A Terraform module input that was never set had an empty-string default. Combined with a wildcard, it produced a policy that granted a reporting service read access to every bucket in the account—not just the two intended buckets. Sergey Shinder says the broad access went unnoticed for five weeks, until a quarterly access review. His account offers a useful security lesson: inspect what an input means when it is absent, and review the rendered policy rather than trusting the intended configuration.
How an unset input widened access
In a first-person account published September 20, 2026, Sergey Shinder describes a pull request intended to give a reporting service read access to two storage buckets. The module constructed resource identifiers by combining a bucket ARN prefix, an input variable intended to hold a team prefix, and an asterisk.
The affected workspace had never set the input, and its default was an empty string. With nothing between the ARN root and the wildcard, the result matched all buckets, according to Shinder. The applied policy therefore granted broader access than the pull request intended. The author says the mismatch remained unnoticed for five weeks, until a quarterly access review. Read Shinder’s account.
Why the change escaped review
The plan displayed the policy as a long, escaped JSON string on one line. Shinder says the consequential difference between the old and new policies was the disappearance of eight characters in the middle of that string. Two reviewers approved the change.
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The review problem was not simply that the policy was hard to read: the rendered result had different access semantics from the change reviewers expected. A wildcard can remain broad even when a concatenated component disappears. Reviewing the final resource pattern makes that difference easier to catch than reasoning only from the intended input.
What Shinder changed
Shinder reports three changes after the incident. They operate at different points in the workflow, from rejecting an unsuitable input to examining the policy produced by a plan:
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| Layer | Reported change | Failure it is meant to catch |
|---|---|---|
| Input validation | Reject a prefix shorter than four characters. | An unset or too-short prefix that could leave a broader pattern than intended. |
| Resource construction | Build ARNs from an explicit list of names instead of assembling them through interpolation; an empty list then produces an empty policy rather than a universal one. | A missing string component leaving a wildcard attached to a broad ARN root. |
| Plan review in the pipeline | Decode policy documents from a plan and print statements in readable rows. Fail the build when a resource ends in a bare wildcard unless an exception is recorded. | A broad resource pattern hidden in an escaped, difficult-to-scan policy string. |
These are changes Shinder says he implemented, not independently tested controls or a guarantee that the same checks cover every policy design. In particular, an exception mechanism is useful only if exceptions remain deliberate and reviewable.
How to apply the lesson to a module
Check the absent-input case
Inspect defaults and determine what the module produces when a caller omits an input or supplies an empty value. Test that case alongside the expected populated input. A value that appears harmless in a complete example may behave differently in a workspace where it was never configured.
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Make allowed resources explicit when appropriate
If the module’s design is based on a set of permitted resource names, represent that set explicitly rather than relying on a string prefix whose absence changes the meaning of a wildcard expression. The right construction depends on the module’s intended behavior; the key is to ensure an empty input cannot silently become a universal scope.
Review rendered policy semantics
Make policy output readable during review, whether through a decoded plan view or another review step. Check the actual resource patterns and statements produced, not only the source expression or the plan’s escaped JSON representation.
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Automate checks for broad patterns
A pipeline check can flag unexpectedly broad resource patterns and require an explicit, auditable exception where one is intended. Such a check is a review aid, not proof that every policy is safe: it needs to match the policy forms the module actually emits.
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Shinder summarizes the habit behind these changes: “The habit I would pass on is to ask what each variable means when it is absent, not when it is filled in.” The incident is his account, rather than independently verified reporting, and its five-week timeline describes this case alone—not a general measure of how long policy errors go unnoticed.
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