This paper builds theory on the process of open-strategy infiltration through an ethnographic study investigating how open strategy entered a financial services firm's strategy process behind the backs of top managers. Based on our analysis, we show how open strategy infiltrates the strategy process through a ‘strategic practice drift’, i.e., a gradual and partly unnoticed shift towards open strategy that occurs through ‘accommodating’ and ‘legitimizing’ the performance of transparency and inclusion in the strategy process. We show how the ‘goal-based ambiguity’ and ‘procedural certainty’ of initiatives that latently imply the performance of transparency and inclusion in the strategy process enable open-strategy infiltration. Furthermore, we show how top managers' ‘goal-based rationalization’ and ‘procedural renegotiation’ of practising transparency and inclusion contribute to the eventual reproduction of open strategy in the strategy process. Our model generates an understanding of how and why open strategy can enter the strategy process behind the backs of top managers and adds nuance to extant understandings of the role of top managers in this process. In addition, our findings contribute to research on strategy as practice by theorizing ‘strategic practice drifts’ and extending our understanding of the role of ambiguity therein.