Man-in-the-browser (MITB, MitB, MIB, MiB), a form of Internet threat related to man-in-the-middle (MITM), is a proxy Trojan horse that infects a web browser by taking advantage of vulnerabilities in browser security to modify web pages, modify transaction content or insert additional transactions, all in a covert fashion invisible to both the user and host web application. A MitB attack will be successful irrespective of whether security mechanisms such as SSL/PKI and/or two- or three-factor authentication solutions are in place. A MitB attack may be countered by using out-of-band transaction verification, although SMS verification can be defeated by man-in-the-mobile (MitMo) malware infection on the mobile phone. Trojans may be detected and removed by antivirus software; this approach scored a 23% success rate against Zeus in 2009 and still low rates in a 2011 report. The 2011 report concluded that additional measures on top of antivirus software were needed.

A related, simpler attack is the boy-in-the-browser (BitB, BITB).

The majority of financial service professionals in a survey considered MitB to be the greatest threat to online banking.