Hermetic Builds

What is a Hermetic Build? Understanding Build Isolation in Software Supply Chain Security

Hermetic builds are an approach to build process security and reproducibility. A hermetic build is a software build process that operates in complete isolation from external network access, relies exclusively on explicitly declared inputs, and produces identical outputs given the same inputs regardless of when or where the build executes. For DevSecOps leaders and security directors managing complex software development lifecycles, hermetic builds offer a powerful defense against supply chain attacks, dependency confusion attacks, and unauthorized code injection.

The term "hermetic" originates from the concept of an airtight seal, similar to hermetically sealed containers that prevent external contamination. When applied to software builds, this isolation creates a controlled environment where every dependency, tool, and input must be declared upfront and verified before the build process begins. This approach stands in stark contrast to traditional build systems that freely download dependencies from external repositories, potentially introducing malicious code or creating inconsistent build results.

For enterprise and mid-size businesses operating in regulated industries or handling sensitive customer data, hermetic builds provide critical guarantees about build integrity. The deterministic nature of these builds means that teams can verify exactly what went into producing a binary, recreate builds from months or years ago, and detect tampering attempts through reproducibility checks.

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Definition of Hermetic Builds: Core Principles and Technical Requirements

The definition of hermetic builds centers on three foundational principles that distinguish them from conventional build processes. Understanding these principles helps security directors and team leads evaluate whether their current build infrastructure meets hermetic standards or requires modernization.

Network Isolation During Build Execution

The first principle requires complete network isolation during the build process itself. Once a hermetic build begins executing, the build environment cannot access external networks, package registries, or download repositories. This isolation prevents several attack vectors that have plagued software supply chains, including man-in-the-middle attacks on dependency downloads, compromised package repositories, and time-of-check-time-of-use vulnerabilities.

Implementation of network isolation typically involves containerized build environments, network namespace restrictions, or dedicated build infrastructure without internet connectivity. The build system must have all required inputs staged before execution begins, creating a clear boundary between the dependency resolution phase and the actual build phase.

Explicitly Declared Inputs and Dependencies

Every input to a hermetic build must be explicitly declared, versioned, and cryptographically verified. This includes not just application dependencies but also compiler toolchains, build scripts, configuration files, and even the operating system environment where the build executes. The declaration typically takes the form of lock files, checksums, or content-addressable storage references that uniquely identify each input.

For code experts implementing hermetic builds, this requirement means moving away from version ranges and "latest" tags toward pinned versions with cryptographic hashes. Dependency manifests must capture the complete dependency graph, including transitive dependencies that traditional build systems might resolve dynamically.

Reproducible Build Outputs

The third principle demands that hermetic builds produce bit-for-bit identical outputs when given identical inputs, regardless of when or where the build executes. This reproducibility enables several security benefits: teams can independently verify that published binaries match their source code, detect unauthorized modifications to build outputs, and establish a chain of custody from source to deployment.

Achieving reproducibility requires eliminating sources of non-determinism in the build process, including timestamps, random number generation, file system ordering, and environmental variables that might vary between build executions. Modern build systems designed for hermetic operation provide mechanisms to control or eliminate these sources of variation.

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How Hermetic Builds Strengthen Software Supply Chain Security

The security benefits of hermetic builds extend throughout the software development lifecycle, addressing vulnerabilities at multiple stages of the supply chain. For security directors evaluating risk mitigation strategies, hermetic builds provide measurable improvements in attack surface reduction and incident response capabilities.

Protection Against Dependency Confusion and Substitution Attacks

Hermetic builds defend against dependency confusion attacks by requiring explicit declaration and verification of all dependencies before the build begins. When a build system must fetch dependencies from external sources during execution, attackers can exploit naming collisions between public and private packages, DNS hijacking, or compromised package repositories to inject malicious code. The isolation model of hermetic builds eliminates this attack window by staging and verifying dependencies in a separate, controlled process.

The software supply chain security landscape has seen numerous high-profile attacks exploiting dynamic dependency resolution. Hermetic builds transform dependency management from a runtime security concern into a verification problem that can be solved with cryptographic tools and policy enforcement.

Detection of Unauthorized Build Modifications

The reproducibility guarantee of hermetic builds creates a powerful audit mechanism. When multiple parties can independently rebuild software from source code and compare outputs, any discrepancy signals potential tampering. This capability proves valuable for incident response, compliance audits, and establishing trust in distributed development environments.

For DevSecOps leaders implementing continuous integration pipelines, hermetic builds enable verification workflows where production releases can be rebuilt from tagged source code and compared against deployed artifacts. Any mismatch triggers investigation rather than silent acceptance of potentially compromised binaries.

Simplified Vulnerability Management and Patch Validation

Hermetic builds maintain a complete inventory of build inputs, creating an accurate software bill of materials that security teams can use for vulnerability scanning and license compliance. When a vulnerability is discovered in a dependency, teams can quickly identify affected builds and validate that patches were correctly applied by reproducing builds before and after the update.

The explicit dependency declarations also prevent phantom dependencies—transitive dependencies that traditional build systems might download without appearing in top-level manifests. These hidden dependencies frequently introduce vulnerabilities that security scanning tools miss because they're not properly inventoried.

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Explanation of Hermetic Build Implementation Strategies

Implementing hermetic builds requires changes to build infrastructure, development workflows, and organizational processes. For team leads planning this transition, understanding the implementation options and their tradeoffs helps create realistic project timelines and resource allocations.

Build System Selection and Configuration

Modern build systems like Bazel, Buck, and Pants were designed with hermetic builds as a core feature. These systems provide dependency management, sandboxing, and caching mechanisms that enforce hermetic principles by default. Teams using these tools benefit from built-in support for content-addressable storage, remote execution, and reproducible builds.

For organizations with existing build infrastructure based on Make, Maven, Gradle, or npm, achieving hermetic builds requires additional tooling and discipline. Containerization platforms like Docker can provide isolation, while dependency management tools can enforce pinned versions and checksum verification. The challenge lies in retrofitting hermetic principles onto systems not originally designed for strict isolation.

Dependency Management and Artifact Storage

Hermetic builds require a robust artifact storage strategy that preserves all build inputs with integrity guarantees. Organizations typically implement private artifact repositories that mirror external dependencies, performing security scanning and verification before admitting packages into the trusted repository. This approach provides several benefits:

• Control over dependency availability, preventing external repository outages from blocking builds
• Centralized security scanning and policy enforcement for all dependencies
• Audit trails showing when dependencies were introduced and by whom
• Protection against package deletion or modification in upstream repositories

The build pipeline security depends heavily on dependency management practices. Private artifact repositories should implement access controls, vulnerability scanning, and provenance tracking to maintain the integrity of the hermetic build process.

Containerization and Sandbox Environments

Containers provide practical isolation for hermetic builds by creating reproducible execution environments with controlled file systems, network configurations, and process namespaces. Teams can define container images that include all necessary build tools, pin these images by cryptographic digest, and execute builds within containers that have network access disabled.

The containerization approach works well for teams already using container orchestration platforms like Kubernetes. Build containers can be scheduled as batch jobs with resource limits, timeout controls, and logging that captures build telemetry for security analysis. The container image itself becomes part of the declared build inputs, subject to the same verification requirements as application dependencies.

Handling Dynamic Build Requirements

Some build processes have legitimate needs for external data that varies over time, such as timestamp services for code signing or certificate revocation lists. Hermetic build systems handle these cases by treating the external data as an explicitly declared input that's fetched and verified before the isolated build begins. The build process itself still operates without network access, but the pre-build phase includes controlled network operations with verification.

Teams must carefully evaluate which external dependencies genuinely require dynamic fetching versus those that represent technical debt from legacy build systems. Moving toward hermetic builds often reveals implicit dependencies that should be made explicit or eliminated entirely.

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How to Transition to Hermetic Builds: A Practical Roadmap

The transition to hermetic builds represents a significant undertaking for established development organizations. For DevSecOps leaders planning this transition, a phased approach reduces risk and allows teams to build expertise gradually while demonstrating value at each stage.

Phase One: Assessment and Dependency Mapping

The first phase involves comprehensive assessment of current build processes to identify all dependencies, external network calls, and sources of non-determinism. Teams should instrument existing builds to capture network activity, file system access patterns, and environmental dependencies that might not be documented.

This assessment typically reveals surprising complexity in build processes that appeared simple. Transitive dependencies, build tool plugins, code generation steps, and post-build processing often introduce dependencies that weren't explicitly managed. Creating a complete dependency graph provides the foundation for hermetic implementation.

Phase Two: Dependency Pinning and Verification

The second phase focuses on converting dynamic dependency resolution to explicitly pinned versions with cryptographic verification. This work includes:

• Generating lock files that capture complete dependency graphs with version pins
• Implementing checksum verification for all downloaded dependencies
• Establishing private artifact repositories with security scanning
• Creating processes for dependency updates that maintain hermetic properties

Teams should prioritize high-risk components and critical build paths during this phase. Not all builds need to become hermetic simultaneously, and focusing on security-critical systems provides immediate risk reduction while building organizational capability.

Phase Three: Build Isolation and Sandboxing

The third phase implements isolation mechanisms that prevent network access during build execution. Depending on existing infrastructure, this might involve containerization, build system migration, or custom sandboxing solutions. The goal is ensuring that builds execute with only their declared inputs available, catching any undeclared dependencies through build failures rather than allowing silent network fetches.

Build failures during this phase indicate successful enforcement of hermetic principles. Each failure represents a previously undeclared dependency that must be added to the explicit dependency list or eliminated. Teams should expect significant troubleshooting during this phase as hidden dependencies surface.

Phase Four: Reproducibility and Verification

The final phase focuses on achieving bit-for-bit reproducible builds and implementing verification workflows. This requires eliminating sources of non-determinism like timestamps, ensuring deterministic file ordering, and controlling compiler optimizations that might vary across platforms.

Organizations should implement continuous verification where builds are periodically reproduced and compared against original outputs. Discrepancies trigger security reviews to determine whether legitimate build changes occurred or whether tampering should be investigated. This verification capability transforms hermetic builds from a development practice into an ongoing security control.

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Hermetic Builds and Modern DevSecOps Practices

Hermetic builds integrate with broader DevSecOps practices to create defense-in-depth for software supply chains. For security directors building comprehensive security programs, hermetic builds complement other security controls while enabling new capabilities that weren't practical with traditional build systems.

Integration with SBOM Generation and Management

The explicit dependency declarations required for hermetic builds provide accurate input for software bill of materials generation. Unlike SBOM tools that must reverse-engineer dependencies from built artifacts, hermetic build systems have complete dependency information as a build requirement. This relationship ensures SBOM accuracy and completeness, addressing a common weakness in SBOM initiatives where transitive dependencies get missed.

Organizations can generate SBOMs directly from build system metadata, creating a reliable link between deployed software and its declared components. The SBOM practices benefit significantly from hermetic build adoption, as the accuracy and completeness of dependency information improves dramatically.

Enhanced CI/CD Pipeline Security

Hermetic builds strengthen continuous integration and continuous delivery pipelines by reducing the attack surface available to compromised build agents or malicious insiders. When build processes cannot access external networks, attackers who compromise build infrastructure face significant limitations in exfiltrating code or introducing backdoors.

The reproducibility of hermetic builds also enables verification stages in deployment pipelines where release candidates are independently rebuilt and compared before production deployment. This verification catch unauthorized modifications that might evade other security controls, providing a final check before software reaches production environments.

Compliance and Audit Requirements

Regulated industries increasingly face requirements to demonstrate software integrity and maintain chain of custody from source code to deployed systems. Hermetic builds provide evidence that audit and compliance teams need to verify build processes, validate dependency management, and demonstrate reproducibility.

The detailed build metadata from hermetic systems supports compliance reporting by documenting exactly what went into each build, when dependencies were updated, and who approved changes to build configurations. This audit trail addresses regulatory requirements while reducing the manual effort needed for compliance documentation.

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Common Challenges and Solutions in Hermetic Build Adoption

Organizations implementing hermetic builds encounter predictable challenges that can derail adoption if not addressed proactively. Understanding these challenges and proven solutions helps team leads set realistic expectations and plan for obstacles.

Legacy Build System Compatibility

Many organizations have mature build systems optimized for developer productivity rather than security. These systems often rely on implicit dependencies, dynamic resolution, and convenience features that conflict with hermetic principles. The challenge lies in modernizing these builds without disrupting development velocity or breaking existing workflows.

Successful approaches typically involve gradual migration rather than wholesale replacement. Teams can start by containerizing existing builds to gain basic isolation, then progressively add dependency pinning and verification. Parallel operation of old and new build systems during transition periods allows teams to validate hermetic builds against known-good outputs from legacy systems.

Build Performance and Caching

Hermetic builds can initially appear slower than traditional builds because they can't opportunistically use cached dependencies from external networks. Organizations address this through aggressive caching strategies that preserve hermetic properties while improving performance.

Content-addressable storage systems allow safe cache sharing between builds and developers. Since inputs are identified by cryptographic hashes rather than mutable version numbers, cache hits guarantee that the same input is being reused. Remote build execution services can provide shared caches across entire development organizations, amortizing the cost of building common dependencies.

Developer Experience and Workflow Changes

Developers accustomed to flexible build systems sometimes view hermetic builds as bureaucratic overhead that slows down iteration. The requirement to explicitly declare and verify dependencies can feel burdensome compared to traditional package managers that automatically resolve dependencies.

Organizations overcome this resistance by automating dependency management tasks and providing clear documentation of the security benefits. Tools that automatically generate lock files, update dependency pins, and verify checksums reduce the manual burden on developers. Education about supply chain attacks and the protection that hermetic builds provide helps development teams understand the rationale behind new processes.

Third-Party Dependencies and Ecosystem Limitations

Some programming language ecosystems make hermetic builds more challenging than others. Dynamic languages with extensive runtime dependency loading, build tools that embed network fetching in core functionality, or dependencies with non-reproducible build processes all create friction for hermetic adoption.

Teams working in these ecosystems should focus on controlling what they can while accepting some compromises. Even partial hermetic builds that reduce attack surface and improve reproducibility provide security benefits over completely traditional approaches. The goal is continuous improvement toward hermetic ideals rather than perfect compliance with an abstract standard.

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Measuring Success: Metrics for Hermetic Build Programs

DevSecOps leaders need metrics to evaluate hermetic build implementation progress and demonstrate value to organizational stakeholders. These measurements should cover both technical implementation status and business outcomes that resonate with decision makers.

Technical Metrics and Indicators

Key technical metrics for hermetic build programs include:

• Percentage of builds executing without network access
• Number of builds with complete dependency pinning and verification
• Reproducibility rate when rebuilding from source
• Time to detect and respond to dependency vulnerabilities
• Number of undeclared dependencies discovered and remediated
• Cache hit rates and build performance metrics

These metrics provide objective measures of implementation progress and help identify areas needing attention. Tracking trends over time demonstrates continuous improvement and helps justify resource investments in build infrastructure.

Business and Risk Metrics

Decision makers care about risk reduction and business impact more than technical implementation details. Relevant business metrics include:

• Reduction in security incidents related to supply chain compromises
• Time saved in incident response through improved build auditing
• Compliance audit effort reduction from automated build documentation
• Developer productivity impact (positive or negative)
• Cost reduction from elimination of security tools made redundant by hermetic builds

Organizations should baseline these metrics before hermetic build adoption to demonstrate measurable improvement. The ability to show concrete risk reduction and efficiency gains helps maintain executive support for ongoing security investments.

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Hermetic Builds in the Broader Security Architecture

Hermetic builds represent one component of comprehensive software supply chain security rather than a complete solution. Security directors should understand how hermetic builds interact with other security controls to create layered defenses.

Relationship with Code Signing and Provenance

Hermetic builds provide the foundation for meaningful code signing and provenance attestations. When builds execute in isolated, reproducible environments, signing keys can be restricted to these controlled environments rather than distributed to individual developers. The combination of hermetic builds and cryptographic signing creates strong assurance about software origins and integrity.

Provenance metadata generated during hermetic builds can document the complete build environment, inputs, and process used to create artifacts. This metadata supports supply chain levels for software artifacts (SLSA) and similar frameworks that require evidence of build security practices.

Complement to Runtime Security Controls

Hermetic builds address supply chain risks during the build phase but don't replace runtime security controls like container scanning, runtime application self-protection, or behavioral monitoring. Organizations need both build-time and runtime defenses to address the full range of potential attacks.

The dependency inventory from hermetic builds feeds into runtime security by providing accurate information about what should be present in deployed systems. Deviations detected at runtime might indicate compromise or configuration drift that requires investigation.

Foundation for Zero Trust Architecture

Zero trust principles apply to build systems just as they do to production infrastructure. Hermetic builds implement zero trust by never implicitly trusting external dependencies, requiring explicit verification of all inputs, and assuming that network connections could be compromised.

Organizations implementing zero trust architecture should extend these principles to development and build infrastructure. Hermetic builds naturally align with zero trust by eliminating implicit trust relationships and requiring verification at every stage.

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Ensure Your Builds Meet Modern Security Standards

Securing your software supply chain requires more than good intentions—it demands robust tooling and processes that enforce security policies throughout your development lifecycle. Hermetic builds provide critical protection against supply chain attacks, but implementing them effectively requires expertise and the right platform capabilities.

Kusari's software supply chain security platform helps organizations implement and maintain hermetic build practices at scale. Our solution provides dependency verification, build isolation enforcement, and reproducibility validation integrated with your existing development tools. Security teams gain visibility into build processes while developers maintain the productivity they need to ship features quickly.

Discover how Kusari can strengthen your software supply chain security and enable hermetic builds across your organization. Schedule a demo to see how leading enterprises are protecting their build processes.

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What Are the Main Benefits of Implementing Hermetic Builds?

The main benefits of implementing hermetic builds center on security, reproducibility, and operational confidence. Hermetic builds eliminate entire classes of supply chain attacks by isolating the build process from external networks and requiring explicit declaration of all dependencies. This isolation prevents dependency confusion attacks, man-in-the-middle attacks on package downloads, and unauthorized code injection that plague traditional build systems.

Beyond security, hermetic builds provide reproducibility guarantees that enable independent verification of software artifacts. When multiple parties can rebuild software from source and produce identical binaries, organizations gain strong assurance that published artifacts match their source code without tampering. This capability proves valuable for incident response, compliance audits, and establishing trust in distributed development environments.

The operational benefits include simplified vulnerability management through accurate dependency inventory, faster incident response enabled by detailed build metadata, and reduced risk of build failures due to external service outages. Organizations with hermetic builds can confidently rebuild years-old releases because all dependencies remain available in controlled artifact repositories rather than depending on external package registries that might change or disappear.

For DevSecOps leaders, hermetic builds provide measurable risk reduction that satisfies both security requirements and business needs for reliable software delivery. The initial investment in hermetic infrastructure pays ongoing dividends through reduced security incidents and improved development efficiency.

How Do Hermetic Builds Differ from Traditional Build Processes?

Hermetic builds differ from traditional build processes primarily in their approach to dependencies and isolation. Traditional build systems typically fetch dependencies dynamically during build execution, connecting to external package repositories like npm, PyPI, or Maven Central to download required libraries. This approach prioritizes developer convenience but creates security vulnerabilities where compromised repositories or network attacks can inject malicious code.

Hermetic builds reverse this model by requiring all dependencies to be declared explicitly, verified cryptographically, and staged before the isolated build begins. Once the build process starts executing, network access is completely disabled, preventing any dynamic dependency fetching. Traditional builds might specify dependency version ranges and resolve the latest compatible version at build time, while hermetic builds pin exact versions with cryptographic hashes ensuring the same dependency is used consistently.

The reproducibility guarantees also distinguish hermetic builds from traditional approaches. Traditional build systems often produce slightly different outputs when run at different times or on different machines due to timestamps, environmental variability, or dependency updates. Hermetic builds eliminate these sources of non-determinism, producing bit-for-bit identical outputs given identical inputs.

For code experts implementing build systems, the difference lies in trading some convenience for significant security and reliability benefits. Hermetic builds require more upfront work to declare dependencies and configure isolation, but they eliminate entire categories of security vulnerabilities and operational failures that plague traditional build systems.

What Tools and Technologies Support Hermetic Build Implementation?

Several modern build tools and technologies were designed specifically to support hermetic build implementation. Bazel, developed by Google and released as open source, provides comprehensive hermetic build capabilities including content-addressable dependency management, sandboxed execution, and remote build execution. Bazel's design assumes hermetic principles by default, making it the gold standard for organizations committing to this approach.

Buck and Pants represent alternative build systems with hermetic capabilities, each offering different tradeoffs in terms of language support, performance characteristics, and migration paths from existing build systems. These tools share the core hermetic principles while differing in implementation details and ecosystem maturity.

For organizations with existing build infrastructure based on Maven, Gradle, npm, or pip, achieving hermetic builds requires supplementary tools and practices. Container platforms like Docker provide isolation when combined with network policies that disable external access. Dependency management tools like Kusari Inspector can automate the generation of lock files and checksum verification that hermetic builds require.

Artifact repository solutions like Artifactory, Nexus, or cloud-native alternatives provide the controlled dependency storage that hermetic builds depend on. These repositories mirror external packages, perform security scanning, and ensure that dependencies remain available even if upstream sources change or disappear.

Build execution platforms with support for hermetic principles include cloud build services that provide isolated execution environments, content-addressable caching, and reproducible build capabilities. The choice of tools depends on existing technology investments, language ecosystems, and organizational constraints around cloud adoption.

How Long Does It Take to Implement Hermetic Builds Across an Organization?

The timeline for implementing hermetic builds across an organization varies significantly based on codebase size, existing build system maturity, and available engineering resources. Small organizations with modern codebases might achieve hermetic builds for critical systems within a few months, while large enterprises with diverse technology stacks and legacy build systems should plan for multi-year initiatives.

A typical implementation following the phased approach begins with a three to six month assessment and pilot phase where teams evaluate current build processes, map dependencies, and implement hermetic builds for a representative project. This pilot provides concrete experience with the challenges and benefits, informing realistic plans for broader rollout.

The dependency pinning and verification phase often requires six to twelve months for organizations with extensive codebases. Teams must generate lock files, implement checksum verification, establish private artifact repositories, and create processes for ongoing dependency management. This work can proceed incrementally across different projects, with high-priority systems converting first.

Build isolation and sandboxing implementation typically takes three to six months per major build system or platform. Organizations with multiple language ecosystems might have several parallel workstreams addressing different technology stacks. The discovery of undeclared dependencies during this phase can extend timelines as teams investigate and remediate issues.

Achieving full reproducibility and implementing verification workflows adds another six to twelve months as teams eliminate sources of non-determinism and establish processes for continuous verification. Organizations should expect the entire journey from initial assessment to comprehensive hermetic builds to span two to three years for large enterprises, with incremental security benefits realized at each phase rather than waiting for complete implementation.

Strengthening Your Software Supply Chain with Hermetic Builds

Hermetic builds represent a fundamental improvement in software supply chain security, addressing vulnerabilities that traditional build systems leave exposed. By isolating build processes from external networks, requiring explicit dependency declarations, and ensuring reproducible outputs, hermetic builds provide defense against dependency confusion, unauthorized code injection, and supply chain tampering.

For DevSecOps leaders, security directors, and team leads responsible for protecting software development lifecycles, hermetic builds offer measurable risk reduction backed by strong technical principles. The implementation requires significant investment in build infrastructure and process changes, but the security benefits justify this effort for organizations handling sensitive data or operating in regulated industries.

The journey toward hermetic builds doesn't require wholesale replacement of existing build systems overnight. Organizations can adopt hermetic principles incrementally, starting with high-risk components and expanding coverage as teams develop expertise. Each step toward greater isolation, explicit dependencies, and reproducibility provides security improvements while building toward comprehensive hermetic builds.

Modern software development faces persistent threats from supply chain attacks that exploit vulnerabilities in dependency management and build processes. Hermetic builds eliminate many of these attack vectors through architectural principles rather than depending on vigilance alone. As supply chain security continues evolving, hermetic builds will become table stakes for organizations serious about protecting their software delivery pipelines and the customers who depend on their products.