RE: CVE selection for IDS/IPS signature rules

["Srinivasa Addepalli" <srao () intoto com>]

You got very good answers from Ron. I try to give some specifics.

1. Generic signatures

There are close to 10000 XSS and SQL injection vulnerabilities (based on
search in www.osvdb.org).  Some IPS/IDS vendors, including us, don't create
signatures for each one of them.  We are able to cover them using 200+
signatures which are generic in nature. 

IPS systems having intelligent application detection may cover many buffer
overflow attacks using few signatures. For example,  we see many HTTP URL,
HTTP request header/response header field, SMTP/FTP/IMAP/NNTP command buffer
overflow attacks. Many of them can be detected with few signatures without
having to develop rules for each CVE.

2. Signature deletion to improve IPS/IDS performance.  This is one of the
reasons you could see some discrepancy between CVE IDs and signatures.

Some vendors tend to delete very old signatures.  Deciding which signatures
to delete is a painful process. Some easier decisions are ones specific to
executing the local applications via malformed java script of pages related
to popular web sites. Once these web sites fix the issue, there is no need
for these signatures.

3. Vulnerabilities which can only be exploited after authentication.  Some
vendors tend to give lower priority for these vulnerabilities. 

4. Vulnerabilities related to services which are typically accessed by other
machines within same network (within one administrative domain).  Some
examples are LDAP and RADIUS servers. These are typically accessed by other
servers within the network, that is, these services are not exposed to wider
network.  Again some vendors tend to give lower priority for these
vulnerabilities.

5. Client side attacks requiring deeper data inspection:  For network
IDS/IPS, it becomes very difficult to develop signatures (requiring deeper
data inspection) which can detect without any false positives and negatives.
File based attacks is one example, where when the file is opened, client
application either crashes or malfunctions. Difficulty of signature
development arise from different methods to get the files (email, http
etc..) and encoding mechanisms used. Due to these difficulties, you might
not see signatures for some of these attacks. (One example: CVE-2008-0105)

6. Lack of information on vulnerabilities:  Yet times, you see some time
difference between disclosure and signature due to this. 

Srini

-----Original Message-----
From: listbounce () securityfocus com [mailto:listbounce () securityfocus com] On
Behalf Of Ron Gula
Sent: Thursday, May 29, 2008 11:36 AM
To: Focus IDS
Subject: Re: CVE selection for IDS/IPS signature rules

Ravi Chunduru wrote:
> Hi,
>
> There are over 30000 CVE vulnerability reports.  Many IDS/IPS devices
> have around 4000-5000 signature rules. My guess is that these
> signatures may cover (detect)around 4000-7000 attacks.  23000 to 26000
> CVEs, that is, significant number of CVEs are not covered by IDS/IPS
> devices.
>
> I am guessing that there is reason for this. IDS/IPS vendors may be
> selecting few CVEs for developing signatures. What is the selection
> criteria followed in industry? One criteria, I know is that Network
> IDS/IPS devices don't need to worry about attacks that can only be
> mounted on the local machine, that is,  NIDS/NIPS devices only need to
> worry about detection of attacks mounted remotely. Are there any other
> considerations?
>
> Thanks
> Ravi


Hi Ravi,

There are several reasons, probably more.

Some NIDS vendors try to code for generic exploit vectors and not
specific vulnerabilities. Some try to do both.

Many of the CVEs not covered are for products that have come and
gone, are very old, don't work over TCP/IP and so on.

Some CVE entries focus on weak encryption and denial of service
attacks which can be difficult to see with NIDS technology.

Ron Gula
Tenable Network Security
http://www.nessus.org









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