Signatures

Bro relies primarily on its extensive scripting language for defining and analyzing detection policies. In addition, however, Bro also provides an independent signature language for doing low-level, Snort-style pattern matching. While signatures are not Bro’s preferred detection tool, they sometimes come in handy and are closer to what many people are familiar with from using other NIDS. This page gives a brief overview on Bro’s signatures and covers some of their technical subtleties.

Contents

Signatures

Basics

Let’s look at an example signature first:

signature my-first-sig {
    ip-proto == tcp
    dst-port == 80
    payload /.*root/
    event "Found root!"
}

This signature asks Bro to match the regular expression .*root on all TCP connections going to port 80. When the signature triggers, Bro will raise an event signature_match of the form:

event signature_match(state: signature_state, msg: string, data: string)

Here, state contains more information on the connection that triggered the match, msg is the string specified by the signature’s event statement (Found root!), and data is the last piece of payload which triggered the pattern match.

To turn such signature_match events into actual alarms, you can load Bro’s base/frameworks/signatures/main.bro script. This script contains a default event handler that raises Signatures::Sensitive_Signature Notices (as well as others; see the beginning of the script).

As signatures are independent of Bro’s policy scripts, they are put into their own file(s). There are two ways to specify which files contain signatures: By using the -s flag when you invoke Bro, or by extending the Bro variable signature_files using the += operator. If a signature file is given without a path, it is searched along the normal BROPATH. The default extension of the file name is .sig, and Bro appends that automatically when necessary.

Signature language

Let’s look at the format of a signature more closely. Each individual signature has the format signature <id> { <attributes> }. <id> is a unique label for the signature. There are two types of attributes: conditions and actions. The conditions define when the signature matches, while the actions declare what to do in the case of a match. Conditions can be further divided into four types: header, content, dependency, and context. We discuss these all in more detail in the following.

Conditions

Header Conditions

Header conditions limit the applicability of the signature to a subset of traffic that contains matching packet headers. For TCP, this match is performed only for the first packet of a connection. For other protocols, it is done on each individual packet.

There are pre-defined header conditions for some of the most used header fields. All of them generally have the format <keyword> <cmp> <value-list>, where <keyword> names the header field; cmp is one of ==, !=, <, <=, >, >=; and <value-list> is a list of comma-separated values to compare against. The following keywords are defined:

src-ip/dst-ip <cmp> <address-list>: Source and destination address, respectively. Addresses can be given as IP addresses or CIDR masks.
src-port/dst-port <int-list>: Source and destination port, respectively.
ip-proto tcp|udp|icmp: IP protocol.

For lists of multiple values, they are sequentially compared against the corresponding header field. If at least one of the comparisons evaluates to true, the whole header condition matches (exception: with !=, the header condition only matches if all values differ).

In addition to these pre-defined header keywords, a general header condition can be defined either as

header <proto>[<offset>:<size>] [& <integer>] <cmp> <value-list>

This compares the value found at the given position of the packet header with a list of values. offset defines the position of the value within the header of the protocol defined by proto (which can be ip, tcp, udp or icmp). size is either 1, 2, or 4 and specifies the value to have a size of this many bytes. If the optional & <integer> is given, the packet’s value is first masked with the integer before it is compared to the value-list. cmp is one of ==, !=, <, <=, >, >=. value-list is a list of comma-separated integers similar to those described above. The integers within the list may be followed by an additional / mask where mask is a value from 0 to 32. This corresponds to the CIDR notation for netmasks and is translated into a corresponding bitmask applied to the packet’s value prior to the comparison (similar to the optional & integer).

Putting it all together, this is an example condition that is equivalent to dst-ip == 1.2.3.4/16, 5.6.7.8/24:

header ip[16:4] == 1.2.3.4/16, 5.6.7.8/24

Internally, the predefined header conditions are in fact just short-cuts and mapped into a generic condition.

Content Conditions

Content conditions are defined by regular expressions. We differentiate two kinds of content conditions: first, the expression may be declared with the payload statement, in which case it is matched against the raw payload of a connection (for reassembled TCP streams) or of each packet (for ICMP, UDP, and non-reassembled TCP). Second, it may be prefixed with an analyzer-specific label, in which case the expression is matched against the data as extracted by the corresponding analyzer.

A payload condition has the form:

payload /<regular expression>/

Currently, the following analyzer-specific content conditions are defined (note that the corresponding analyzer has to be activated by loading its policy script):

http-request /<regular expression>/: The regular expression is matched against decoded URIs of HTTP requests. Obsolete alias: http.
http-request-header /<regular expression>/: The regular expression is matched against client-side HTTP headers.
http-request-body /<regular expression>/: The regular expression is matched against client-side bodys of HTTP requests.
http-reply-header /<regular expression>/: The regular expression is matched against server-side HTTP headers.
http-reply-body /<regular expression>/: The regular expression is matched against server-side bodys of HTTP replys.
ftp /<regular expression>/: The regular expression is matched against the command line input of FTP sessions.
finger /<regular expression>/: The regular expression is matched against finger requests.

For example, http-request /.*(etc/(passwd|shadow)/ matches any URI containing either etc/passwd or etc/shadow. To filter on request types, e.g. GET, use payload /GET /.

Note that HTTP pipelining (that is, multiple HTTP transactions in a single TCP connection) has some side effects on signature matches. If multiple conditions are specified within a single signature, this signature matches if all conditions are met by any HTTP transaction (not necessarily always the same!) in a pipelined connection.

Dependency Conditions

To define dependencies between signatures, there are two conditions:

requires-signature [!] <id>: Defines the current signature to match only if the signature given by id matches for the same connection. Using ! negates the condition: The current signature only matches if id does not match for the same connection (using this defers the match decision until the connection terminates).
requires-reverse-signature [!] <id>: Similar to requires-signature, but id has to match for the opposite direction of the same connection, compared to the current signature. This allows to model the notion of requests and replies.

Context Conditions

Context conditions pass the match decision on to other components of Bro. They are only evaluated if all other conditions have already matched. The following context conditions are defined:

eval <policy-function>

The given policy function is called and has to return a boolean confirming the match. If false is returned, no signature match is going to be triggered. The function has to be of type function cond(state: signature_state, data: string): bool. Here, content may contain the most recent content chunk available at the time the signature was matched. If no such chunk is available, content will be the empty string. signature_state is defined as follows:

type signature_state: record {
    id: string;          # ID of the signature
    conn: connection;    # Current connection
    is_orig: bool;       # True if current endpoint is originator
    payload_size: count; # Payload size of the first packet
    };

payload-size <cmp> <integer>

Compares the integer to the size of the payload of a packet. For reassembled TCP streams, the integer is compared to the size of the first in-order payload chunk. Note that the latter is not very well defined.

same-ip

Evaluates to true if the source address of the IP packets equals its destination address.

tcp-state <state-list>

Imposes restrictions on the current TCP state of the connection. state-list is a comma-separated list of the keywords established (the three-way handshake has already been performed), originator (the current data is send by the originator of the connection), and responder (the current data is send by the responder of the connection).

Actions

Actions define what to do if a signature matches. Currently, there are two actions defined:

event <string>

Raises a signature_match event. The event handler has the following type:

event signature_match(state: signature_state, msg: string, data: string)

The given string is passed in as msg, and data is the current part of the payload that has eventually lead to the signature match (this may be empty for signatures without content conditions).

enable <string>

Enables the protocol analyzer <string> for the matching connection ("http", "ftp", etc.). This is used by Bro’s dynamic protocol detection to activate analyzers on the fly.

Things to keep in mind when writing signatures

Each signature is reported at most once for every connection, further matches of the same signature are ignored.
The content conditions perform pattern matching on elements extracted from an application protocol dialogue. For example, http /.*passwd/ scans URLs requested within HTTP sessions. The thing to keep in mind here is that these conditions only perform any matching when the corresponding application analyzer is actually active for a connection. Note that by default, analyzers are not enabled if the corresponding Bro script has not been loaded. A good way to double-check whether an analyzer “sees” a connection is checking its log file for corresponding entries. If you cannot find the connection in the analyzer’s log, very likely the signature engine has also not seen any application data.
As the name indicates, the payload keyword matches on packet payload only. You cannot use it to match on packet headers; use the header conditions for that.
For TCP connections, header conditions are only evaluated for the first packet from each endpoint. If a header condition does not match the initial packets, the signature will not trigger. Bro optimizes for the most common application here, which is header conditions selecting the connections to be examined more closely with payload statements.
For UDP and ICMP flows, the payload matching is done on a per-packet basis; i.e., any content crossing packet boundaries will not be found. For TCP connections, the matching semantics depend on whether Bro is reassembling the connection (i.e., putting all of a connection’s packets in sequence). By default, Bro is reassembling the first 1K of every TCP connection, which means that within this window, matches will be found without regards to packet order or boundaries (i.e., stream-wise matching).
For performance reasons, by default Bro stops matching on a connection after seeing 1K of payload; see the section on options below for how to change this behaviour. The default was chosen with Bro’s main user of signatures in mind: dynamic protocol detection works well even when examining just connection heads.
Regular expressions are implicitly anchored, i.e., they work as if prefixed with the ^ operator. For reassembled TCP connections, they are anchored at the first byte of the payload stream. For all other connections, they are anchored at the first payload byte of each packet. To match at arbitrary positions, you can prefix the regular expression with .*, as done in the examples above.
To match on non-ASCII characters, Bro’s regular expressions support the \x<hex> operator. CRs/LFs are not treated specially by the signature engine and can be matched with \r and \n, respectively. Generally, Bro follows flex’s regular expression syntax. See the DPD signatures in base/frameworks/dpd/dpd.sig for some examples of fairly complex payload patterns.
The data argument of the signature_match handler might not carry the full text matched by the regular expression. Bro performs the matching incrementally as packets come in; when the signature eventually fires, it can only pass on the most recent chunk of data.

Options

The following options control details of Bro’s matching process:

dpd_reassemble_first_packets: bool (default: T): If true, Bro reassembles the beginning of every TCP connection (of up to dpd_buffer_size bytes, see below), to facilitate reliable matching across packet boundaries. If false, only connections are reassembled for which an application-layer analyzer gets activated (e.g., by Bro’s dynamic protocol detection).
dpd_match_only_beginning : bool (default: T): If true, Bro performs packet matching only within the initial payload window of dpd_buffer_size. If false, it keeps matching on subsequent payload as well.
dpd_buffer_size: count (default: 1024): Defines the buffer size for the two preceding options. In addition, this value determines the amount of bytes Bro buffers for each connection in order to activate application analyzers even after parts of the payload have already passed through. This is needed by the dynamic protocol detection capability to defer the decision which analyzers to use.

So, how about using Snort signatures with Bro?

There was once a script, snort2bro, that converted Snort signatures automatically into Bro’s signature syntax. However, in our experience this didn’t turn out to be a very useful thing to do because by simply using Snort signatures, one can’t benefit from the additional capabilities that Bro provides; the approaches of the two systems are just too different. We therefore stopped maintaining the snort2bro script, and there are now many newer Snort options which it doesn’t support. The script is now no longer part of the Bro distribution.

Signatures

Basics

Signature language

Conditions

Header Conditions

Content Conditions

Dependency Conditions

Context Conditions

Actions

Things to keep in mind when writing signatures

Options

So, how about using Snort signatures with Bro?

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Signatures

Basics

Signature language

Conditions

Header Conditions

Content Conditions

Dependency Conditions

Context Conditions

Actions

Things to keep in mind when writing signatures

Options

So, how about using Snort signatures with Bro?

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