  While deep learning based image retrieval is reported to be vulnerable to
adversarial attacks, existing works are mainly on image-to-image retrieval with
their attacks performed at the front end via query modification. By contrast,
we present in this paper the first study about a threat that occurs at the back
end of a text-to-image retrieval (T2IR) system. Our study is motivated by the
fact that the image collection indexed by the system will be regularly updated
due to the arrival of new images from various sources such as web crawlers and
advertisers. With malicious images indexed, it is possible for an attacker to
indirectly interfere with the retrieval process, letting users see certain
images that are completely irrelevant w.r.t. their queries. We put this thought
into practice by proposing a novel Trojan-horse attack (THA). In particular, we
construct a set of Trojan-horse images by first embedding word-specific
adversarial information into a QR code and then putting the code on benign
advertising images. A proof-of-concept evaluation, conducted on two popular
T2IR datasets (Flickr30k and MS-COCO), shows the effectiveness of the proposed
THA in a white-box mode.

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