A slight defect in the truncated HMAC code...
Perry E. Metzger
perry at piermont.com
Tue Jun 10 17:18:47 EDT 2008
>From the No Comment Department:
Date: Tue, 10 Jun 2008 11:01:06 -0400
Message-Id: <TA08-162A.1166 at us-cert.gov>
From: CERT Advisory <cert-advisory at cert.org>
Subject: US-CERT Technical Cyber Security Alert TA08-162A -- SNMPv3 Authentication Bypass Vulnerability
National Cyber Alert System
Technical Cyber Security Alert TA08-162A
SNMPv3 Authentication Bypass Vulnerability
Original release date: June 10, 2008
Last revised: --
* Multiple Implementations of SNMPv3
A vulnerability in the way implementations of SNMPv3 handle specially
crafted packets may allow authentication bypass.
The Simple Network Management Protocol (SNMP) is a widely deployed
protocol that is commonly used to monitor and manage network devices.
SNMPv3 ( RFC 3410) supports a user-based security model (RFC 3414)
that incorporates security features such as authentication and privacy
control. Authentication for SNMPv3 is done using keyed-hash message
authentication code (HMAC), a message authentication code calculated
using a cryptographic hash function in combination with a secret key.
Implementations of SNMPv3 may allow a shortened HMAC code in the
authenticator field to authenticate to an agent or a trap daemon using
a minimum HMAC of one byte. Reducing the HMAC to one-byte HMAC makes
brute-force authentication trivial. This issue is known to affect
Net-SNMP and UCD-SNMP. Other SNMP implementations may also be
This vulnerability allows attackers to read and modify any SNMP object
that can be accessed using the authentication credentials that got
them into the system. Attackers exploiting this vulnerability can view
and modify the configuration of these devices. Attackers must gain
access using credentials with write privileges in order to modify
Please consult your vendor for more information.
Apply a patch
Net-SNMP has released a patch to address this issue. For more
information, refer to SECURITY RELEASE: Multiple Net-SNMP Versions
Released. Users are encouraged to apply the patch as soon as possible.
Note that patch should apply cleanly to UCD-snmp too.
Enable the SNMPv3 privacy subsystem
The configuration should be modified to enable the SNMPv3 privacy
subsystem to encrypt the SNMPv3 traffic using a secret, private key.
This option does not encrypt the HMAC, but does minimize the possible
affects from this vulnerability.
* RFC 3410 - <http://tools.ietf.org/html/rfc3410>
* RFC 3414 - <http://tools.ietf.org/html/rfc3414>
* SECURITY RELEASE: Multiple Net-SNMP Versions Released -
* US-CERT Vulnerability Note -
The most recent version of this document can be found at:
Feedback can be directed to US-CERT Technical Staff. Please send
email to <cert at cert.org> with "TA08-162A Feedback VU#878044" in the
For instructions on subscribing to or unsubscribing from this
mailing list, visit <http://www.us-cert.gov/cas/signup.html>.
Produced 2008 by US-CERT, a government organization.
June 10 2008: Initial release
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