DoS and NTLM Brute force protection for HTTP(s) traffic

Problem this snippet solves:

This snippet has been designed to mainly protect against NTLM's Denial of Service and brute force attacks against web application that use this authentication mecanism.

This irule help companies to fight against brute force attacks at the HTTP layer. You can combine this irule with another one on non-http traffic to provide a brute force protection across multiple layers.

For pure HTTP application, you should prefer the Brute force protection provided by the ASM module.

How to use this snippet:

This irule should be installed on each Virtual Server that require NTLM protection. In a Microsoft Skype for Business deployment, you may need to protect following services :

  • Web Services
  • Conf
  • Autodiscover
  • Exchange Web Services

TO BE DONE : Provide a way to unlock blocked users

External links

Github :

Related Articles


Code :

when RULE_INIT {
    array set NTLMFlags {
        unicode        0x00000001
        oem            0x00000002
        req_target     0x00000004
        unknown1       0x00000008
        sign           0x00000010
        seal           0x00000020
        datagram       0x00000040
        lmkey          0x00000080
        netware        0x00000100
        ntlm           0x00000200
        unknown2       0x00000400
        unknown3       0x00000800
        ntlm_domain    0x00001000
        ntlm_server    0x00002000
        ntlm_share     0x00004000
        NTLM2          0x00008000
        targetinfo     0x00800000
        128bit         0x20000000
        keyexch        0x40000000
        56bit          0x80000000
set static::irule_name   "irule-ntlm-bruteforce"
set static::email_domain   ""
set static::user_domain    "DOMAIN"
    set static::log_server     ""
    set static::log_pri        "local0."
    set static::fail_tab       "NTLMfails"
    set static::blacklist_tab  "NTLMblackhole"
    set static::userfail_tab       "NTLMUserfails"
    set static::userblacklist_tab  "NTLMUserblackhole"
    set static::max_failures   5
    set static::fail_memory    300
    set static::block_duration 300


    if {[table lookup -subtable $static::blacklist_tab [IP::client_addr]] == 1} {
        log $static::log_pri "[virtual] - BLACKHOLED IPADDR [IP::client_addr]:[TCP::client_port] (Reputation=[IP::reputation [IP::client_addr]])"


    if {[table lookup -subtable $static::blacklist_tab [IP::client_addr]] == 1} {
        log $static::log_pri "[virtual] - BLACKHOLED IPADDR [IP::client_addr]:[TCP::client_port] (Reputation=[IP::reputation [IP::client_addr]])"

    if {[HTTP::header Authorization] starts_with "NTLM "} {
        set ntlm_msg [ b64decode [split [lindex [HTTP::header Authorization] 1] ] ]
        binary scan $ntlm_msg a7ci protocol zero type
        if { $type eq 3 } {
                binary scan $ntlm_msg @12ssissississississii \
                    lmlen lmlen2 lmoff \
                    ntlen ntlen2 ntoff \
                    dlen  dlen2  doff  \
                    ulen  ulen2  uoff \
                    hlen  hlen2  hoff \
                    slen  slen2  soff \
                set ntlm_domain {}; binary scan $ntlm_msg @${doff}a${dlen} ntlm_domain
                set ntlm_user {};   binary scan $ntlm_msg @${uoff}a${ulen} ntlm_user
                set ntlm_host {};   binary scan $ntlm_msg @${hoff}a${hlen} ntlm_host
                set unicode [expr {$flags & 0x00000001}]
                if {$unicode} {
                    set ntlm_domain_convert ""
                    foreach i [ split $ntlm_domain ""] {
                        scan $i %c c
                        if {$c>1} {
                            append ntlm_domain_convert $i
                        } elseif {$c<128} {
                            set ntlm_domain_convert $ntlm_domain_convert
                        } else {
                            append ntlm_domain_convert \\u[format %04.4X $c]
                    set ntlm_domain $ntlm_domain_convert
                    set ntlm_user_convert ""
                    foreach i [ split $ntlm_user ""] {
                        scan $i %c c
                        if {$c>1} {
                            append ntlm_user_convert $i
                        } elseif {$c<128} {
                            set ntlm_user_convert $ntlm_user_convert
                        } else {
                            append ntlm_user_convert \\u[format %04.4X $c]
                    set ntlm_user   $ntlm_user_convert
                    set ntlm_host_convert ""
                    foreach i [ split $ntlm_host ""] {
                        scan $i %c c
                        if {$c>1} {
                            append ntlm_host_convert $i
                        } elseif {$c<128} {
                            set ntlm_host_convert $ntlm_host_convert
                        } else {
                            append ntlm_host_convert \\u[format %04.4X $c]
                    set ntlm_host   $ntlm_host_convert
                binary scan $ntlm_msg @${ntoff}a${ntlen} ntdata
                binary scan $ntlm_msg @${lmoff}a${lmlen} lmdata
                binary scan $ntdata H* ntdata_h
                binary scan $lmdata H* lmdata_h
                set interesting 1

if { ($ntlm_domain equals $static::user_domain or [string tolower $ntlm_user] ends_with $static::email_domain) } {
set attack 1
if {[table lookup -subtable $static::userblacklist_tab $ntlm_user] == 1} {
# Block ntlm_user exceeding the number of failed logons in the timeout period
log $static::log_pri "[virtual] - BLACKHOLED $ntlm_domain\\$ntlm_user from $ntlm_host at [IP::client_addr]:[TCP::client_port] (Reputation=[IP::reputation [IP::client_addr]])"
} else {
log $static::log_pri "[virtual] - Login attempt by $ntlm_domain\\$ntlm_user from $ntlm_host for [HTTP::uri]."
} else {
set attack 0
log $static::log_pri "[virtual] - Not a valid user - Login attempt by $ntlm_domain\\$ntlm_user from $ntlm_host for [HTTP::uri]."

                return [list type $type flags [format 0x%08x $flags] \
                    ntlm_domain $ntlm_domain ntlm_host $ntlm_host ntlm_user $ntlm_user \
                    lmhash $lmdata nthash $ntdata]
   if {[info exists interesting] && $interesting == 1} {
        set client [IP::client_addr]:[TCP::client_port]
        set node [IP::server_addr]:[TCP::server_port]
        set nodeResp [HTTP::status]

        if { $nodeResp == 401 and ([info exists attack] and $attack == 1)} {
log $static::log_pri "[virtual] - invalid credentials detected for $ntlm_user"
            table set -subtable $static::fail_tab -notouch -excl [IP::client_addr] 0 indef $static::fail_memory
            table incr -subtable $static::fail_tab [IP::client_addr]

            if {[table lookup -subtable $static::fail_tab [IP::client_addr]] >= $static::max_failures} {
                set now [clock seconds]
                set now_date [split [clock format $now -format {%X %x}] " "]

set later [expr {$now + $static::block_duration}]
set later_date [split [clock format $later -format {%X %x}] " "]

                log $static::log_pri "[virtual] - BLACKHOLING IPADDR - [IP::client_addr] (Reputation=[IP::reputation [IP::client_addr]]) at $now_date until $later_date"
                table set -subtable $static::blacklist_tab -excl [IP::client_addr] 1 indef $static::block_duration

if {[info exists ntlm_user]} {
table set -subtable $static::userfail_tab -notouch -excl $ntlm_user 0 indef $static::fail_memory
table incr -subtable $static::userfail_tab $ntlm_user

if {[table lookup -subtable $static::userfail_tab $ntlm_user] >= $static::max_failures} {
set now [clock seconds]
set later [expr {$now + $static::block_duration}]
log $static::log_pri "[virtual] - BLACKHOLING USER - $ntlm_user at $now_date until $later_date"
table set -subtable $static::userblacklist_tab -excl $ntlm_user 1 indef $static::block_duration


Tested this on version:

Published Jun 03, 2016
Version 1.0

Was this article helpful?


  • Hi Yann,


    I have a customer with Skype for Business and they are worried about brute-force attacks on logins so this snippet is perfect. I have added the i-rule to the reverse proxy VS and when I log in with incorrect credentials I see a syslog 'invalid credentials detected for ' sent but I never see the IP blacklisted. Can you advise why this may not be working. I'm on Thanks, Antony


  • Hi Antony,

    It looks like you are not using NTLM authentication on your backend. The following line of code log an invalid authentication attempt for an ntlm user:

    log $static::log_pri "[virtual] - invalid credentials detected for $ntlm_user"

    If you are using different kind of authentication than NTLM, you should change this code snippet to match your needs

    Hope it helps