ESP-IDF SBOM Tool (esp-idf-sbom)

:warning: This is experimental implementation, which is subject to change without prior notice and no backward compatibility is guaranteed.

The esp-idf-sbom tool creates Software bill of materials (SBOM) files in the Software Package Data Exchange (SPDX) format for applications generated by the Espressif IoT Development Framework (ESP-IDF). It also allows to check generated SBOM files for know vulnerabilities against the National Vulnerability Database (NVD) based on the Common Platform Enumeration (CPE) provided in the SBOM.

Required ESP-IDF versions

All release branches of currently supported ESP-IDF versions allow to generate the SBOM file.

Older versions, e.g. v5.0.2, do not have the required code merged and the following error message will be printed.

E: Project description file "build/project_description.json" does not support SBOM generation. Please see the list of IDF versions required by esp-idf-sbom.

If you see this error message and want to try esp-idf-sbom, you can

1. switch to the release branch for version you are using. For example release/v5.0 if you are using v5.0.2.

2. use future ESP-IDF versions to experiment with esp-idf-sbom.

3. use git-cherry-pick and apply commits for your release from the table above. For example for v5.0.2 use

   $ git cherry-pick 30735b33efabd6cf038bcb258b674cf828ad5ecf 9156bbb55c920d6704329975311c331b931ed6bc

Installation

Currently esp-idf-sbom is not integrated into ESP-IDF and needs to by installed separately from Python Package Index (PyPI) with

pip install esp-idf-sbom

After installation the esp-idf-sbom command should be available or the esp_idf_sbom python module can be used with

python -m esp_idf_sbom

Creating SBOM

The application has to be built before the SBOM file can be created. This step should not be required in the future once esp-idf-sbom is integrated into ESP-IDF.

The SBOM file can be created with

esp-idf-sbom create <project description file>

The project description file is a path to the project_description.json file, which is by default created by the ESP-IDF build system in the project's build directory. The generated SBOM file is printed to the standard output stream by default or can be saved in a file if the --output-file option is used.

Checking vulnerabilities

Vulnerabilities are checked based on the Common Platform Enumeration (CPE) information included in the SBOM file for SPDX packages. While checking for vulnerabilities, only packages with direct or indirect relationship to the project package are examined. For example if mbedtls component is compiled, due to component dependecies, but it's actually not linked into the final binary, it will be by default presented in the SBOM file, but it will not be reachable from the root project package and hence it will not be checked for vulnerabilities. The reason for this is to avoid possible false positives, because such packages have no direct impact on the resulting application. This can be changed with the --check-all-packages option. If used, all packages in the SBOM file will be checked for possible vulnerabilities regardless their relationships to the application binary.

SBOM file generated by the esp-idf-sbom tool can be checked for known vulnerabilities with

esp-idf-sbom check [SBOM file]

If SBOM file is not provided, the standard input stream is used.

The default report format consists of multiple tables:

1. Report summary
2. Packages with identified vulnerabilities
3. Packages with excluded vulnerabilities
4. Packages with no identified vulnerabilities
5. Packages without CPE information not applicable for vulnerability check

The output format may be changed with the --format option, which supports exporting the report into json, csv or markdown format.

If package is not vulnerable to a specific CVE, it can be added to the manifest cve-exclude-list list and checker will not report it as identified vulnerability, but as excluded vulnerability.

Usage example

This is an example of basic usage for the blink project, which is part of the ESP-IDF. It's expected that ESP-IDF is installed and set.

$ cd examples/get-started/blink/ # In esp-idf directory go to the blink example
$ idf.py build                   # Project has to be built first
$ esp-idf-sbom create -o blink.spdx build/project_description.json
$ esp-idf-sbom check blink.spdx

$ esp-idf-sbom create build/project_description.json | esp-idf-sbom check

The resulting blink.spdx sbom file can be found in the blink project directory.

SPDX SBOM layout

The SBOM file is created based on application sources, build artefacts, information provided by the ESP-IDF build system and SBOM manifest files. The resulting SBOM file contains SPDX packages for the final project application, used toolchain, components used during build, git submodules and subpackages. The subpackages are created based on sbom.yml manifest files found in submodules and subpackages sub-directories or referenced manifest files. Please see Manifest file.

Packages are linked together with SPDX DEPENDS_ON relationships with the project package as the root package. By default packages for configuration only components and components not linked into the application are present in SBOM, but are not linked through SPDX relationships. In other worlds dependencies on such packages are removed. This behaviour can be altered with --add-config-deps and --add-unused-deps command line options.

Manifest file

During SBOM generation the esp-idf-sbom tool is looking for sbom.yml manifest files. They are used as a source of information for the corresponding SPDX package in the SBOM file as described above.

The manifest file may be present at root of project, component, submodule or in any of their sub-directories. If sbom.yml is found in a sub-directory a new subpackage SPDX package is created and linked with the parent SPDX package. This can be used in cases where e.g. one component contains multiple libraries and they should be represented by separate SPDX packages.

Example of multiple sbom.yml files usage for the console component.

console
├── argtable3
│   └── sbom.yml
├── linenoise
│   └── sbom.yml
└── sbom.yml

The esp-idf-sbom tool will create main console component package, which will contain two subpackages for argtable3 and linenoise libraries. Please note that the manifest file in the console component root directory is not necessary to create SPDX package, because esp-idf-sbom automatically creates SPDX package for each component. The sbom.yml files may be placed at any sub-directory depth and esp-idf-sbom will create proper SPDX package hierarchy for them.

The sbom.yml is a simple yaml file, which may contain the following entries.

• name: Package name that will be used in the SPDX package.
• version: Package version.
• description: Short package description.
• repository: Link to git repository.
• url: Link to package download location.
• cpe: CPE used for vulnerabilities check against NVD. This can be single CPE value or a list of CPEs.
• supplier: Package supplier. Person or organization distributing the package. Should be prefixed with Person: or Organization: as described in SPDX specification.
• originator: Package originator. If the package comes from another person or organization that has been identified as a supplier. For example if a component is based on 3rd party code with some modifications, the originator is the 3rd party code author, but the supplier is the person or organization distributing the final component. For more detailed information please see the SPDX specification. As for supplier, Person: or Organization: prefix should be used for originator value.
• hash: SHA of the directory(git-tree object) the manifest file describes or HEAD SHA of a submodule. This value is used during the manifest file validation to check if the hash in the manifest file matches the SHA recorded in the git-tree. Its purpose is to make sure that the information in the manifest file is up-to-date. For example if a submodule or 3rd party library is updated, the version in the manifest file should be probably updated too. The SHA value can be obtained e.g. with git ls-tree HEAD <path>, where <path> is a package directory, which is described by the manifest file. Please note that a hash value for a directory, not a submodule, cannot be placed in manifest file, which is stored within the same directory, because the directory SHA in git-tree will change every time the manifest file changes(chicken egg problem). To make the hash variable work, it needs to be placed in a referenced manifest, which is not stored within a directory it describes. For example freertos component can have main sbom.yml manifest file, which refers to sbom_FreeRTOS-Kernel.yml manifest describing the FreeRTOS-Kernel package in the FreeRTOS-Kernel directory.

components/freertos/sbom.yml

manifests:
  - path: sbom_FreeRTOS-Kernel.yml
    dest: FreeRTOS-Kernel

components/freertos/sbom_FreeRTOS-Kernel.yml

name: 'freertos'
version: '10.4.3'
cpe: cpe:2.3:o:amazon:freertos:{}:*:*:*:*:*:*:*
supplier: 'Organization: Espressif Systems (Shanghai) CO LTD'
originator: 'Organization: Amazon Web Services'
description: An open-source, real-time operating system (RTOS) with additional features and patches from Espressif.
hash: 4e8101b6f57a0640ae54c6da605c1b532c0f8f89
cve-exclude-list:
  - cve: CVE-2021-43997
    reason: This vulnerability only affects ARMv7-M and ARMv8-M ports of FreeRTOS and hence does not affect Espressif SoCs which are not based on these architectures.

• license: License expression explicitly declared by the author.

• copyright: Copyright explicitly declared by the author. This can be single string or a list of copyrights.

• cve-exclude-list: List of already evaluated CVEs, which do not apply to this package. This can be used to exclude CVEs from the esp-idf-sbom checker report in case the package is not vulnerable to specific CVEs. Each CVE in the exclude list is represented as dictionary with the cve and reason keys. Information about excluded CVEs is added to the generated SBOM file into PackageComment SPDX tag and later used by the checker.

  • cve: CVE-ID
  • reason : description why this package is not vulnerable to this CVE

      version: 0.1.0
      description: Blink application example
      cve-exclude-list:
        - cve: CVE-2023-1234
          reason: Description why this package is not vulnerable

• manifests: List of manifest files which cannot be added directly into the component or submodule sub-directories to create subpackage. For example the following will create a new SPDX package for the subpackage directory with information from the subpackage.yml manifest file. This manifest file is treated as it would be actually stored in the subpackage directoery.

  • path: path of manifest file relatitve to the sbom.yml
  • dest: destination directory for path, again relative to sbom.yml

      version: 0.1.0
      description: Blink application example
      manifests:
        - path: subpackage.yml
          dest: subpackage

Example of the sbom.yml manifest file for the ESP-IDF blink project.

version: 0.1.0
description: Blink application example
url: https://blink.org/blink-0.1.0.tar.gz # non-existing package download URL example
cpe: cpe:2.3:a:hrbata:blink:{}:*:*:*:*:*:*:* # non-existing CPE example
supplier: 'Person: Frantisek Hrbata (frantisek.hrbata@espressif.com)'
originator: 'Organization: Espressif Systems (Shanghai) CO LTD'
cve-exclude-list:
   - cve: CVE-2023-1234
     reason: Description why this package is not vulnerable
   - cve: CVE-2023-1235
     reason: Description why this package is not vulnerable

Information from the sbom.yml manifest file are mapped to the following SPDX tags.

Even though the sbom.yml file is the primary source of information, the esp-idf-sbom tool is also looking at other places if it's not present. The idf_component.yml manifest file, used for components managed by the component manager, may contain sbom dictionary/namespace, which will be used by esp-idf-sbom if presented. This dictionary may contain the same information as sbom.yml.

Example of the idf_component.yml manifest file for led_strip managed component.

dependencies:
  idf:
    version: '>=5.0'
description: Driver for Addressable LED Strip (WS2812, etc)
url: https://github.com/espressif/idf-extra-components/tree/master/led_strip
version: 2.4.1
sbom:
  cpe: cpe:2.3:a:hrbata:led_strip:{}:*:*:*:*:*:*:* # non-existing CPE example
  supplier: 'Organization: Espressif Systems (Shanghai) CO LTD'
  cve-exclude-list:
     - cve: CVE-2023-1234
       reason: Description why this package is not vulnerable
     - cve: CVE-2023-1235
       reason: Description why this package is not vulnerable

If the sbom dictionary is not presented in idf_component.yml or it's missing some information, the version, description, maintainers and url information from idf_component.yml manifest is used.

Information from the idf_component.yml manifest file are mapped to the following SPDX tags.

Component version may be guessed based on git-describe and Espressif as a default supplier may be guessed based on git repository or package URL. The guessing may be disabled by using the '--no-guess' option.

For git submodules, the .gitmodules configuration file is also checked for additional submodule information. Submodule configuration may contain variables with the sbom- prefix, which are considered as SBOM manifest information. All keys used in the sbom.yml manifest file can also be specified in .gitmodules with the git-config format instead of yaml.

Example of manifest information added for the micro-ecc submodule in .gitmodules.

[submodule "components/bootloader/subproject/components/micro-ecc/micro-ecc"]
        path = components/bootloader/subproject/components/micro-ecc/micro-ecc
        url = ../../kmackay/micro-ecc.git
        sbom-version = 1.0
        sbom-cpe = cpe:2.3:a:micro-ecc_project:micro-ecc:{}:*:*:*:*:*:*:*
        sbom-supplier = Person: Ken MacKay
        sbom-url = https://github.com/kmackay/micro-ecc
        sbom-description = A small and fast ECDH and ECDSA implementation for 8-bit, 32-bit, and 64-bit processors
        sbom-hash = d037ec89546fad14b5c4d5456c2e23a71e554966
        sbom-cve-exclude-list = CVE-2023-1234 Description why this package is not vulnerable
        sbom-cve-exclude-list = CVE-2023-1235 Description why this package is not vulnerable

Manifest information is gathered in the following order and only missing values are added. If e.g. version is found in sbom.yml any other version value found e.g. in .gitmodules is ignored.

1. referenced manifest from parent package
2. sbom.yml
3. sbom dictionary/namespace in idf_component.yml
4. sbom information contained in submodule configuration in .gitmodules
5. idf_component.yml information provided for component manager

Validating manifest files

Manifest files are validated while the SBOM is generated. They can be also validated explicitly with the esp-idf-sbom manifest validate command.

esp-idf-sbom manifest validate [PATH_TO_VALIDATE...]

PATH_TO_VALIDATE is an optional path to a manifest file(sbom.yml, idf_manifest.yml or .gitmodules) or directory, which will be searched for manifest files. If PATH_TO_VALIDATE is not provided, the current working directory is used.

Usage example:

$ esp-idf-sbom manifest validate ~/work/esp-idf ~/work/idf-extra-components/
$ esp-idf-sbom manifest validate ~/work/esp-idf/.gitmodules ~/work/esp-idf/components/freertos/FreeRTOS-Kernel/sbom.yml

Checking manifest files for vulnerabilities

The esp-idf-sbom tool uses the generated SBOM SPDX file to check for possible vulnerabilities. It also allows to scan for vulnerabilities based on the information presented in manifest files. This can be used e.g. to scan a whole repository without a need to generate the SBOM SPDX file.

esp-idf-sbom validate-submodule-hash [PATH_TO_CHECK...]

PATH_TO_CHECK is an optional path to a manifest file(sbom.yml, idf_manifest.yml or .gitmodules) or directory, which will be searched for manifest files. If PATH_TO_CHECK is not provided, the current working directory is used.

Usage example:

$ esp-idf-sbom manifest check ~/work/esp-idf ~/work/idf-extra-components/
$ esp-idf-sbom manifest check ~/work/esp-idf/.gitmodules ~/work/esp-idf/components/freertos/FreeRTOS-Kernel/sbom.yml

Licenses and Copyrights

Adding licenses and copyrights information into the SBOM file has to be explicitly requested by using the --file-tags option. This requires to scan all files and may take some time. It also may result in a quite big SBOM file.

All component and submodule files are scanned for the SPDX-License-Identifier, SPDX-FileCopyrightText and SPDX-FileContributor SPDX file tags. Information from these tags is used in the generated SBOM file to specify licenses and copyrights for SPDX packages which represent project, component or submodule. The project's final license expression is a logical AND of all licenses concluded from components and submodules used in the final project binary.

The license can be also explicitly declared by the author in the sbom.yml file with the license variable. This information is used as a value for the PackageLicenseDeclared SPDX tag for given project, component or submodule.

Return Values

• 0: No error, no vulnerability found, manifest file(s) valid.
• 1: Vulnerability found or manifest file not valid.
• 128: Fatal error.