SBOM Adoption Guide: Part 4 - Using External SBOM References

Overview and Introduction

Welcome to the fourth article in the “Care and Feeding of SBOMs” series! We started by exploring the strategic importance of SBOMs in “SBOM Adoption at Scale: Burden or Opportunity?”, then delved into practical generation techniques in “How to Build SBOMs at Scale” and tackled the complexities of intake in “How to Ingest and Manage SBOMs at Scale”. (Need an intro-level background? Check out “SBOM 101: What is an SBOM?”).

This installment covers a slightly more technical aspect of managing SBOMs but one that is still strategic, nesting external SBOM references inside your own for scale, accuracy, and efficiency. As software becomes more deeply integrated with and dependent upon outside and shared software services, listing every dependency in one monolithic SBOM becomes impractical, error-prone, and expensive to manage. External references provide a mechanism for an SBOM to point to another SBOM (which in turn may point to others), creating a dependency graph representing the complete software inventory that can efficiently incorporate components and dependencies maintained elsewhere.

This article is for software developers, DevOps engineers, third-party software integrators, and AppSec leaders who need to understand why effective external SBOM reference management has become a necessity, how these references behave, the requirements they introduce, and the capabilities and best practices best suited to meet these requirements effectively.

Why External SBOM References Are Necessary: Common Use Cases

In modern software development, completely self-contained applications are rare. Dependencies often have their own complex dependency trees. Including every single item from every dependency directly in your application’s primary SBOM can lead to massive, unwieldy documents and potential inaccuracies when (not if) the underlying dependency’s own SBOM changes. The following common scenarios make it all too clear that external references are the only way to go:

1. Container Base Images:

Scenario: Your application is packaged as a container (e.g., using Docker). It builds upon a base OS image like Alpine, Ubuntu, or Red Hat UBI. These base images contain hundreds or thousands of system libraries and packages.
Forcing function: Duplicating the entire component list of the base image into every application container’s SBOM is highly inefficient and prone to error. The base image maintainer (e.g., Canonical for Ubuntu, Docker for official images) is the authoritative source and increasingly provides SBOMs for specific image versions. Your application’s SBOM should reference this official base image SBOM (when available) rather than trying to replicate its contents.
Why external references offer the best solution:

Authority: Ensures you rely on the official software inventory from the maintainer who actually built and understands the base image components.
Efficiency: Avoids massive duplication and bloat by not repeating potentially thousands of base OS components in every single application SBOM.
Maintainability: Updates to the base image’s composition (and its SBOM) are managed by the maintainer; consumers only need to update the reference to the appropriate version.

2. Bundled Commercial Off-The-Shelf (COTS) Software or Complex Libraries:

Scenario: Your product incorporates a significant piece of commercial software (like a database engine, a specialized analytics library) or a large, complex open-source component (like components within the Kubernetes ecosystem). The vendor or maintainer of that component provides their own detailed SBOM.
Forcing function: The vendor/maintainer is the authoritative source for their component’s inventory. Copying their list into your SBOM risks inaccuracies if (not when) their component updates. Referencing their provided SBOM ensures you point to the correct, authoritative source, maintaining accuracy and traceability.
Why external references offer the best solution:
- Authority & Accuracy: Leverages the vendor’s or maintainer’s detailed knowledge of their own complex component, avoiding errors from attempting to re-analyze a black-box or intricate dependency.
- Completeness: Ensures reliance on the vendor’s official component list and associated metadata (like specific license declarations) within their authoritative SBOM.
- Focus: Allows your primary SBOM to focus on your application’s structure and direct dependencies, keeping it more manageable.

3. Large Internal Projects & Monorepos (monolithic repositories):

Scenario: Within a large organization, different teams develop shared internal libraries, microservices, or platform components. Each team generates an SBOM for its specific artifact. An application built by composing these internal components needs to account for their dependencies.
Forcing function: Instead of each application team copying the full SBOM from every internal library they use (which might change frequently), the application’s SBOM can simply reference the official, version-specific SBOM published internally for that library or service. This promotes modularity and ensures consistency across the organization.
Why external references offer the best solution:
- Consistency: Ensures all internal consumers are referencing the exact same, official software inventory for a given version of a shared library or service.
- Modularity & Independence: Allows teams to manage and update SBOMs for their components independently without forcing widespread, duplicative changes in consuming applications’ SBOMs.
- Velocity: Reduces friction and redundant work, as consuming teams don’t need to constantly re-analyze or copy SBOM data for shared internal components.

Distillation: The Need for References

In these common and ubiquitous scenarios, attempting to create a single, fully “flattened” SBOM by duplicating all components from all dependencies is impractical leading to:

Bloated SBOMs: Documents become excessively large and difficult to process.
Maintenance Nightmares: Keeping duplicated information synchronized with upstream changes is extremely difficult.
Loss of Authority: Duplication undermines the principle of using the most authoritative source for component information.

Using external references is not just a convenience; it’s a necessity for accurately representing complex software inventory resulting from modern software composition in a manageable and scalable way.

How Standards Support Referencing

Recognizing the need for modularity and relying on authoritative sources, the primary SBOM standards, SPDX and CycloneDX, include built-in mechanisms to formally link one SBOM document to another.

This capability is fundamental to managing complex software inventories efficiently without excessive duplication and SBOM management tools leverage these technical specifications to trace dependencies across referenced documents, enabling the scalable analysis required for complex software.

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Technical Note: Referencing Mechanisms

While the exact syntax differs, both major standards provide ways to link SBOMs:

SPDX: Primarily uses document-level references (externalDocumentRef) to link to an entire external SBOM, including unique identifiers and checksums for integrity verification.
CycloneDX: Often employs a component-centric approach, using external references (externalReferences of type bom) within a component’s data to point to its specific SBOM, or relying on component identifiers and relationships defined across potentially multiple BOMs.

Detailed syntax examples illustrating these mechanisms in JSON format can be found in Sample_External_Reference_Syntax.txt file. (see accompanying file – NOTE the file needs to be reviewed)

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Challenges and Requirements for Managing References

While necessary, external references are not “free” as they come with their own challenges and requirements.

Trust & Authenticity

How do you know the referenced SBOM is accurate, comes from the legitimate supplier, and hasn’t been tampered with? Relying on an untrusted or modified referenced SBOM could introduce major security blind spots.

Requirements: Source Verification & Integrity Checks.

Mechanisms to verify the origin and integrity of referenced SBOMs are needed. This could involve checking digital signatures on the referenced SBOMs, retrieving them only from trusted, authenticated repositories or vendor portals, or validating checksums provided in the reference link.

Reference Rot & Availability

External references often use URLs or other locators. What happens if the URL becomes invalid, the document is moved, or the hosting server goes down? The link “rots,” and you lose visibility into that part of your software inventory.

Requirements: Reference Validation & Caching.

Tooling and processes must validate the accessibility of external references during SBOM ingestion. For critical dependencies, organizations need the ability to cache or mirror referenced SBOMs locally, creating a fallback to mitigate the risk of external unavailability. Periodic re-validation of links is also required.

Recursive Depth & Completeness

Referenced SBOMs can themselves contain external references, creating a chain or tree of dependencies. How deep should analysis tools follow these chains? What happens if a link is broken mid-chain? An incomplete traversal means an incomplete view of the full dependency graph and potential vulnerabilities.

Requirements: Controlled Reference Resolution & Gap Handling.

Tools need configurable depth limits for traversing references to prevent infinite loops and manage performance. They must also be able to detect and report broken links or missing references within the chain. Organizations need a strategy for assessing the risk associated with incomplete data.

Version Synchronization

How do you ensure the referenced SBOM corresponds precisely to the version of the component (e.g., base image, library) actually being used in your software? Referencing an SBOM for the wrong version provides inaccurate information.

Requirements: Precise Linking & Validation.

The referencing mechanism must use specific identifiers that include version information (like PURLs, CPEs with versions, or specific document checksums). Tooling should validate that the referenced SBOM matches the expected version of the component declared in the primary SBOM.

Aggregated Analysis (Vulnerabilities/Licenses)

Your security and compliance posture depends on the entire software stack. How do you get a consolidated view of all vulnerabilities and licenses across your primary SBOM and all nested, referenced SBOMs? Analyzing each document separately is inefficient and error prone.

Requirements: Automated Reference Resolution & Composite Reporting.

Effective SBOM management requires tooling that can automatically follow valid references (to the configured depth), parse the referenced SBOMs, and aggregate the component, vulnerability, and license data into a single, unified view for analysis, policy checking, and reporting. This process is often called “rehydrating” or “flattening” the SBOM for analysis purposes.

Best Practices for Managing External References

Overcoming the challenges above requires implementing specific best practices that are, in turn, enabled through purpose-built tooling:

Validate References During Ingestion: Don’t accept SBOMs with broken or untrusted references blindly. Use tools to automatically check the availability and, where possible, the integrity (checksums, signatures) of linked external SBOMs as part of your intake process (covered in Article 3: How to Ingest and Manage SBOMs at Scale).
Establish Trust Policies: Define criteria for acceptable sources of referenced SBOMs. Prioritize references that are digitally signed or hosted on secure, authenticated vendor portals over unsecured HTTP links.
Implement Caching/Mirroring: For critical dependencies or references prone to availability issues, automatically download and store local copies of referenced SBOMs within your SBOM management platform.
Track Changes and Updates: Monitor sources of referenced SBOMs for updates. When a referenced component (like a base image or library) releases a new version with a new SBOM, ensure your tooling ingests it and updates your composite view.
Leverage Tools for Composite Analysis: Use an SBOM management platform capable of automatically resolving references (“rehydrating” or “flattening” the view) to provide a complete, aggregated picture of the entire software inventory (all components) along with associated licenses and vulnerabilities across the entire dependency chain for accurate risk assessment and policy enforcement.
Configure Analysis Depth: Set sensible limits within your tooling for how deep into recursive references the analysis should go, balancing completeness with performance and complexity management.

Benefits of Mastering External References

Fortunately, effectively managing external SBOM references isn’t solely about confronting a new crop of DevOps requirements; it’s also very much about enabling scalable and accurate software supply chain management. Organizations with mature processes and tooling for handling external SBOM references will find that they have:

More Manageable SBOMs: Primary SBOMs remain focused and concise.
Improved Accuracy: Relies on authoritative SBOMs from original suppliers.
Enhanced Scalability: Handles complex dependencies without manual duplication.
Better Risk Assessment: Achieves a more complete view of vulnerabilities and licenses across the full stack through automated aggregation.

Beyond these operational advantages, demonstrating this capability significantly enhances transparency during critical business events like mergers and acquisitions (M&A). Providing potential acquirers or investors with accurate, manageable software inventories that properly handle external dependencies allows them to assess technical and Intellectual Property and licensing risk faster and with greater confidence. For sellers, mature external reference management streamlines due diligence and signals a higher level of operational maturity, potentially impacting overall company valuation.

Knowledge Review

Q1: Why are external SBOM references often necessary for modern software?
A: Because software often relies on complex components (like container base images, large libraries, COTS software, internal microservices) that have their own detailed SBOMs. Duplicating all components in the main SBOM is impractical, inefficient, and error prone. References allow pointing to the authoritative source.
Q2: What is “reference rot” in the context of SBOMs?
A: It occurs when the link (URL or other locator) pointing to an external SBOM becomes invalid or inaccessible, breaking the chain of visibility into that component’s dependencies.
Q3: What capability is needed to get a complete vulnerability picture when using external references?
A: Tooling that supports automated reference resolution (or “rehydration”) is needed to follow the links, parse the referenced SBOMs (to a defined depth), and aggregate all component and vulnerability data into a single, composite view for analysis.
Q4: Name two key challenges associated with trusting referenced SBOMs.
A: Ensuring the referenced SBOM actually came from the claimed, legitimate source (authenticity) and verifying that it hasn’t been tampered with since publication (integrity). Checking digital signatures or using trusted repositories helps address this.
Q5: Why is simply listing external references not enough for comprehensive risk analysis?
A: Because the risks (vulnerabilities, license issues) reside within the components detailed inside the referenced SBOMs. Without resolving the references and analyzing the aggregated data, the full risk picture remains hidden.

Bonus Section – How SOOS Helps

(This section highlights relevant SOOS capabilities based on the challenges and practices discussed.)

Managing external SBOM references effectively requires robust tooling designed for the complexities involved. SOOS provides capabilities to address the key challenges:

Detects and Verifies External References: During ingestion, SOOS identifies references in formats like SPDX and CycloneDX and can validate their accessibility. (Addresses Challenge 2: Rot/Availability; Supports Best Practice 1)
Alerts on Broken or Stale References: SOOS monitors the status of external links and can alert users if a previously valid reference becomes inaccessible. (Addresses Challenge 2: Rot/Availability; Supports Best Practice 4)
Maintains Linked Relationships: SOOS understands the relationships between a primary SBOM and its external references, preserving the modular structure for traceability. (Supports underlying need for references)
Allows Optional Rehydration for Analysis: For comprehensive vulnerability and license scanning, SOOS can resolve external references (to a configurable depth) and analyze the aggregated component data, providing a complete risk picture. (Addresses Challenge 5: Aggregated Analysis; Supports Best Practice 5)
Tracks Reference Origin and Metadata: SOOS helps manage information about the source and trustworthiness of referenced SBOMs. (Addresses Challenge 1: Trust; Supports Best Practice 2)

By integrating these capabilities, SOOS helps organizations leverage the benefits of external references while mitigating the associated risks. Learn more about SOOS SBOM Manager.