ||
| Temporal | ||
| Environmental |
References
- <http://www.cpni.gov.uk/Docs/Vulnerability_Advisory_SSH.txt>
- <http://isc.sans.org/diary.html?storyid=5366>
- <http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation>
Acknowledgements
Thanks to CPNI for reporting this vulnerability.
This document was written by Chris Taschner.
Other Information
| CVE IDs: | None |
|---|---|
| Severity Metric: | 0.30 Date Public: |