[Snort-sigs] LDAPv3 with simple authentication

FOULDE Damien damien.foulde at ...4205...
Wed Jan 25 12:54:47 EST 2017


Hello,

 

I finally wrote two working rules through a full decoding of the LDAP BER data :

alert tcp any any -> any 389 (sid:1000000; gid:1; flow:established,to_server; content:"|30|"; depth:1; byte_test:1,=,0,0,relative,bitmask 0x80; content:"|02|"; distance:1; within:1; byte_test:1,=,0,0,relative,bitmask 0x80; byte_jump:1,0,relative,bitmask 0x7f; content:"|60|"; distance:0; within:1; byte_test:1,=,0,0,relative,bitmask 0x80; content:"|02 01 03 04|"; fast_pattern; distance:1; within:4; byte_test:1,=,0,0,relative,bitmask 0x80; byte_jump:1,0,relative,bitmask 0x7f; content:"|80|"; distance:0; within:1; msg:"LDAPv3 simple Authentication SSSS"; classtype:policy-violation; rev:1;)

 

alert tcp any any -> any 389 (sid:1000001; gid:1; flow:established,to_server; content:"|30|"; depth:1; byte_test:1,=,1,0,relative,bitmask 0x80; byte_jump:1,0,relative,bitmask 0x7f; content:"|02|"; distance:0; within:1; byte_test:1,=,0,0,relative,bitmask 0x80; byte_jump:1,0,relative,bitmask 0x7f; content:"|60|"; distance:0; within:1; byte_test:1,=,1,0,relative,bitmask 0x80; byte_jump:1,0,relative,bitmask 0x7f; content:"|02 01 03 04|"; fast_pattern; distance:0; within:4; byte_test:1,=,0,0,relative,bitmask 0x80; byte_jump:1,0,relative,bitmask 0x7f; content:"|80|"; distance:0; within:1; msg:"LDAPv3 simple Authentication LSLS"; classtype:policy-violation; rev:1;)

 

I’ve some difficulties to write other ones as I just noticed that a value extracted through “byte_extract” can only be supplied to the “offset” argument of the “byte_jump” rule keyword and not to the “bytes_to_convert” argument.

 

For example I have these bytes and I need to check the 0x80 byte :

82 00 05 12 24 56 78 12 80

0x82 = 10000010

The MSB is set to 1, so the value of the 7 other bits is not the length of the data but the number of bytes used to describe the length of the data, in this example, the number of bytes to describe the length of the data is 0000010 = 2

We can get this value through “byte_extract:1,0,var_length,relative,bitmask 0x7f;”.

Then we would need to get the “00 05” = 5 value, to jump over the 5 following bytes : “12 24 56 78 12” and finally test the 0x80 content we need to check.

This could be achieved through “byte_jump:long_length,0,relative;” if the “byte_jump” rule keyword would accept an extracted value for the “bytes_to_convert” argument, unfortunately this is not the case.

How should we proceed ?

Any help would be really appreciated.

 

If someone from Talos could take some time to review the simpler rule provided previously it would be great.

I think that detecting / blocking all LDAP simple authentication, with cleartext password over the network, could benefit to everyone security.

 

Thank you & regards,

 

Damien

 

De : FOULDE Damien [mailto:damien.foulde at ...4205...] 
Envoyé : mercredi 4 janvier 2017 17:36
À : snort-sigs at lists.sourceforge.net
Objet : Re: [Snort-sigs] LDAPv3 with simple authentication

 

Hello,


You’ll find a packet capture attached to this email.

It would be really great if the signature provided previously could be reviewed.

 

Thank you & regards,


Damien

 

De : FOULDE Damien [mailto:damien.foulde at ...4205...] 
Envoyé : jeudi 29 décembre 2016 16:37
À : snort-sigs at lists.sourceforge.net
Objet : Re: [Snort-sigs] LDAPv3 with simple authentication

 

Hello,

 

The previous message is currently waiting for a moderator approval.

As it’s not released for the moment, I registered to the maillinglist and here it is.

 

Regards,

 

Damien

 

De : FOULDE Damien 
Envoyé : mercredi 21 décembre 2016 10:51
À : snort-sigs at lists.sourceforge.net
Objet : RE: LDAPv3 with simple authentication

 

Hello,

 

Any ideas / suggestions / advices will be greatly appreciated regarding this question.

 

In the meantime, here’s a working signature without fully decoding the BER data :

alert tcp any any -> any 389 (sid:1000000; gid:1; flow:established,to_server; content:"|30|"; depth:1; content:"|02|"; distance:1; within:127; content:"|60|"; distance:1; within:5; content:"|02 01 03 04|"; fast_pattern; distance:1; within:127; content:"|80|"; distance:1; within:127; content:!"|02 01 03 04 00 a3|"; offset:7; depth:257; msg:"LDAPv3 simple Authentication"; classtype:policy-violation; rev:1; )

 

It would be great if it could be reviewed by Talos.

I can provide a packet capture if needed.

 

Regards,

 

Damien

 

De : FOULDE Damien [mailto:damien.foulde at ...4205...] 
Envoyé : lundi 19 décembre 2016 12:39
À : snort-sigs at lists.sourceforge.net
Objet : [Snort-sigs] LDAPv3 with simple authentication

 

Hello,

 

We need to write a signature to match on LDAPv3 with simple authentication.

LDAPv3 is described in the RFC 2251 through Abstract Syntax Notation 1 (ASN.1) and encoded through a subset of Basic Encoding Rules (BER) in the packets.

You may have a look to this great website http://www.selfadsi.org/ldap.htm#Frame to have a quick look over the encoding.

https://en.wikipedia.org/wiki/X.690#BER_encoding is also a good source of information.

As you should have seen the length can be encoded in a short or long form.

When the short form is used the MSB is set to 0 and the 7 remaining bits are used to encode the length directly from 0 to 127.

Using the byte_jump function we should be able to jump to the next encoded data.

When the long form is used the MSB is set to 1 and the 7 remaining bits are used to encode the number of bytes that follow from 1 to 126 which will contains the actual length.

Using byte_extract and byte_jump functions with bitmask we should be able to jump to the next encoded data.

Before reaching the point where the LDAPv3 authentication is set to simple (encoded to 0) or sasl (encoded to 3) there’re 5 short or long length bytes.

Is there a way through the subset of snort packet dissection functions to match on this without writing 32 (2^5) different signatures to match all short / long possibilities ?

The BER encoding is also used to encode SNMP, the same kind of issue may have been seen there also.

 

Thank you for your help,

 

Damien

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