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cve: fix sandbox path bypass in file helper through symlink attack - #2143
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The file template helper enforces sandbox_path during template evaluation, but the later dependency fetch reads the original raw path without re-checking the sandbox. This creates a time-of-check to time-of-use gap. If an attacker can retarget a symlink or equivalent link target after validation but before dependency fetch, consul-template can read an out-of-sandbox file and then render the cached contents even after the link is restored to an in-sandbox target.
PCI review checklist
I have documented a clear reason for, and description of, the change I am making.
If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
If applicable, I've documented the impact of any changes to security controls.
Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.