Nessus Scan Showing SSLv3 on NS Client

Posted on by Jordansphere

Problem

During a Nessus scan of a network it was showing lots of insecurities with an NSClient configuarion on a windows machine we were monitoring via Nagios. 

OS:           Windows 2016
NSClient:     0.3.x
Nagios:       4.0.7

 

Looking at one particular issue (SSLv2 and SSLv3) showing on the report:

nessus1

Nessus Output:

The remote service accepts connections encrypted using SSL 2.0 and/or SSL 3.0. These versions of SSL are affected by several cryptographic flaws, including:

– An insecure padding scheme with CBC ciphers.

– Insecure session renegotiation and resumption schemes.

An attacker can exploit these flaws to conduct man-in-the-middle attacks or to decrypt communications between the affected service and clients.

Although SSL/TLS has a secure means for choosing the highest supported version of the protocol (so that these versions will be used only if the client or server support nothing better), many web browsers implement this in an unsafe way that allows an attacker to downgrade a connection (such as in POODLE). Therefore, it is recommended that these protocols be disabled entirely.

NIST has determined that SSL 3.0 is no longer acceptable for secure communications. As of the date of enforcement found in PCI DSS v3.1, any version of SSL will not meet the PCI SSC's definition of 'strong cryptography'.

Solution

Consult the application's documentation to disable SSL 2.0 and 3.0.
Use TLS 1.1 (with approved cipher suites) or higher instead.

See Also

https://www.schneier.com/academic/paperfiles/paper-ssl.pdf
http://www.nessus.org/u?0bb7b67d
http://www.nessus.org/u?247c4540
https://www.openssl.org/~bodo/ssl-poodle.pdf
http://www.nessus.org/u?5d15ba70
https://www.imperialviolet.org/2014/10/14/poodle.html
https://tools.ietf.org/html/rfc7507
https://tools.ietf.org/html/rfc7568

Output
  • - SSLv3 is enabled and the server supports at least one cipher.
    Port Hosts
    5666 / tcp

    10.0.200.2

 

Resolution:

Our Nagios server connects to the NS Client on the Windows machine via 12489. Port 5666 is not required. However NRPE listener was seemingly started by default. As we did not require this port to be listening we just stopped the NRPE service from starting:

1) Open the nsclient/NCSA config file (eg C:\Program Files\NSClient++\NSC) and comment out the NRPEListener.dll like so

[modules]
;NRPEListener.dll
NSClientListener.dll
FileLogger.dll

 

Note: in later versions of the NSClient you can just disable the SSL values but as we didnt even require NRPE to be listening we simply stopped the service.