Annotation Process	We developed our annotation guidelines through six preliminary rounds of annotation, covering 560 posts. Each round was followed by discussion and resolution of every post with disagreements. We benefited from members of our team who brought extensive domain expertise to the task. As well as refining the annotation guidelines, the development process trained annotators who were not security experts. The data annotated during this process is not included in Table TABREF3 . Once we had defined the annotation standard, we annotated datasets from Darkode, Hack Forums, Blackhat, and Nulled as described in Table TABREF3 . Three people annotated every post in the Darkode training, Hack Forums training, Blackhat test, and Nulled test sets; these annotations were then merged into a final annotation by majority vote. The development and test sets for Darkode and Hack Forums were annotated by additional team members (five for Darkode, one for Hack Forums), and then every disagreement was discussed and resolved to produce a final annotation. The authors, who are researchers in either NLP or computer security, did all of the annotation. We preprocessed the data using the tokenizer and sentence-splitter from the Stanford CoreNLP toolkit  .

Note that many sentences in the data are already delimited by line breaks, making the sentence-splitting task much easier. We performed annotation on the tokenized data so that annotations would be consistent with surrounding punctuation and hyphenated words. Our full annotation guide is available with our data release. Our basic annotation principle is to annotate tokens when they are either the product that will be delivered or are an integral part of the method leading to the delivery of that product. Figure FIGREF2 shows examples of this for a deliverable product (bot) as well as a service (cleaning). Both a product and service may be annotated in a single example: for a post asking to hack an account, hack is the method and the deliverable is the account, so both are annotated. In general, methods expressed as verbs may be annotated in addition to nominal references.   When the product is a multiword expression (e.g., Backconnect bot), it is almost exclusively a noun phrase, in which case we annotate the head word of the noun phrase (bot).

Type-level Annotation	Another approach following Hypothesis 1 is to use small amounts of supervised data, One cheap approach for annotating data in a new domain is to exploit type-level annotation  ,  . Our token-level annotation standard is relatively complex to learn, but a researcher could quite easily provide a few exemplar products for a new forum based on just a few minutes of reading posts and analyzing the forum. Given the data that we've already annotated, we can simulate this process by iterating through our labeled data and collecting annotated product names that are sufficiently common. Specifically, we take all (lowercased, stemmed) product tokens and keep those occurring at least 4 times in the training dataset (recall that these datasets are INLINEFORM0 700 posts). This gives us a list of 121 products in Darkode and 105 products in Hack Forums. To incorporate this information into our system, we add a new feature on each token indicating whether or not it occurs in the gazetteer. At training time, we use the gazetteer scraped from the training set. At test time, we use the gazetteer from the target domain as a form of partial type-level supervision.

While prior work has dealt with these messy characteristics for syntax  and for discourse  ,  ,  , our work is the first to tackle forum data (and marketplace forums specifically) from an information extraction perspective. Having annotated a dataset, we examine supervised and semi-supervised learning approaches to the product extraction problem. Binary or CRF classification of tokens as products is effective, but performance drops off precipitously when a system trained on one forum is applied to a different forum: in this sense, even two different cybercrime forums seem to represent different “fine-grained domains.” Since we want to avoid having to annotate data for every new forum that might need to be analyzed, we explore several methods for adaptation, mixing type-level annotation  ,  , token-level annotation  , and semi-supervised approaches  ,  . We find little improvement from these methods and discuss why they fail to have a larger impact. Overall, our results characterize the challenges of our fine-grained domain adaptation problem in online marketplace data. We believe that this new dataset provides a useful testbed for additional inquiry and investigation into modeling of fine-grained domain differences.

Introduction	NLP can be extremely useful for enabling scientific inquiry, helping us to quickly and efficiently understand large corpora, gather evidence, and test hypotheses  ,  . One domain for which automated analysis is particularly useful is Internet security: researchers obtain large amounts of text data pertinent to active threats or ongoing cybercriminal activity, for which the ability to rapidly characterize that text and draw conclusions can reap major benefits  ,  . However, conducting automatic analysis is difficult because this data is out-of-domain for conventional NLP models, which harms the performance of both discrete models  and deep models  . Not only that, we show that data from one cybercrime forum is even out of domain with respect to another cybercrime forum, making this data especially challenging. In this work, we present the task of identifying products being bought and sold in the marketplace sections of these online cybercrime forums. We define a token-level annotation task where, for each post, we annotate references to the product or products being bought or sold in that post.

Dataset and Annotation	We consider several forums that vary in the nature of products being traded: Table TABREF3 gives some statistics of these forums. These are the same forums used to study product activity in PortnoffEtAl2017. We collected all available posts and annotated a subset of them. In total, we annotated 130,336 tokens; accounting for multiple annotators, our annotators considered 478,176 tokens in the process of labeling the data. Figure FIGREF2 shows two examples of posts from Darkode. In addition to aspects of the annotation, which we describe below, we see that the text exhibits common features of web text: abbreviations, ungrammaticality, spelling errors, and visual formatting, particularly in thread titles. Also, note how some words that are not products here might be in other contexts (e.g., Exploits).

Identifying Products in Online Cybercrime Marketplaces: A Dataset for Fine-grained Domain Adaptation	One weakness of machine-learned NLP models is that they typically perform poorly on out-of-domain data. In this work, we study the task of identifying products being bought and sold in online cybercrime forums, which exhibits particularly challenging cross-domain effects. We formulate a task that represents a hybrid of slot-filling information extraction and named entity recognition and annotate data from four different forums. Each of these forums constitutes its own"fine-grained domain"in that the forums cover different market sectors with different properties, even though all forums are in the broad domain of cybercrime. We characterize these domain differences in the context of a learning-based system: supervised models see decreased accuracy when applied to new forums, and standard techniques for semi-supervised learning and domain adaptation have limited effectiveness on this data, which suggests the need to improve these techniques. We release a dataset of 1,938 annotated posts from across the four forums.

Token-level Annotation	We now turn our attention to methods that might address Hypothesis 2. If we assume the domain transfer problem is more complex, we really want to leverage labeled data in the target domain rather than attempting to transfer features based only on type-level information. Specifically, we are interested in cases where a relatively small number of labeled posts (less than 100) might provide substantial benefit to the adaptation; a researcher could plausibly do this annotation in a few hours. We consider two ways of exploiting labeled target-domain data. The first is to simply take these posts as additional training data. The second is to also employ the “frustratingly easy” domain adaptation method of Daume2007. In this framework, each feature fired in our model is actually fired twice: one copy is domain-general and one is conjoined with the domain label (here, the name of the forum). In doing so, the model should gain some ability to separate domain-general from domain-specific feature values, with regularization encouraging the domain-general feature to explain as much of the phenomenon as possible. For both training methods, we upweight the contribution of the target-domain posts in the objective by a factor of 5.

Conclusion	We present a new dataset of posts from cybercrime marketplaces annotated with product references, a task which blends IE and NER. Learning-based methods degrade in performance when applied to new forums, and while we explore methods for fine-grained domain adaption in this data, effective methods for this task are still an open question. Our datasets used in this work are available at https://evidencebasedsecurity.org/forums/ Code for the product extractor can be found at https://github.com/ccied/ugforum-analysis/tree/master/extract-product

Having the ability to automatically tag posts in this way lets us characterize the composition of a forum in terms of what products it deals with, identify trends over time, associate users with particular activity profiles, and connect to price information to better understand the marketplace. Some of these analyses only require post-level information (what is the product being bought or sold in this post?) whereas other analyses might require token-level references; we annotate at the token level to make our annotation as general as possible. Our dataset has already proven enabling for case studies on these particular forums  , including a study of marketplace activity on bulk hacked accounts versus users selling their own accounts. Our task has similarities to both slot-filling information extraction (with provenance information) as well as standard named-entity recognition (NER). Compared to NER, our task features a higher dependence on context: we only care about the specific product being bought or sold in a post, not other products that might be mentioned. Moreover, because we are operating over forums, the data is substantially messier than classical NER corpora like CoNLL  .

Annotating single tokens instead of spans meant that we avoided having to agree on an exact parse of each post, since even the boundaries of base noun phrases can be quite difficult to agree on in ungrammatical text. If multiple different products are being bought or sold, we annotate them all. We do not annotate: Features of products Generic product references, e.g., this, them Product mentions inside “vouches” (reviews from other users) Product mentions outside of the first and last 10 lines of each post Table TABREF3 shows inter-annotator agreement according to our annotation scheme. We use the Fleiss' Kappa measurement  , treating our task as a token-level annotation where every token is annotated as either a product or not. We chose this measure as we are interested in agreement between more than two annotators (ruling out Cohen's kappa), have a binary assignment (ruling out correlation coefficients) and have datasets large enough that the biases Krippendorff's Alpha addresses are not a concern. The values indicate reasonable agreement.

Discussion	Because we annotate entities in a context-sensitive way (i.e., only annotating those in product context), our task resembles a post-level information extraction task. The product information in a post can be thought of as a list-valued slot to be filled in the style of TAC KBP  ,  , with the token-level annotations constituting provenance information. However, we chose to anchor the task fully at the token level to simplify the annotation task: at the post level, we would have to decide whether two distinct product mentions were actually distinct products or not, which requires heavier domain knowledge. Our approach also resembles the fully token-level annotations of entity and event information in the ACE dataset  .

Phrase-level Evaluation	Another axis of variation in metrics comes from whether we consider token-level or phrase-level outputs. As noted in the previous section, we did not annotate noun phrases, but we may actually be interested in identifying them. In Figure FIGREF2 , for example, extracting Backconnect bot is more useful than extracting bot in isolation, since bot is a less specific characterization of the product. We can convert our token-level annotations to phrase-level annotations by projecting our annotations to the noun phrase level based on the output of an automatic parser. We used the parser of ChenManning2014 to parse all sentences of each post. For each annotated token that was given a nominal tag (N*), we projected that token to the largest NP containing it of length less than or equal to 7; most product NPs are shorter than this, and when the parser predicts a longer NP, our analysis found that it typically reflects a mistake. In Figure FIGREF2 , the entire noun phrase Backconnect bot would be labeled as a product. For products realized as verbs (e.g., hack), we leave the annotation as the single token.

Figure FIGREF41 shows learning curves for both of these methods in two adaptation settings as we vary the amount of labeled target-domain data. The system trained on Hack Forums is able to make good use of labeled data from Darkode: having access to 20 labeled posts leads to gains of roughly 7 F INLINEFORM0 . Interestingly, the system trained on Darkode is not able to make good use of labeled data from Hack Forums, and the domain-specific features actually cause a drop in performance until we include a substantial amount of data from Hack Forums (at least 80 posts). We are likely overfitting the small Hack Forums training set with the domain-specific features.

Basic Results	Table TABREF30 shows development set results on Darkode for each of the four systems for each metric described in Section SECREF3 . Our learning-based systems substantially outperform the baselines on the metrics they are optimized for. The post-level system underperforms the binary classifier on the token evaluation, but is superior at not only post-level accuracy but also product type F INLINEFORM0 . This lends credence to our hypothesis that picking one product suffices to characterize a large fraction of posts. Comparing the automatic systems with human annotator performance we see a substantial gap. Note that our best annotator's token F INLINEFORM1 was 89.8, and NP post accuracy was 100%; a careful, well-trained annotator can achieve very high performance, indicating a high skyline. The noun phrase metric appears to be generally more forgiving, since token distinctions within noun phrases are erased. The post-level NP system achieves an F-score of 78 on product type identification, and post-level accuracy is around 88%. While there is room for improvement, this system is accurate enough to enable analysis of Darkode with automatic annotation. Throughout the rest of this work, we focus on NP-level evaluation and post-level NP accuracy.

Acknowledgments	This work was supported in part by the National Science Foundation under grants CNS-1237265 and CNS-1619620, by the Office of Naval Research under MURI grant N000140911081, by the Center for Long-Term Cybersecurity and by gifts from Google. We thank all the people that provided us with forum data for our analysis; in particular Scraping Hub and SRI for their assistance in collecting data for this study. Any opinions, findings, and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors.

Brown Clusters	To test Hypothesis 1, we investigate whether additional lexical information helps identify product-like words in new domains. A classic semi-supervised technique for exploiting unlabeled target data is to fire features over word clusters or word vectors  . These features should generalize well across domains that the clusters are formed on: if product nouns occur in similar contexts across domains and therefore wind up in the same cluster, then a model trained on domain-limited data should be able to learn that that cluster identity is indicative of products. We form Brown clusters on our unlabeled data from both Darkode and Hack Forums (see Table TABREF3 for sizes). We use Liang2005's implementation to learn 50 clusters. Upon inspection, these clusters do indeed capture some of the semantics relevant to the problem: for example, the cluster 110 has as its most frequent members service, account, price, time, crypter, and server, many of which are product-associated nouns. We incorporate these as features into our model by characterizing each token with prefixes of the Brown cluster ID; we used prefixes of length 2, 4, and 6.

Moreover, Table TABREF33 shows that the Hack Forums-trained system achieves a 21% error reduction on Hack Forums compared to a Darkode-trained system, while a Darkode-trained system obtains a 38% error reduction on Darkode relative to a Hack Forums-trained system; this greater error reduction means that Darkode has better coverage of Hack Forums than vice versa. Darkode's better product coverage also helps explain why Section SECREF40 showed better performance of adapting Hack Forums to Darkode than the other way around: augmenting Hack Forums data with a few posts from Darkode can give critical knowledge about new products, but this is less true if the forums are reversed. Duplicating features and adding parameters to the learner also has less of a clear benefit when adapting from Darkode, when the types of knowledge that need to be added are less concrete. Note, however, that these results do not tell the full story. Table TABREF39 reports recall values, but not all systems have the same precision/recall tradeoff: although they were tuned to balance precision and recall on their respective development sets, the Hack Forums-trained system is slightly more precision-oriented on Nulled than the Darkode-trained system.

Models	We consider several baselines for product extraction, two supervised learning-based methods (here), and semi-supervised methods (Section SECREF5 ).

Throughout the rest of this work, we will evaluate sometimes at the token-level and sometimes at the NP-level (including for the product type evaluation and post-level accuracy); we will specify which evaluation is used where.

Domain Adaptation	Table TABREF30 only showed results for training and evaluating within the same forum (Darkode). However, we wish to apply our system to extract product occurrences from a wide variety of forums, so we are interested in how well the system will generalize to a new forum. Tables TABREF33 and TABREF38 show full results of several systems in within-forum and cross-forum evaluation settings. Performance is severely degraded in the cross-forum setting compared to the within-forum setting, e.g., on NP-level F INLINEFORM0 , a Hack Forums-trained model is 14.6 F INLINEFORM1 worse at the Darkode task than a Darkode-trained model (61.2 vs. 75.8). Differences in how the systems adapt between different forums will be explored more thoroughly in Section SECREF43 . In the next few sections, we explore several possible methods for improving results in the cross-forum settings and attempting to build a more domain-general system. These techniques generally reflect two possible hypotheses about the source of the cross-domain challenges:

Tables TABREF33 and TABREF38 show the results of incorporating Brown cluster features into our trained models. These features do not lead to statistically-significant gains in either NP-level F INLINEFORM0 or post-level accuracy, despite small improvements in some cases. This indicates that Brown clusters might be a useful feature sometimes, but do not solve the domain adaptation problem in this context.

Tables TABREF33 and TABREF38 shows the results of incorporating the gazetteer into the system. Gazetteers seem to provide somewhat consistent gains in cross-domain settings, though many of these individual improvements are not statistically significant, and the gazetteers can sometimes hurt performance when testing on the same domain the system was trained on.

Analysis	In order to understand the variable performance and shortcomings of the domain adaptation approaches we explored, it is useful to examine our two initial hypotheses and characterize the datasets a bit further. To do so, we break down system performance on products seen in the training set versus novel products. Because our systems depend on lexical and character INLINEFORM0 -gram features, we expect that they will do better at predicting products we have seen before. Table TABREF39 confirms this intuition: it shows product out-of-vocabulary rates in each of the four forums relative to training on both Darkode and Hack Forums, along with recall of an NP-level system on both previously seen and OOV products. As expected, performance is substantially higher on in-vocabulary products. OOV rates of a Darkode-trained system are generally lower on new forums, indicating that that forum has better all-around product coverage. A system trained on Darkode is therefore in some sense more domain-general than one trained on Hack Forums. This would seem to support Hypothesis 1.

In fact, Table TABREF33 shows that the Hack Forums-trained system actually performs better on Nulled, largely due to better performance on previously-seen products. This indicates that there is some truth to Hypothesis 2: product coverage is not the only important factor determining performance.

Evaluation Metrics	In light of the various views on this task and its different requirements for different potential applications, we describe and motivate a few distinct evaluation metrics below. The choice of metric will impact system design, as we discuss in the following sections.

Question: Who annotated the data?
Answer: annotators who were not security experts, researchers in either NLP or computer security