The concept of Runtime Contracts is an effective way to add behavioral safety tests to CI so teams can detect malicious dependencies and transitive dependency threats before they become backdoors in production. This article explains what runtime contracts are, why behavioral testing complements static supply‑chain tools, and how to implement practical CI checks that stop suspicious or unexpected dependency behavior early in the development lifecycle.
Why runtime contracts matter for supply‑chain security
Traditional supply‑chain tooling—SBOM generation, signature verification, and vulnerability scanners—focuses on provenance and known vulnerabilities. Those tools are essential, but they often miss novel or deliberately malicious behavior embedded deep in transitive dependencies. Runtime contracts express the expected behavior of third‑party components (network targets, file system access, spawned processes, environment mutation, and API usage). When enforced as tests in CI, they provide a behavioral gate: if a dependency acts outside its contract, the build fails and the issue is investigated.
Core principles of behavioral safety tests
- Least privilege by behavior: Define not just which packages are allowed, but what those packages are allowed to do at runtime.
- Small, testable contracts: Contracts should be precise (e.g., “this library may call github.com:443” or “this module must not create new processes”).
- Deterministic tests: Keep tests reproducible in CI using containers, mocks, and deterministic inputs to avoid flaky failures.
- Observable assertions: Assert on logs, network traffic, file access events, and syscall traces rather than just exit codes.
- Fail-fast and actionable: Fail CI early with clear diagnostic artifacts (traces, pcap, logs) so engineers can triage quickly.
Designing runtime contracts
Start by inventorying how your application and its dependencies are expected to behave. Typical contract elements include:
- Allowed network destinations and protocols (hostnames, IP ranges, ports)
- Allowed filesystem paths and file modes (read vs write, config directories)
- Process creation rules (allowed executables or none at all)
- External resource access (databases, secret stores) and expected authentication flows
- Time/cron behavior (scheduled background jobs should not spawn in short‑lived CI runs)
Represent contracts in a machine‑readable policy (YAML/JSON) that your CI jobs and runtime monitors can enforce. Keep policies scoped to the smallest module possible to reduce false positives.
How to add behavioral safety tests to CI
Integrate behavioral testing into your existing CI pipeline as separate jobs that run after dependency install but before merge. A typical workflow includes:
- Install dependencies and produce an SBOM to map direct and transitive packages.
- Start a controlled runtime (container, VM, or process sandbox) with the code and its dependencies.
- Execute a scripted workload that exercises common code paths and imports the dependency graph.
- Use instrumentation (syscall tracing, network capture, file access hooks, or eBPF probes) to collect behavioral data.
- Compare observed behavior against defined runtime contracts; if any assertion fails, fail the CI job and publish artifacts for investigation.
Implementation tips:
- Run tests in an isolated container with no external network by default, then selectively allow expected endpoints using a whitelist for test cases that require connectivity.
- Collect rich evidence: process trees, syscall logs, network captures (pcap), and a copy of the SBOM for correlation.
- Automate artifact upload (to CI job logs or artifact storage) so triage teams can reproduce locally.
Practical tooling and building blocks
You don’t need bespoke tooling to start—combine existing open tools for fast wins:
- SBOM tools: CycloneDX, SPDX generators (to enumerate transitive dependencies)
- Provenance and signing: Sigstore, in‑toto for verifying build provenance
- Static checks: Dependency checkers (OWASP Dependency‑Check, Snyk, Trivy) as the first filter
- Runtime observability: Strace or syscall tracing, Falco for rule‑based detection, lightweight eBPF probes for Linux
- Network control: Container network policies, network namespaces, or local proxy mocks to detect unexpected external calls
Compose these pieces in CI: generate an SBOM during dependency install, run a small integration harness that exercises imports, and enforce policies with an assertion engine that reads the policy file and observed telemetry.
Detecting transitive dependency threats specifically
Transitive dependencies are tricky because they may be pulled in indirectly and rarely exercised in unit tests. To catch them:
- Use SBOM mapping to identify newly introduced transitive packages on dependency updates and require a manual approval step or extra behavioral tests for any new transitive entries.
- Targeted fuzzing: run lightweight fuzz or mutation tests that exercise less common code paths in dependencies (fuzz inputs often trigger unexpected behavior).
- Canary execution: run new or changed dependencies in an even stricter sandbox (no network, read-only filesystem) and watch for contract violations before allowing them to the main branch.
Balancing security and developer velocity
Behavioral safety tests must be fast and deterministic to avoid slowing development. Strategies for balance:
- Run quick smoke contracts on pull requests and full behavioral suites in nightly gates or pre‑release pipelines.
- Classify dependencies: low‑risk core libraries get lighter checks; new or rarely‑used transitive libs get stricter scrutiny.
- Provide clear remediation guidance in CI failures (which policy failed, what evidence was captured, and recommended next steps).
Operationalizing post‑detection
When a runtime contract is violated, the CI job should:
- Fail the build and block merging
- Attach diagnostic artifacts and a succinct incident summary
- Create an automated ticket containing the SBOM diff, the offending package, and the failing assertion
- Trigger an incident response playbook if high‑risk behavior (exfiltration, remote code execution) is observed
Having an established triage path prevents noisy alerts from becoming a bottleneck and ensures real threats are escalated quickly.
Getting started checklist
- Create a minimal runtime contract template for your app (network, file, process rules)
- Generate SBOMs on every dependency change and compare diffs in CI
- Implement a test harness that runs dependencies in a container and captures syscalls and network traffic
- Integrate policy enforcement into CI jobs and fail fast with helpful artifacts
- Iterate on policies to reduce false positives and expand coverage to new dependency classes
Runtime Contracts are not a silver bullet, but when combined with provenance checks and static scanning they form a resilient, layered defense that catches behavior attackers often rely on to turn dependencies into backdoors.
Conclusion: Embedding behavioral safety tests in CI through clear runtime contracts raises the cost for attackers and dramatically improves the ability to catch malicious or unexpected transitive dependency behavior before it reaches production. Start small, automate the evidence collection, and iterate on policies to protect the supply chain while keeping developer velocity high.
Ready to stop backdoors before they start? Add a runtime contract smoke test to your CI pipeline this week and watch your supply‑chain risk drop.
