Theory suggests that attachment figures' regulation of child arousal during stress is central to the development of secure attachment, that is, trust in the attachment figure, and knowledge about care-related interactions, known as the Secure Base Script (SBS). The current study tested the hypothesis that hair cortisol concentration (HCC), reflecting cumulative cortisol production over time, would be associated with attachment development in middle childhood during the first COVID-19 lockdown. HCC was measured in 55 West-European children (56.4% girls) aged 9-11 years (M = 9.48, SD = 0.47) as a biomarker of chronic stress during the lockdown. SBS knowledge and self-reported trust in the attachment figure were measured both before and after the lockdown period to assess attachment development. The results supported the hypothesis that higher HCC was associated with a decrease in trust in the attachment figure ([partial eta-squared] = 0.08). However, contrary to expectations, HCC did not moderate changes in SBS knowledge. Instead, all children showed an increase in SBS knowledge over time, regardless of HCC ([partial eta-squared] = 0.954). These findings suggest that cognitive (i.e., SBS knowledge) and affective (i.e., trust) dimensions of attachment may be differentially affected by stress. Further research is needed to clarify the role of endocrinological processes in distinct dimensions of attachment development.