Preprocessor Documentation


ASN.1 Detection Capability

Author: Daniel Roelker

The asn.1 detection plugin decodes a packet or a portion of a packet, and looks for various malicious encodings.

The general configuration of the asn.1 rule option is as follows:

asn1: [keyword [argument]], . . .

Multiple keywords can be used in an ‘asn1’ option and the implied logic is boolean OR. So if any of the arguments evaluate as true, the whole option evaluates as true.

ASN.1 Keywords

The ASN.1 keywords provide programmatic detection capabilities as well as some more dynamic type detection. Most of the keywords don’t have arguments as the detection is looking for non-configurable information. If a keyword does have an argument, the keyword is followed by a comma and the argument is the next word. If a keyword has multiple arguments, then a command separates each.


The bitstring_overflow detects invalid bitstring encodings that are known to be remotely exploitable.


The double_overflow detects an double ascii encoding that is larger than a standard buffer. This is known to be an exploitable function in Microsoft, but it is unknown at this time which services may be exploitable.


This detection keyword compares ASN.1 type lengths with the supplied argument. The syntax looks like, “oversize_length 500”. This means that if an ASN.1 type is greater than 500, then this keyword is evaluated as true. This keyword must have one argument and that is the length to compare against.


This is the absolute offset from the beginning of the packet. For example, if you wanted to decode snmp packets, you would say “absolute_offset, 0”.
absolute_offset has one argument and that is the offset. Offset may be positive or negative.


This is the relative offset from the last content match, pcre or byte_jump. relative_offset has one argument and that is the offset number. So if you wanted to start decoding an ASN.1 sequence right after the content “foo”, you would specify ‘content:”foo”; asn1: bitstring_overflow, relative_offset, 0’. Offset may be positive or negative.


alert udp any any -> any 161 (msg:”Oversize SNMP Length”; \ asn1: oversize_length, 10000, absolute_offset, 0;)

alert tcp any any -> any 80 (msg:”ASN1 Relative Foo”; content:”foo”; \ asn1: bitstring_overflow, print, relative_offset, 0;)

Background on ASN.1

The following comments were compiled by Chris Reid circa Feb 2003.

ASN.1 - Abstract Syntax Notation number One

An interesting site about ASN.1 (and its primary author Professor John Larmouth) can be found here: including an electronic copy of Professor Larmouth’s book titled “ASN.1 Complete”.

The following descriptions are copied from the website: “”. I have no affiliation with this website, but reference it here because they provide a large amount of useful information about ASN.1.

“Abstract Syntax Notation number One (ASN.1) is an international standard that aims at specifying data used in communication protocols. It is a powerful and complex language: its features are designed to describe accurately and efficiently communications between homogeneous or heterogeneous systems.”

“ASN.1 is a formal notation used for describing data transmitted by telecommunications protocols, regardless of language implementation and physical representation of these data, whatever the application, whether complex or very simple.

 Abstract Syntax Notation number One
 is a standard that defines a formalism
 for the specification of abstract data types. 

“The notation provides a certain number of pre-defined basic types such as: - integers (INTEGER), - booleans (BOOLEAN), - character strings (IA5String, UniversalString…), - bit strings (BIT STRING), - etc.,

“and makes it possible to define constructed types such as:
- structures (SEQUENCE), - lists (SEQUENCE OF), - choice between types (CHOICE), - etc. “

My own notes about ASN.1, as understood from skimming through Section 3 (“Encodings”) of the book “ASN.1 Complete”…

  • Data in ASN.1 is represented by groupings composed of T-L-V (Type, Length, and Value)

  • Datatypes are identified by the bottom six bits of a single byte. Datatypes values 0 through 30 are represented in a single byte. Value “31” (0x1f) is an escape marker to indicate the datatype value continues in the following bytes – as long as successive bytes have the high bit set.

  • The length of a given datatype can be represented in any of three ways: Short Length, Long Length, and Indefinite Length.

  • Short Lengths are stored in one byte, and can indicate data lengths of up to 127 bytes. Note that Short Lengths are identified by an upper bit of zero.

  • Long Lengths are stored in multiple bytes. The first byte indicates how many bytes are required to contain the Long Length, and the successive bytes contain the value of the Long Length. Note that the first byte of Long Lengths are identified by an upper bit of one, and that only the bottom seven bits of the first byte are used to represent how many bytes are required to contain the Long Length. Also, note that the maximum allowed value of the bottom seven bits is 126 (0x7e), and that 127 (0x7f) is reserved for future use.

  • Indefinite Lengths are conceptually like BLOB data. The upper bit of the first byte is set to one, and the bottom seven bits are zero. The data value follows immediately, and continues until two zero-bytes are encountered.


ASN.1 uses generator ID 115 and can produce the following alerts:

SID Description — ———– 1 Indefinite ASN.1 length encoding 2 Invalid ASN.1 length encoding 3 ASN.1 oversized item, possible overflow 4 ASN.1 spec violation, possible overflow 5 Datum length > packet length