The vulnerability was discovered by cloud security firm Wiz and reported to Microsoft in September.
The issue was fixed in November, and Microsoft has spent the last few weeks investigating how many customers were impacted.
The issue, nicknamed NotLegit, resides in Azure App Service, a feature of the Azure cloud that allows customers to deploy websites and web apps from a source code repository.
Wiz researchers said that in situations where Azure customers selected the "Local Git" option to deploy their websites from a Git repository hosted on the same Azure server, the source code was also exposed online.
All PHP, Node, Ruby, and Python applications deployed via this method were impacted, Microsoft said in a blog post today.
Only apps deployed on Linux-based Azure servers were impacted, but not those hosted on Windows Server systems.
Apps deployed as far back as 2013 were impacted, although the exposure began in September 2017, when the vulnerability was introduced in Azure's systems, the Wiz team said in a report today.
The most dangerous exposure scenarios are situations where the exposed source code contained a .git configuration file that, itself, contained passwords and access tokens for other customer systems, such as databases and APIs.