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Comment by tptacek | Hacker News Reader

There's this person you are thinking of who is actually vulnerable to something an SSH scanner is looking for. What I'd like to know is, what is that person reasonably going to do when his portscan detector goes off? Why didn't he do it a month ago instead of waiting?

I hope my comments here are of the nature of "My mind is boggling as to why smart people think these measures are anything but laetrile." Also: do you think I'm just making it up? Don't be passive-aggressive. If you think I'm full of it on this topic, just say so.

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> There's this person you are thinking of who is actually vulnerable to something an SSH scanner is looking for.

The point is that if a vulnerability gets discovered in future, he doesn't want to be on a list of "servers that are now vulnerable".

When he hears about the exploit, he'll fix it. But if the bad guys hear about it first, not being on that list is useful obscurity.

> If you think I'm full of it on this topic, just say so.

You're probably not full of it, but it's hard to extract much value from your comments.

Once every ~4-6 years or so someone will find a vulnerability in OpenSSH. When that happens, the list of vulnerable servers is "everyone running SSH". Maybe there's some attacker that is keeping a database of every SSH server on the Internet, but I doubt it.

I don't think it's hard to extract value from my comments. I think you don't want the value that's in my comment, because that value is "you are spending time and energy doing something that isn't providing the security benefit you think it is", and that's not fun to hear.

You don't think an attacker would hold on to the results of scanning for SSH servers? They just scan for days then throw it all away?

I had absolutely no idea what you were trying to say with your first reply to me. It's only by digging through lots of your other replies that I got the hint that it may be an attempt at sarcasm, but I'm still not sure. You are not supporting your assertions, or even making your assertions clear.

Yes. They scan for days and throw away all the results that aren't "machine I can log into".

If a company with a large and complex web property, e.g. Microsoft.com or Google.com, asked you if it mattered whether they allow DNS AXFR, what would your answer be?

Nobody enables AXFR. AXFR isn't a default. I wouldn't advise someone to override that default.

If one of my clients then asked me, "because I don't enable AXFR, is wide-open-dev-machine.customer.com safer?", I would say "no, it is not".

So you agree that disabling AXFR, if AXFR is enabled, is a good idea?

Could you explain why you think wide-open-dev-machine.customer.com is NOT safer without AXFR (assuming it blocks robots, and the web server only serves the site for "wide-open-dev-machine.customer.com")?

I'm just trying to understand how it could not be considered beneficial in any way, however much (which is what you're claiming).

I don't have to think about this much because nobody has AXFR enabled. If you had AXFR enabled, I would in fact tell you to turn it off. It isn't an operational win (unlike SSH on port 22), so what possible reason could there be for enabling it?

wide-open-dev-machine isn't safer because there's a whole variety of ways to learn about it without AXFR.

For what it's worth, I would not flag a customer for running SSH on port 22; I would certainly flag a customer for having port 22 exposed where it didn't need to be; for most SaaS/ASP-type companies, I'd flag them for having more than one SSH port exposed, and I'd recommend that they invest in a VPN instead of exposed SSH.

I think there are far more nameservers allowing AXFR than you think. Write a little app that attempts a zone transfer for the top 1000 of the Alexa list (http://s3.amazonaws.com/alexa-static/top-1m.csv.zip) and see for yourself. (Note to anyone reading: Please don't try this before you've checked if it's legal in your state/country or not.)

Could you explain how you would learn about wide-open-dev-machine as simply as by using AXFR in your scenario (you aren't on the network, so no *casting).

Edit: "so what possible reason could there be for enabling it?" is eerily similar to my message :)

Here's your answer:

    imageshack.com
    about.com
    ehow.com
    sparkstudios.com 
    yfrog.com
    liveinternet.ru
    pgmediaserve.com
    wikimedia.com
    adfly.com
    thefreedictionary.com
    amung.us
    btjunkie.com
    bluehost.com
    drtuber.com
    ero-advertising.com
    autohome.com.cn
    exblog.jp
    inetglobal.com
    milliyet.com.tr
    imagebam.com
    naukri.com
    bigpoint.com
    altervista.com
    hypergames.com
    gsmarena.com
    excite.co.jp
    admagnet.com
    macrumors.com
    linkbucks.com
    cracked.com
    traidnt.com
    radikal.ru
    paper.li
    eluniversal.com.mx
    wiktionary.org
    indianrail.gov.in
    docin.com
    123rf.com
    perezhilton.com
    mangafox.com
    hostmonster.com
    myfreecams.com
    giveawayoftheday.com
    ultimate-guitar.com
    itpro1.nikkeibp.co.jp
    pantip.com
    hawaaworld.com
    120ask.com

4.8%. That's more than I would have thought, but it's still a small number: 95.2% of the Alexa 1000 keep AXFR disabled. Also: someone should tell some of these people not to do this.

Indeed. Keep in mind it's the top, so these are the people you really expect to keep something like that hidden. I promise you the stats are much worse further down the list, or if you do the same, but for a specific country's TLD.

> When that happens, the list of vulnerable servers is "everyone running SSH".

Agreed, but in between "bad guys discover vulnerability" and "he fixes vulnerability", can't he reduce the chance that he will be attacked by having SSH on a different port? It won't stop someone who's targeting him, but it might keep him off the radar of someone who's just looking for easy targets. From another post:

> "32,000" sounds like a large number on a message board, but write an evented port scanner† and see how big that number really is.

So is your claim that anyone looking for vulnerable SSH servers will scan every port they can on a given IP, rather than just looking at port 22 and moving on?

> I don't think it's hard to extract value from my comments.

With respect, you are not the best judge of this. (I personally don't administer a server; if anything, I want you to be correct because that will save me effort if I ever do.) When I read your posts, I learn your opinion, but I often can't work out why you hold that opinion.

Possibly you just write your posts under assumptions that seem obvious to you, but not to others.

One assumption I may have that needs to be stated: OpenSSH vulnerabilities are infrequent, bordering on rare. The discovery of a remote auth bypass or remote code exec flaw in OpenSSH is a big deal; a "drop everything and block all access to port 22" deal.

Another assumption: the gap between the rumor of an OpenSSH remote and the time it hits "people so dumb as to burn the exploit on a scanner" is large; probably weeks, possibly months.

Another assumption: anyone running OpenSSH has turned off the dumb stuff, like password authentication. If you are using passwords to log into SSH servers, you have problems port numbers can't fix.

The only attacker SSH-on-port-51122 defends against is the one so far back in line for SSH exploits that she'll stick it on the business end of an Internet-wide scanner. That vulnerability is worth many tens of thousands of dollars. Nobody that has it in the window between announce and "public downloadable exploit" is scanning with it.

Finally, you really should only have one machine exposed to SSH, if that. If you have to manage 50 servers, set up 1 bastion server and relay through it.

Auto-ban the source of the portscanning, and limit what the now-known-hostile source IP can do next.

This person is probably doing nothing more than an automated nightly security update. Someone running an exploit against SSH which may work is bad, someone unable to run an exploit against SSH because they didn't find it before being blocked by an intrusion detection system is better.

And, because it wasn't scanning a month ago.

They are of that "lol, dumbasses" nature. That's annoying and unhelpful. The submission is an article where the author explains some specific thoughts on some specific potential security measures, your replies are of the nature "lol no; ha ha nope; wrong again". I don't claim your view is wrong, but neither are you giving any of us mere mortals anything more to go on than trusting your reputation.

Great, "SHA-512 is plutonium", that's gonna help me what ... be afraid of it? "None of these security measures do anything to improve security"? Well that's literally not true, so are you merely claiming that they do to little to be worth it? Or what?

"People do things they think are heplful but which aren't helpful so we feel less powerless"? No, we do them because we think they help. That's just an ad-hom.

Changing the SSH port to a random one makes a scan to find it 32,000 times more effort, on average. The author claims that makes it too expensive for people to scan for. You claim it's "obvious" where you put it so it doesn't help. Well it isn't obvious where you put it. It just isn't. You do have to scan for it.

1. Your comments seems premised on the notion that OpenSSH vulnerabilities are routine. They are not routine. They are not hard to keep up with. And if you do not keep up with them, nothing you do is going to protect your server. I am reading tea leaves to arrive at this conclusion about your mindset, yes, but it's still worth pointing out.

2. Blocking sources is a silly way to defend against a server. Anyone who is actually using SSH exploits to break into servers has lots of sources. Meanwhile: anything a scanner is looking for, you're not vulnerable to; if you were, you're already owned. All blocking the source does is make you feel btter.

3. Yes, you should be afraid of using SHA-512. Yes, that is the point of that comment.

4. "32,000" sounds like a large number on a message board, but write an evented port scanner† and see how big that number really is.

† Programming Challenge Level: 3 (Casual Comment On Hacker News - http://news.ycombinator.com/item?id=2317547)*.

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