Vulnerability Exploitability Exchange (VEX)

EXPERIMENTAL

This feature might change without preserving backwards compatibility.

Trivy supports filtering detected vulnerabilities using the Vulnerability Exploitability Exchange (VEX), a standardized format for sharing and exchanging information about vulnerabilities. By providing VEX during scanning, it is possible to filter vulnerabilities based on their status. Currently, Trivy supports the following three formats:

• CycloneDX
• OpenVEX
• CSAF

CycloneDX

Target	Supported
Container Image
Filesystem
Code Repository
VM Image
Kubernetes
SBOM	✅

There are two VEX formats for CycloneDX:

• Independent BOM and VEX BOM
• BOM With Embedded VEX

Trivy only supports the Independent BOM and VEX BOM format, so you need to provide a separate VEX file alongside the SBOM. The input SBOM format must be in CycloneDX format.

The following steps are required:

1. Generate a CycloneDX SBOM
2. Create a VEX based on the SBOM generated in step 1
3. Provide the VEX when scanning the CycloneDX SBOM

Generate the SBOM

You can generate a CycloneDX SBOM with Trivy as follows:

$ trivy image --format cyclonedx --output debian11.sbom.cdx debian:11

Create the VEX

Next, create a VEX based on the generated SBOM. Multiple vulnerability statuses can be defined under vulnerabilities. Take a look at the example below.

$ cat <<EOF > trivy.vex.cdx
{
  "bomFormat": "CycloneDX",
  "specVersion": "1.5",
  "version": 1,
  "vulnerabilities": [
    {
      "id": "CVE-2020-8911",
      "analysis": {
        "state": "not_affected",
        "justification": "code_not_reachable",
        "response": ["will_not_fix", "update"],
        "detail": "The vulnerable function is not called"
      },
      "affects": [
        {
          "ref": "urn:cdx:3e671687-395b-41f5-a30f-a58921a69b79/1#pkg:golang/github.com/aws/aws-sdk-go@1.44.234"
        }
      ]
    }
  ]
}
EOF

This is a VEX document in the CycloneDX format. The vulnerability ID, such as a CVE-ID or GHSA-ID, should be placed in vulnerabilities.id. When the analysis.state is set to not_affected, Trivy will not detect the vulnerability.

BOM-Links must be placed in affects.ref. The BOM-Link has the following syntax and consists of three elements:

urn:cdx:serialNumber/version#bom-ref

• serialNumber
• version
• bom-ref

These values must be obtained from the CycloneDX SBOM. Please note that while the serialNumber starts with urn:uuid:, the BOM-Link starts with urn:cdx:.

The bom-ref must contain the BOM-Ref of the package affected by the vulnerability. In the example above, since the Go package github.com/aws/aws-sdk-go is affected by CVE-2020-8911, it was necessary to specify the SBOM's BOM-Ref, pkg:golang/github.com/aws/aws-sdk-go@1.44.234.

For more details on CycloneDX VEX and BOM-Link, please refer to the following links:

• CycloneDX VEX
• BOM-Link
• Examples

Scan SBOM with VEX

Provide the VEX when scanning the CycloneDX SBOM.

$ trivy sbom trivy.sbom.cdx --vex trivy.vex.cdx
...
2023-04-13T12:55:44.838+0300    INFO    Filtered out the detected vulnerability {"VEX format": "CycloneDX", "vulnerability-id": "CVE-2020-8911", "status": "not_affected", "justification": "code_not_reachable"}

go.mod (gomod)
==============
Total: 1 (UNKNOWN: 0, LOW: 1, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

┌───────────────────────────┬───────────────┬──────────┬───────────────────┬───────────────┬────────────────────────────────────────────────────────────┐
│          Library          │ Vulnerability │ Severity │ Installed Version │ Fixed Version │                           Title                            │
├───────────────────────────┼───────────────┼──────────┼───────────────────┼───────────────┼────────────────────────────────────────────────────────────┤
│ github.com/aws/aws-sdk-go │ CVE-2020-8912 │ LOW      │ 1.44.234          │               │ aws-sdk-go: In-band key negotiation issue in AWS S3 Crypto │
│                           │               │          │                   │               │ SDK for golang...                                          │
│                           │               │          │                   │               │ https://avd.aquasec.com/nvd/cve-2020-8912                  │
└───────────────────────────┴───────────────┴──────────┴───────────────────┴───────────────┴────────────────────────────────────────────────────────────┘

CVE-2020-8911 is no longer shown as it is filtered out according to the given CycloneDX VEX document.

OpenVEX

Target	Supported
Container Image	✅
Filesystem	✅
Code Repository	✅
VM Image	✅
Kubernetes	✅
SBOM	✅

Trivy also supports OpenVEX that is designed to be minimal, compliant, interoperable, and embeddable. OpenVEX can be used in all Trivy targets, unlike CycloneDX VEX.

The following steps are required:

1. Create a VEX document
2. Provide the VEX when scanning your target

Create the VEX document

Please see also the example. Trivy requires the Package URL (PURL) as the product identifier.

$ cat <<EOF > debian11.openvex.json
{
  "@context": "https://openvex.dev/ns/v0.2.0",
  "@id": "https://openvex.dev/docs/public/vex-2e67563e128250cbcb3e98930df948dd053e43271d70dc50cfa22d57e03fe96f",
  "author": "Aqua Security",
  "timestamp": "2023-08-29T19:07:16.853479631-06:00",
  "version": 1,
  "statements": [
    {
      "vulnerability": {"name": "CVE-2019-8457"},
      "products": [
        {"@id": "pkg:deb/debian/libdb5.3@5.3.28+dfsg1-0.8"}
      ],
      "status": "not_affected",
      "justification": "vulnerable_code_not_in_execute_path"
    }
  ]
}
EOF

In the above example, PURLs, pkg:deb/debian/libdb5.3@5.3.28+dfsg1-0.8 are used for the product identifier. You can find PURLs in the JSON report generated by Trivy. This VEX statement is applied if the PURL specified in the VEX matches the PURL found during the scan. See here for more details of PURL matching.

Trivy also supports OpenVEX subcomponents, which allow for more precise specification of the scope of a VEX statement, reducing the risk of incorrect filtering. Let's say you want to suppress vulnerabilities within a container image. If you only specify the PURL of the container image as the product, the resulting VEX would look like this:

OpenVEX products only

"statements": [
  {
    "vulnerability": {"name": "CVE-2024-32002"},
    "products": [
      {"@id": "pkg:oci/trivy?repository_url=ghcr.io%2Faquasecurity%2Ftrivy"}
    ],
    "status": "not_affected",
    "justification": "vulnerable_code_not_in_execute_path"
  }
]

However, this approach would suppress all instances of CVE-2024-32002 within the container image. If the intention is to declare that the git package distributed by Alpine Linux within this image is not affected, subcomponents can be utilized as follows:

OpenVEX subcomponents

"statements": [
  {
    "vulnerability": {"name": "CVE-2024-32002"},
    "products": [
      {
        "@id": "pkg:oci/trivy?repository_url=ghcr.io%2Faquasecurity%2Ftrivy",
        "subcomponents": [
          {"@id": "pkg:apk/alpine/git"}
        ]
      }
    ],
    "status": "not_affected",
    "justification": "vulnerable_code_not_in_execute_path"
  }
]

By declaring the subcomponent in this manner, Trivy will filter the results, considering only the git package within the ghcr.io/aquasecurity/trivy container image as not affected. Omitting the version in the PURL applies the statement to all versions of the package. More details about PURL matching can be found here.

Furthermore, the product specified in a VEX statement does not necessarily need to be the target of the scan. It is possible to specify a component that is included in the scan target as the product. For example, you can designate a specific Go project as the product and its dependent modules as subcomponents.

In the following example, the VEX statement declares that the github.com/docker/docker module, which is a dependency of the github.com/aquasecurity/trivy Go project, is not affected by CVE-2024-29018.

OpenVEX intermediate components

"statements": [
  {
    "vulnerability": {"name": "CVE-2024-29018"},
    "products": [
      {
        "@id": "pkg:golang/github.com/aquasecurity/trivy",
        "subcomponents": [
          { "@id": "pkg:golang/github.com/docker/docker" }
        ]
      }
    ],
    "status": "not_affected",
    "justification": "vulnerable_code_not_in_execute_path"
  }
]

This VEX document can be used when scanning a container image as well as other targets. The VEX statement will be applied when Trivy finds the Go binary within the container image.

$ trivy image ghcr.io/aquasecurity/trivy:0.50.0 --vex trivy.openvex.json

VEX documents can indeed be reused across different container images, eliminating the need to issue separate VEX documents for each image. This is particularly useful when there is a common component or library that is used across multiple projects or container images.

You can see the appendix for more details on how VEX is applied in Trivy.

Scan with VEX

Provide the VEX when scanning your target.

$ trivy image debian:11 --vex debian11.openvex.json
...
2023-04-26T17:56:05.358+0300    INFO    Filtered out the detected vulnerability {"VEX format": "OpenVEX", "vulnerability-id": "CVE-2019-8457", "status": "not_affected", "justification": "vulnerable_code_not_in_execute_path"}

debian11.spdx.json (debian 11.6)
================================
Total: 80 (UNKNOWN: 0, LOW: 58, MEDIUM: 6, HIGH: 16, CRITICAL: 0)

CVE-2019-8457 is no longer shown as it is filtered out according to the given OpenVEX document.

CSAF

Target	Supported
Container Image	✅
Filesystem	✅
Code Repository	✅
VM Image	✅
Kubernetes	✅
SBOM	✅

Trivy also supports CSAF format for VEX. Since CSAF aims to be SBOM format agnostic, both CycloneDX and SPDX formats are available for use as input SBOMs in Trivy.

The following steps are required:

1. Create a CSAF document
2. Provide the CSAF when scanning your target

Create the CSAF document

Create a CSAF document in JSON format as follows:

CSAF VEX

$ cat <<EOF > debian11.vex.csaf
{
  "document": {
    "category": "csaf_vex",
    "csaf_version": "2.0",
    "notes": [
      {
        "category": "summary",
        "text": "Example Company VEX document. Unofficial content for demonstration purposes only.",
        "title": "Author comment"
      }
    ],
    "publisher": {
      "category": "vendor",
      "name": "Example Company ProductCERT",
      "namespace": "https://psirt.example.com"
    },
    "title": "AquaSecurity example VEX document",
    "tracking": {
      "current_release_date": "2024-01-01T11:00:00.000Z",
      "generator": {
        "date": "2024-01-01T11:00:00.000Z",
        "engine": {
          "name": "Secvisogram",
          "version": "1.11.0"
        }
      },
      "id": "2024-EVD-UC-01-A-001",
      "initial_release_date": "2024-01-01T11:00:00.000Z",
      "revision_history": [
        {
          "date": "2024-01-01T11:00:00.000Z",
          "number": "1",
          "summary": "Initial version."
        }
      ],
      "status": "final",
      "version": "1"
    }
  },
  "product_tree": {
    "branches": [
      {
        "branches": [
          {
            "branches": [
              {
                "category": "product_version",
                "name": "5.3",
                "product": {
                  "name": "Database Libraries 5.3",
                  "product_id": "LIBDB-5328",
                  "product_identification_helper": {
                    "purl": "pkg:deb/debian/libdb5.3@5.3.28%2Bdfsg1-0.8?arch=amd64\u0026distro=debian-11.8"
                  }
                }
              }
            ],
            "category": "product_name",
            "name": "Database Libraries"
          }
        ],
        "category": "vendor",
        "name": "Debian"
      }
    ]
  },
  "vulnerabilities": [
    {
      "cve": "CVE-2019-8457",
      "notes": [
        {
          "category": "description",
          "text": "SQLite3 from 3.6.0 to and including 3.27.2 is vulnerable to heap out-of-bound read in the rtreenode() function when handling invalid rtree tables.",
          "title": "CVE description"
        }
      ],
      "product_status": {
        "known_not_affected": [
          "LIBDB-5328"
        ]
      },
      "threats": [
        {
          "category": "impact",
          "details": "Vulnerable code not in execute path.",
          "product_ids": [
            "LIBDB-5328"
          ]
        }
      ]
    }
  ]
}
EOF

Trivy also supports CSAF relationships, reducing the risk of incorrect filtering. It works in the same way as OpenVEX subcomponents. At present, the specified relationship category is not taken into account and all the following categories are treated internally as "depends_on".

• default_component_of
• installed_on
• installed_with

You can see the appendix for more details on how VEX is applied in Trivy.

Scan with CSAF VEX

Provide the CSAF document when scanning your target.

$ trivy image debian:11 --vex debian11.vex.csaf
...
2024-01-02T10:28:26.704+0100    INFO    Filtered out the detected vulnerability {"VEX format": "CSAF", "vulnerability-id": "CVE-2019-8457", "status": "not_affected"}

debian11.spdx.json (debian 11.6)
================================
Total: 80 (UNKNOWN: 0, LOW: 58, MEDIUM: 6, HIGH: 16, CRITICAL: 0)

CVE-2019-8457 is no longer shown as it is filtered out according to the given CSAF document.

Appendix

PURL matching

In the context of VEX, Package URLs (PURLs) are utilized to identify specific software packages and their versions. The PURL matching specification outlines how PURLs are interpreted for vulnerability exception processing, ensuring precise identification and broad coverage of software packages.

Note

The following PURL matching rules are not formally defined within the current official PURL specification. Instead, they represent a community consensus on how to interpret PURLs.

Below are the key aspects of the PURL matching rules:

Matching Without Version

A PURL without a specified version (e.g., pkg:maven/com.google.guava/guava) matches all versions of that package. This rule simplifies the application of vulnerability exceptions to all versions of a package.

Example: pkg:maven/com.google.guava/guava matches:

• All versions of guava, such as com.google.guava:guava:24.1.1, com.google.guava:guava:30.0.

Matching Without Qualifiers

A PURL without any qualifiers (e.g., pkg:maven/com.google.guava/guava@24.1.1) matches any variation of that package, irrespective of qualifiers. This approach ensures broad matching capabilities, covering all architectural or platform-specific variations of a package version.

Example: pkg:maven/com.google.guava/guava@24.1.1 matches:

• pkg:maven/com.google.guava/guava@24.1.1?classifier=x86
• pkg:maven/com.google.guava/guava@24.1.1?type=pom

Matching With Specific Qualifiers

A PURL that includes specific qualifiers (e.g., pkg:maven/com.google.guava/guava@24.1.1?classifier=x86) matches only those package versions that include the same qualifiers.

Example: pkg:maven/com.google.guava/guava@24.1.1?classifier=x86 matches:

• pkg:maven/com.google.guava/guava@24.1.1?classifier=x86&type=dll
  • Extra qualifiers (e.g., type=dll) are ignored.

does not match:

• pkg:maven/com.google.guava/guava@24.1.1
  • classifier=x86 is missing.
• pkg:maven/com.google.guava/guava@24.1.1?classifier=sources
  • classifier must have the same value.

Applying VEX to Dependency Trees

Trivy internally generates a dependency tree and applies VEX statements to this graph. Let's consider a project with the following dependency tree, where Module C v2.0.0 is assumed to have a vulnerability CVE-XXXX-YYYY:

graph TD;
  modRootA(Module Root A v1.0.0)
  modB(Module B v1.0.0) 
  modC(Module C v2.0.0)

  modRootA-->modB
  modB-->modC

Now, suppose a VEX statement is issued for Module B as follows:

"statements": [
  {
    "vulnerability": {"name": "CVE-XXXX-YYYY"},
    "products": [
      {
        "@id": "pkg:golang/module-b@1.0.0",
        "subcomponents": [
          { "@id": "pkg:golang/module-c@2.0.0" }
        ]
      }
    ],
    "status": "not_affected",
    "justification": "vulnerable_code_not_in_execute_path"  
  }
]

It declares that Module B is not affected by CVE-XXXX-YYYY on Module C.

Note

The VEX in this example defines the relationship between Module B and Module C. However, as Trivy traverses all parents from vulnerable packages, it is also possible to define a VEX for the relationship between a vulnerable package and any parent, such as Module A and Module C, etc.

Mapping this VEX onto the dependency tree would look like this:

graph TD;
  modRootA(Module Root A v1.0.0)

  subgraph "VEX (Not Affected)"
  modB(Module B v1.0.0)
  modC(Module C v2.0.0)
  end

  modRootA-->modB
  modB-->modC

In this case, it's clear that Module Root A is also not affected by CVE-XXXX-YYYY, so this vulnerability is suppressed.

Now, let's consider another project:

graph TD;
  modRootZ(Module Root Z v1.0.0)
  modB'(Module B v1.0.0)
  modC'(Module C v2.0.0)
  modD'(Module D v3.0.0)

  modRootZ-->modB'
  modRootZ-->modD'
  modB'-->modC'
  modD'-->modC'

Assuming the same VEX as before, applying it to this dependency tree would look like:

graph TD;
  modRootZ(Module Root Z v1.0.0)

  subgraph "VEX (Not Affected)"
  modB'(Module B v1.0.0)
  modC'(Module C v2.0.0)
  end

  modD'(Module D v3.0.0)

  modRootZ-->modB'
  modRootZ-->modD'
  modB'-->modC'
  modD'-->modC'

Module Root Z depends on Module C via multiple paths. While the VEX tells us that Module B is not affected by the vulnerability, Module D might be. In the absence of a VEX, the default assumption is that it is affected. Taking all of this into account, Trivy determines that Module Root Z is affected by this vulnerability.