Vulnerability Note VU#905795

OpenSSH fails to properly apply source IP based access control restrictions

Original Release date: 07 Dec 2001 | Last revised: 10 Dec 2001

Overview

OpenSSH is an implementation of the Secure Shell protocol. A user may be able to bypass the IP based access control restriction feature specified in a key when two keys of varying types are specified.

Description

Versions of OpenSSH between 2.5.x - 2.9.x may fail to enforce the IP based access control restriction feature. A user may specify from which IP's a key may be used. They may have several entries for several keys. Expected behavior of this feature can be demonstrated as follows. If the authorized_keys2 file contained an entry for a key A that was an RSA key and restricted to 10.0.0.1 via the "from=" line option and key B was a DSA key and restricted to 10.0.0.2, then key B would not be of any use if compromised unless it was used from the machine with an IP address of 10.0.0.2.

Due to the flaw in this feature, when a user specifies two keys of differing types in their ~/.ssh/authorized_keys2, OpenSSH may fail to apply the proper source IP based access control restrictions specified by the "from=" line. For example, assume key A was an RSA key and restricted to 10.0.0.1 via the "from=" line and key B was a DSA key and restricted to 10.0.0.2. Now assume that key B is compromised. One would expect that key B could only be used from 10.0.0.1. However, since key A is specified on the line immediately before the line containing the entry for the compromised key and is of a different type and "from=", then the intruder can access the network from the IP address of key A (10.0.0.1) using the compromised key B.

Likewise a systems administrator could set up a single authorized_keys2 file and direct the individual users ssh clients to this file via a symbolic link. If the systems administrator kept the file world readable, but not writable, then he could control the contents of the file. In this case, a malicious user could use their key in the same method as described above to bypass any IP restrictions that the systems administrator may have placed on them.

Impact

An attacker with a compromised key, or authorized users can circumvent the security policies and login from IP addresses that are not permitted to access the system.

Solution

This vulnerability is fixed in OpenSSH 2.9.9. Upgrade to a version 2.9.9 or later.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
ConectivaAffected-05 Dec 2001
ImmunixAffected-05 Dec 2001
MandrakeSoftAffected-05 Dec 2001
OpenSSHAffected-05 Dec 2001
Red HatAffected-05 Dec 2001
TrustixAffected-05 Dec 2001
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A

References

Credit

This vulnerability was discovered by the OpenSSH team.

This document was written by Jason Rafail.

Other Information

  • CVE IDs: Unknown
  • Date Public: 27 Sep 2001
  • Date First Published: 07 Dec 2001
  • Date Last Updated: 10 Dec 2001
  • Severity Metric: 0.30
  • Document Revision: 13

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