sec(iac/build): close the shell-injection surface on docker buildx build

[cristim]

Summary

registry_login_command and extra_build_args on terraform/modules/build were consumed unquoted inside a local-exec heredoc, so shell metacharacters in either value would execute as script text rather than data.

Root cause

• extra_build_args was interpolated as literal ${var.extra_build_args} text into the heredoc, so its value became shell source at apply time: unquoted ;, backticks, or $() in that value would execute with the CI deploy identity attached.
• registry_login_command's documented example is a pipe into docker login, but the module never stated whether a static credential was an acceptable value for it, and its rendered command wasn't redacted anywhere.

Fix

• extra_build_args now flows through the local-exec provisioner's environment map (EXTRA_BUILD_ARGS) instead of being interpolated into the heredoc text, so its value is never re-parsed as shell source. Unquoted $EXTRA_BUILD_ARGS still preserves the original whitespace-split, multi-flag behavior and the original empty-default no-op (see Regression test).
• registry_login_command's variable description now states the contract explicitly: it must be an identity-based login (the CLI resolves/mints the credential itself - az acr login, aws ecr get-login-password | docker login, gcloud auth configure-docker) and must never embed a static, long-lived credential literal.
• Quoted the remaining heredoc interpolations that are safe to quote as single values (image_uri, git_commit, timestamp build args).

Revision note: sensitive = true was reverted

An earlier revision of this PR marked both variables sensitive = true. That was wrong and has been reverted: Terraform suppresses a resource's entire local-exec output when any of its config contains a sensitive value ((output suppressed due to sensitive value in config)) - verified locally on Terraform 1.14.7 with a minimal terraform_data/local-exec repro. That means a failing build or registry login would show nothing in CI, and deploy-aws-lambda.yml / deploy-azure.yml / deploy-gcp.yml tee and grep exactly this log to detect build failures. After #5 merged (ACR now authenticates via az acr login, an Entra identity, not the registry admin password), no caller in any of the three environments passes a static credential through registry_login_command, and extra_build_args was never a secret - so sensitive = true was blocking diagnostics without protecting anything live. The variable description carries the "identity-based only" contract instead, and the injection fix (environment-map pass-through + quoting) holds without marking anything sensitive.

registry_login_command itself still has to be interpolated as literal shell source, since its contract requires it to run as a (possibly piped) shell command. --network=host on the same local-exec is filed and fixed separately as #122 (merged as #351).

Regression test

No automated Terraform test exercises the rendered local-exec script. Verified manually by rendering the exact heredoc shape with bash -c / sh -c (the interpreters local-exec actually uses):

• Empty extra_build_args (the default in all three environments today): argc=2 (--push .), matching the pre-fix unquoted-and-empty behavior - no stray empty-string positional argument (confirmed this would otherwise have broken every build: quoting "${var.extra_build_args}" directly, the naive fix, produces argc=3 with an empty-string arg that docker buildx build would reject as more than one context path).
• Multi-flag value (--no-cache --build-arg FOO=bar): splits into separate argv entries, preserving intended multi-flag usage.
• Injection attempt (; touch /tmp/x) passed via the environment: comes through as a single inert argv word, not executed.
• sensitive = true removal: a standalone terraform_data/local-exec repro shows visible provisioner output without sensitive, and (output suppressed due to sensitive value in config) with it.

Verification

• terraform fmt -check -diff on terraform/modules/build and the touched environment file: clean.
• terraform init -backend=false && terraform validate in terraform/modules/build, terraform/environments/{aws,azure,gcp}: all Success! The configuration is valid.
• tflint --config=.tflint.hcl on terraform/modules/build and terraform/environments/azure: no findings.
• go test ./terraform/... (GOWORK=off, repo root): ok for both packages.
• Repo pre-commit hooks (fmt, validate, tflint, trivy config scan, secret scan, etc.) all passed on the commit.
• No plan/apply was run against real clouds, per policy.

Notes

• No bootstrap (ci-cd-permissions) re-apply is required for this change; it only touches the build module and its callers.
• extra_build_args is whitespace-split with no shell quoting honored: a value like --build-arg LABEL=hello world splits into two docker buildx arguments, not one. No caller sets this variable today; documented in its description rather than fixed, since fixing it would mean changing its type to list(string) for a case nothing currently needs.
• Rebased onto current main (post sec(iac/azure): pull and push ACR with Entra identities, disable the admin account #5, post sec(iac/build): drop --network=host from docker buildx build #351) during review; the build.tf hunk that removed a nonsensitive() wrapper around the ACR admin password was dropped as moot, since sec(iac/azure): pull and push ACR with Entra identities, disable the admin account #5 replaced that login with the identity-based az acr login (no secret value at all).

Closes #127

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📝 Walkthrough

Walkthrough

The build provisioner now passes extra build arguments through an environment variable and quotes the image URI and Git metadata values in the Docker command. The variable descriptions and Azure build comment document build-argument and registry-login requirements.

Changes

Terraform build configuration

Layer / File(s)	Summary
Build argument handling terraform/modules/build/main.tf, terraform/modules/build/variables.tf	The provisioner passes extra_build_args through EXTRA_BUILD_ARGS and expands it in the Docker command. The command quotes the image URI and Git commit and build-date values. The variable description documents whitespace splitting and the limitation for values with embedded spaces.
Registry login guidance terraform/modules/build/variables.tf, terraform/environments/azure/build.tf	The registry-login description gives Azure, AWS, and Google Cloud examples. It specifies identity-based authentication, warns against static credentials, and describes plan/apply visibility. The Azure comment documents local az login and the caller’s AcrPush requirement.

Priority: ⬆️ High

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: High

Merge Risk: 🔵 Low · up to f45fc

Build arguments now pass through the environment instead of being written into the script, and the Azure build uses identity-based registry login without an embedded password. One limitation remains: an extra build argument whose value contains spaces is split into separate arguments. This is mergeable if the maintainers are aware of that limitation or plan a follow-up.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	Issue #127 requires registry_login_command and extra_build_args to use sensitive = true, and requires remaining heredoc interpolations to be quoted. At the reviewed head, neither variable is sen…	Mark both module variables as sensitive = true. Remove the direct unquoted registry_login_command heredoc interpolation or provide a safe command transport that preserves the required login behavior. Quote all remaining single-value her…

✅ Passed checks (4 passed)

Check name	Status	Explanation
Out of Scope Changes check	✅ Passed	The reviewed changes remain within Issue #127. They modify the build module's registry-login handling, extra build argument transport, heredoc command construction, variable documentation, and the Azu…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: reducing shell-injection risk in the Terraform Docker Buildx invocation. It is concise and directly related to the changeset.

Full details: Linked Issues check

Explanation

Issue #127 requires registry_login_command and extra_build_args to use sensitive = true, and requires remaining heredoc interpolations to be quoted. At the reviewed head, neither variable is sensitive in terraform/modules/build/variables.tf. registry_login_command remains a direct, unquoted heredoc interpolation in terraform/modules/build/main.tf. The extra_build_args environment transport addresses shell parsing of its value, and terraform/environments/azure/build.tf no longer uses nonsensitive(), but these changes do not satisfy the sensitivity and quoting requirements.

Resolution

Mark both module variables as sensitive = true. Remove the direct unquoted registry_login_command heredoc interpolation or provide a safe command transport that preserves the required login behavior. Quote all remaining single-value heredoc interpolations. Update any caller or logging behavior that depends on unsuppressed sensitive provisioner output.

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Inline comments:
Review comments at @terraform/modules/build/main.tf:
- Line 123: Update the build command’s use of EXTRA_BUILD_ARGS to pass extra
build arguments as a structured argument list, preserving values containing
spaces as single arguments instead of relying on unquoted shell expansion.

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[cristim]

Independent adversarial review (two rounds) + local verification: MERGE at f45fc79. Injection half of #127 re-proven on the exact provisioner shape: a payload with ;, $(...) and backticks in extra_build_args reaches argv as inert words and runs nothing, multi-flag use still works, empty default unchanged. sensitive dropped because it suppressed the entire docker build log in every environment; the identity-based-login contract is documented on the variable instead. fmt/validate/tflint and terraform tests clean; checks green.