Lawrence Sheed // 23 Dec 2010 // 3 Comments

Comment spam is a big load on servers – recently troubleshooting some intermittent load issues on one of our webservers, I discovered that one page was getting hammered by comment spam abuse – the page had already generated over 400,000 comments, which was causing the server to be slightly unhappy loadwise.

After clearing out the comment table for that clients site, I looked into solutions.

mod_security was an obvious one, and the atomic corp rules seem to be better than the default mod_security ones (which break most popular apps, sigh).  However, although the gotroot rules were good at blocking comment spam, they don’t block the ip’s, so persistent spammer bots will still hammer the server.

So, whats a solution?
Blocking the ip’s dynamically for a short period.
Assuming you have a standard setup, the below should be of interest.

I stick my ruleset into /etc/modsecurity2, so amend your url’s accordingly.

My default initialise code is below (pretty much stock except for the end part)

/etc/modsecurity2/modsecurity_crs_10_config.conf

SecRuleEngine On

 SecRequestBodyAccess On

 SecResponseBodyAccess On

 SecResponseBodyMimeType (null) text/html text/plain text/xml

 SecResponseBodyLimit 2621440

 SecServerSignature Apache

 SecComponentSignature 200911012341

 SecUploadDir /var/asl/data/suspicious

 SecUploadKeepFiles Off

 SecAuditEngine RelevantOnly

 SecAuditLogRelevantStatus "^(?:5|4(?!04))"

 SecAuditLogType Concurrent

 SecAuditLog /var/log/apache2/modsec_audit.log

 SecAuditLogParts ABIFHZ

 SecArgumentSeparator "&"

 SecCookieFormat 0

 SecRequestBodyInMemoryLimit 131072

 SecDataDir /var/asl/data/msa

 SecTmpDir /tmp

 SecAuditLogStorageDir /var/asl/data/audit

 SecResponseBodyLimitAction ProcessPartial

SecPcreMatchLimit 100000

SecPcreMatchLimitRecursion 100000

SecDebugLog /var/log/apache2/modsec_debug.log

SecDebugLogLevel 3

### Ruleset additions for blocking ########################################

# Make sure to clear the default action

SecDefaultAction phase:1,pass

# Initialize collection and deprecate by 3 points per day (86400 seconds)

SecAction phase:1,initcol:IP=%{REMOTE_ADDR},deprecatevar:IP.spam=3/86400,nolog

# If there are already >15 spam points for this IP, then drop

# the connection and add 1 point (instead of 3, as below).

SecRule IP:spam "@gt 15" phase:1,setvar:IP.spam=+1,drop,setenv:spam=spam

# Clear the default action for any mod_security rules later in httpd.conf.

SecDefaultAction phase:1,pass

### End Ruleset additions for blocking ####################################

What does it do?  Basically we keep score of any ip address that visits the site.
Every day any scores depreciate by 3 points per ip.
If any ip scores more than 15 points, we block it.

Now, as it stands, its pretty useless, as we don’t do any scoring.

So, next we’ll need to add some scoring to some of the rules.

In our case, we were getting seriously hammered by pharmacy spam (mostly out of latvia)

So, I went into the default gotroot ruleset that I installed into /etc/modsecurity2

Comment spam is in this file: 30_asl_antispam.conf

I added some scoring there.
In my case, we saw hundreds of thousands of attempts for levitra and tramadol spam, so I went to the pharmacy scoring, and added this:

setvar:IP.spam=+15 in the SecRule check.
This means that any positive lookup for that rule will give the ip a 15 point score.
That will block that attacker ip from the site for 5 days (as we decrement 3 points per day for a given ip).

The setvar needs to go after the “capture,…” part of the rule.

eg

(don’t copy, paste this, you’ll need to add “setvar:IP.spam=+15″ into your existing file)
# Rule 300040:
SecRule ARGS|!ARGS:/domain/|!ARGS:description|!ARGS:redirect_to|!ARGS:setting[banemail]|!ARGS:/username/|!ARGS:/user_name/|!ARGS:/page_content/|!ARGS:/search/|!ARGS:/email/!ARGS:Mensaje|!ARGS:/product/|!ARGS:/domain/|!ARGS:description$
"capture,setvar:IP.spam=+15,id:300040,rev:7,severity:2,msg:'Atomicorp.com - FREE UNSUPPORTED DELAYED FEED - WAF Rules: Spam: Pharmacy',logdata:'%{TX.0}'"

I added this to appropriate places, and watched for our first victim^Mspammer in our site logs.
Bingo – here we have a spammer trying to add to a url on one of our sites:

[23/Dec/2010:11:39:45 +0800] [www.chou.cn/sid#bcf3f338][rid#b339b670][/gallery/papratiti/img_5993.jpg.php][1] Access denied with code 403 (phase 2). Pattern match “(?:buy[-_ ]?(cheap)?[-_ ]?(?:adipex|suboxone|pseudovent|topamax|trazodone|prevacid|zyrtec|xenical|toprol|zoloft…” at ARGS:comment. [file "/etc/modsecurity2/30_asl_antispam.conf"] [line "194"] [id "300061"] [rev "10"] [msg "Atomicorp.com - FREE UNSUPPORTED DELAYED FEED - WAF Rules: Spam: Pharmacy"] [data "xanax"] [severity "CRITICAL"]

[23/Dec/2010:11:43:16 +0800] [www.chou.cn/sid#bcf3f338][rid#bfac9d68][/gallery/papratiti/img_5993.jpg.php][1] Access denied with connection close (phase 1). Operator GT matched 15 at IP:spam. [file "/etc/modsecurity2/modsecurity_crs_10_config.conf"] [line "40"]

Bam, that ip is blocked (for 5 days).  Next time around, as its the first rule to run, it will block immediately without processing other rules.

You can amend the block times by decreasing the increments or scoring as per your requirements.

All in all, this is an easy amendment to make to the gotroot rules, and makes for faster experience for users.
Thanks to http://linux.icydog.net/apache/commentspam.php for his implementation, this is pretty much based off that.

Lawrence