Utilizing SiLK and Mothra to Determine Information Exfiltration by way of the Area Title Service

Quite a lot of fashionable community threats contain information theft by way of abuse of community providers, which is termed information exfiltration. To trace such threats, analysts monitor information transfers out of the group’s community, notably information transfers occurring by way of community providers not primarily meant for bulk switch providers. One such service is the Area Title System (DNS), which is important for a lot of different Web providers. Sadly, attackers can manipulate DNS to exfiltrate information in a covert method.

This SEI weblog submit focuses on how the DNS protocol could be abused to exfiltrate information by including bytes of knowledge onto DNS queries or making repeated queries that comprise information encoded into the fields of the question. The submit additionally examines the overall site visitors analytic we are able to use to establish this abuse and applies a number of instruments obtainable to implement the analytic. The combination dimension of DNS packets can present a prepared indicator of DNS abuse. Nevertheless, as a result of the DNS protocol has grown from a easy deal with decision mechanism to distributed database help for community connectivity, decoding the mixture dimension requires understanding of the context of queries and responses. By understanding the amount of DNS site visitors, each in isolation and in mixture, analysts might higher match outgoing queries and incoming responses.

The info used on this weblog submit is the CIC-BELL-DNS-EXF 2021 information set, as printed along side the paper Light-weight Hybrid Detection of Information Exfiltration utilizing DNS primarily based on Machine Studying by Samaneh Mahdavifar et al.

The Function of DNS

DNS helps a number of varieties of queries. These queries are described in a wide range of Web Engineering Activity Pressure (IETF) Request for Remark (RFC) paperwork. These RFCs embrace the next:

  • A and AAAA queries for IP deal with equivalent to a site title (e.g., “which deal with corresponds to www.instance.com?” with a response like “”)
  • pointer document (PTR) queries for title equivalent to an IP deal with (e.g., “which title corresponds to” with a response of “www.instance.com”)
  • title server (NS), mail change (MX), and service locator (SRV) queries for the id of key servers in a given area
  • begin of authority (SOA) queries for details about addresses on which the queried server might converse authoritatively
  • certificates (CERT) queries for encryption certificates pertaining to the server’s coated domains
  • textual content document (TXT) queries for extra info (as configured by the community administrator) in a textual content format

A given DNS question packet will request info on a given area from a specific server, however the response from that server might embrace a number of useful resource data. The scale of the response will rely on what number of useful resource data are returned and the kind of every document.

As soon as analysts perceive the explanations for monitoring DNS site visitors and the context wanted for decoding the monitoring outcomes, they will then decide what info is desired from the monitoring. This weblog submit assumes the analyst needs to trace exterior hosts which may be receiving exfiltrated info.

Overview of the Analytic for Figuring out Information Exfiltration

The analytic coated on this weblog submit assumes that the networks of curiosity are coated by site visitors sensors that produce community movement data or a minimum of packet captures that may be aggregated into community movement data. There are a number of instruments obtainable to generate these movement data. As soon as produced, the movement data are archived in a movement repository or acceptable database tables, relying on the evaluation software suite.

The strategy taken on this analytic is, first, to mixture DNS site visitors related to exterior locations performing like servers and, second, to profile the site visitors for these locations. Step one (affiliation) entails figuring out DNS site visitors (both by service port or by precise examination of the applying protocol), then figuring out the exterior locations concerned. The second step (profiling) examines what number of sources are speaking with every of the locations, the mixture byte rely, packet rely, and different revealing info as described within the following sections.

A number of completely different instruments can be utilized for this evaluation. This weblog submit will talk about two units of SEI-developed instruments:

  • The System for Web-Degree Data (SiLK) is a group of site visitors evaluation instruments developed to facilitate safety evaluation of enormous networks. The SiLK software suite helps the environment friendly assortment, storage, and evaluation of community movement information, enabling community safety analysts to quickly question giant historic site visitors information units. SiLK is ideally suited to analyzing site visitors on the spine or border of a giant, distributed enterprise or mid-sized ISP.
  • Mothra is a group of Apache Spark libraries that help evaluation of community movement data in Web Protocol Stream Data Export(IPFIX) format with deep packet inspection fields.

Every of the next sections will current an analytic for detecting exfiltration by way of DNS queries within the corresponding software set.

Implementing the Analytic by way of SiLK

Determine 1 beneath presents a collection of SiLK instructions to implement an analytic to detect exfiltration. The primary command applies a filter to regular, benign DNS site visitors, isolating DNS site visitors (recognized by protocol recognition as indicated by the applying label of 53) coming from the inner community (classless inter-domain routing [CIDR] block and of comparatively lengthy (70 bytes or extra) packets. The output of the filter is then summarized by vacation spot deal with and transport protocol, counting bytes, movement data, and packets for every mixture of deal with and protocol. The ensuing counts are solely proven if the accrued bytes are 500 or extra. After making use of the analytic to benign DNS information, it’s utilized within the second sequence to DNS information encompassing compressed information for exfiltration.

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Determine 1: SiLK Analytic and Outcomes

The ends in Determine 1 present that the community talks to a major DNS server, a secondary DNS server, and a public server. Within the benign case, the information is principally directed to the first DNS server and the general public server. Within the exfiltration case, the information is principally directed to the first DNS server and the secondary DNS server. This shift of vacation spot, in isolation, shouldn’t be sufficient to make the exfiltration site visitors suspicious or present a foundation for shifting past suspicion into investigation. Within the benign case, there’s a notable fraction of the site visitors directed to the general public DNS server at Within the site visitors labeled as abusive, this fraction is lessened, and the fraction to a non-public DNS server (the exfiltration goal) at is elevated. Sadly, given the restricted nature of SiLK movement data, safety analysts have a tough time exfiltrating extra site visitors. To go additional, extra DNS-specific fields are required. These fields are supplied by deep packet inspection (DPI) information in expanded movement data in IPFIX format. Whereas SiLK can’t course of IPFIX movement data, different instruments corresponding to Mothra and databases can.

Implementing the Analytic by way of Mothra

The code pattern beneath reveals the analytic applied in Spark utilizing the Mothra libraries. These libraries enable definition and loading of knowledge frames with community movement document information in both SiLK or IPFIX format. A knowledge body is a assortment of knowledge organized into named columns. Information frames could be manipulated by Spark capabilities to isolate flows of curiosity and to summarize these flows. Defining the information frames entails figuring out the columns and the information to populate the columns. Within the code pattern, the information frames are outlined by the spark.learn.area operate and populated by information from both the captured benign site visitors or the captured exfiltration site visitors by way of Mothra’s ipfix operate. Collectively, these capabilities set up the information information body.

The consequence information body is constructed from the information information body by way of a collection of filtering and summarization capabilities. The preliminary filter restricts it to site visitors labeled as DNS site visitors, adopted by one other filter that ensures the data comprise DNS useful resource document queries or responses. The choose operate that follows isolates particular document options for summarization: time, site visitors supply and vacation spot, byte and packet volumes, DNS names, DNS flags, and DNS useful resource document sorts. The groupBy operate generates the summarization for every distinctive DNS title and useful resource document sort mixture. The agg operate specifies that the summarization comprise the rely of movement data, the counts of supply and vacation spot IP addresses, and the totals for bytes and packets. The filter operate (after the summarization) restricts output to only these displaying a bytes-per-packet ratio of greater than 70 with fewer than three entries within the DNS Title checklist. This final filter excludes summarizations of site visitors that’s giant solely as a result of size of the response checklist fairly than to the size of particular person queries.

This filtering and summarization course of creates a profile of enormous DNS requests and responses (separated by DNS flag values). Using DNS names as a grouping worth permits the analytic to tell apart repeated queries to related domains. The counts of supply and vacation spot IP addresses enable the analyst to tell apart repeated site visitors to some areas as a substitute of uncommon site visitors to a number of areas or from a number of sources.

val data_dir = ".../path/to/information"
import org.cert.netsa.mothra.datasources._
import org.cert.netsa.mothra.datasources.ipfix.IPFIXFields
import org.apache.spark.sql.capabilities._

// In dnsIDBenign.sc:
val data_file = s"$data_dir/light_benign.ipfix"
// In dnsIDAbuse.sc:
// val data_file =
//   s"$data_dir/light_compressed.ipfix"

val information = {
    IPFIXFields.default, IPFIXFields.dpi.dns

val consequence = {
    .filter(($"silkAppLabel" === 53) &&
       $"dnsRecordList.dnsRRType" as "dnsRRType",
       $"dnsRecordList.dnsQueryResponse" as "dnsQR",
       $"dnsRecordList.dnsResponseCode" as "dnsResponse",
       $"dnsRecordList.dnsName" as "dnsName")
     .agg(rely($"*") as "flows",
          countDistinct($"sourceIPAddress") as "#sIP",
          countDistinct($"destinationIPAddress") as "#dIP",
          sum($"octetCount") as "bytes",
          sum($"packetCount") as "packets")
 //    .filter($"packets" > 20)
     .filter($"bytes"/$"packets" > 70)
     .filter(dimension($"dnsName") < 3)

The code pattern beneath reveals the output of dnsIDExfil.sc on benign and on compressed information, the information units used within the previous SiLK dialogue. The presence of multicast (224/8 and 239/8 CIDR blocks) and RFC1918 personal addresses (192.168/16 CIDR blocks) is because of this information coming from a synthetic assortment surroundings as a substitute of dwell Web site visitors seize.

Contrasting the benign output towards the abuse output, we see a smaller variety of lookup addresses being queried within the abuse outcomes and a a lot faster drop-off within the variety of queries per host. Within the benign outcomes, there are six DNSNames which can be queried repeatedly; within the abuse outcomes, there are two. The entire queries proven are PTR (reverse. RRType=12) queries, and all are going to the identical server. Within the high-volume DNSName queries, the utmost common packet size is barely bigger for the abuse information than for the benign information (81 vs. 78). Taken collectively, these variations present a slow-and-steady launch of extra information as a part of the DNS information switch, which displays the file switch happening.

dnsIDBenign.sc output:
|dnsName                              |dnsRRType|flows|#sIP|#dIP|bytes |packets|
|[]          |[12]     |2835 |1   |1   |416539|5901   |
|[]       |[12]     |982  |1   |1   |242585|3125   |
|[]       |[12]     |895  |1   |1   |134756|1836   |
|[]        |[12]     |901  |1   |1   |133490|1844   |
|[]       |[12]     |757  |1   |1   |112173|1533   |
|[]         |[12]     |635  |1   |1   |91734 |1288   |
|[]         |[12]     |315  |1   |1   |45438 |640    |
|[_ipps._tcp.local., _ipp._tcp.local.]|[12, 12] |122  |32  |1   |13161 |136    |
|[]      |[12]     |74   |1   |1   |11328 |152    |
|[]       |[12]     |31   |1   |1   |4666  |64     |
solely displaying high 10 rows

dnsIDAbuse.sc output:
|dnsName                              |dnsRRType|flows|#sIP|#dIP|bytes |packets|
|[]          |[12]     |1260 |1   |1   |191398|2696   |
|[]         |[12]     |255  |1   |1   |130725|1615   |
|[]       |[12]     |416  |1   |1   |63606 |866    |
|[]       |[12]     |388  |1   |1   |57686 |788    |
|[]        |[12]     |379  |1   |1   |56492 |781    |
|[]       |[12]     |340  |1   |1   |50738 |694    |
|[]         |[12]     |125  |1   |1   |17750 |250    |
|[]      |[12]     |32   |1   |1   |4736  |64     |
|[_ipps._tcp.local., _ipp._tcp.local.]|[12, 12] |46   |30  |1   |4467  |51     |
|[_ipp._tcp.local., _ipps._tcp.local.]|[12, 12] |13   |9   |1   |1782  |19     |
solely displaying high 10 rows

Understanding Information Exfiltration

Whichever type of tooling is used, analysts usually want an understanding of the information transfers from their community. Repetitive queries for DNS decision ought to be fairly uncommon—caching ought to get rid of many of those repetitions. As repetitive queries for decision are recognized, a number of teams of hosts could also be discovered:

  • Hosts that generate repetitive queries not indicative of exfiltration of knowledge are prone to exist, characterised by very constant question dimension, periodic timing, and the usage of anticipated title servers.
  • Hosts that generate repetitive queries with uncommon title servers or timing might require additional investigation.
  • Hosts that generate repetitive queries with uncommon title servers or question sizes ought to be examined fastidiously to establish potential exfiltration.

The influence of those hosts on community safety will differ relying on the vary and criticality of property these hosts entry, however among the site visitors might demand fast response.

What Would possibly a Safety Analyst Wish to Know

This submit is a part of a collection addressing a easy query: What would possibly a safety analyst need to know firstly of every shift relating to the community? In every submit we’ll talk about one reply to this query and software of a wide range of instruments that will implement that reply. Our purpose is to offer some key observations that assist analysts monitor and defend their networks, specializing in helpful ongoing measures, fairly than these particular to at least one occasion, incident, or situation.

We won’t deal with signature-based detection, since there are a selection of sources for such together with intrusion detection programs (IDS)/intrusion prevention programs (IPS) and antivirus merchandise. The instruments utilized in these articles will primarily be a part of the CERT/NetSA Evaluation Suite, however we’ll embrace different instruments if useful. Earlier posts examined instruments for monitoring software program updates and proxy bypass.

Our strategy will likely be to spotlight a given analytic, talk about the motivation behind the analytic, and supply the applying as a labored instance. The labored instance, by intention, is illustrative fairly than exhaustive. The choice of what analytics to deploy, and the way, is left to the reader.

If there are particular behaviors that you just wish to recommend, please ship them by e mail to [email protected] with “SOC Analytics Thought” within the topic line.

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