Scanning images for vulnerabilities in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of scanning Docker images for vulnerabilities?

Scanning Docker images helps find security weaknesses before using them in production. It protects your applications from known security risks.

beginner

Name a popular tool used to scan Docker images for vulnerabilities.

Trivy is a popular, easy-to-use tool that scans Docker images for known vulnerabilities.

beginner

What does the command trivy image your-image-name do?

It scans the specified Docker image for security vulnerabilities and shows a report with found issues.

intermediate

Why should you scan images regularly, not just once?

New vulnerabilities are discovered all the time. Regular scanning helps catch new risks and keeps your software safe.

intermediate

What is a common output detail in vulnerability scan reports?

Reports usually show vulnerability severity (like LOW, MEDIUM, HIGH), affected packages, and fixed versions if available.

Which tool is commonly used to scan Docker images for vulnerabilities?

A Kubernetes

B Git

C Trivy

D Docker Compose

Trivy is a dedicated vulnerability scanner for container images.

What does a vulnerability scanner check inside a Docker image?

A Installed packages and their known security issues

B Network traffic

C CPU usage

D Container logs

It checks installed software packages for known vulnerabilities.

Why is it important to scan images before deployment?

A To speed up the container startup

B To find and fix security risks early

C To reduce image size

D To improve network speed

Early detection of vulnerabilities helps prevent security breaches.

Which severity level indicates the most critical vulnerability in scan reports?

A CRITICAL

B MEDIUM

C HIGH

D LOW

CRITICAL severity means the highest risk level.

How often should you scan your Docker images for vulnerabilities?

A Only once when creating the image

B Never, it's not necessary

C Only after deployment

D Regularly, to catch new vulnerabilities

Regular scans help keep your images secure over time.

Explain why scanning Docker images for vulnerabilities is important and how it helps keep applications safe.

Think about how finding problems early can protect your app.

Describe how to use a tool like Trivy to scan a Docker image and interpret the results.

Imagine you are checking your image for problems before using it.

Practice

1. What is the primary purpose of running docker scan <image>?

easy

A. To find security vulnerabilities in the container image

B. To build the container image faster

C. To delete unused images from the system

D. To start a container from the image

Solution

1. Step 1: Understand the command purpose

   The docker scan <image> command is designed to analyze container images for security issues.

2. Step 2: Identify the correct use case

   It does not build, delete, or run images but scans them for vulnerabilities.

3. Final Answer:

   To find security vulnerabilities in the container image -> Option A

4. Quick Check:

   docker scan = vulnerability check [OK]

Hint: Scan images to catch security issues early [OK]

Common Mistakes:

• Confusing scanning with building images
• Thinking scan deletes images
• Assuming scan runs containers

2. Which of the following is the correct syntax to scan a Docker image named myapp:latest?

easy

A. docker scan -image myapp:latest

B. docker scan myapp:latest

C. docker scan --img myapp:latest

D. docker scan image=myapp:latest

Solution

1. Step 1: Recall the correct command syntax

   The correct syntax is docker scan <image-name> without extra flags.

2. Step 2: Verify options

   Options B, C, and D use incorrect flags or formats not supported by docker scan.

3. Final Answer:

   docker scan myapp:latest -> Option B

4. Quick Check:

   docker scan + image name = correct syntax [OK]

Hint: Use simple: docker scan image_name [OK]

Common Mistakes:

• Adding unsupported flags like -image or --img
• Using equal signs in command
• Misplacing image name

3. You run docker scan alpine:3.15 and get a report showing 2 vulnerabilities. What does this output mean?

medium

A. The scan failed and found no issues

B. The image is safe and has no vulnerabilities

C. The Alpine image has 2 known security issues to fix

D. The scan only checks for outdated packages, not vulnerabilities

Solution

1. Step 1: Interpret scan results

   The scan output listing 2 vulnerabilities means the image has 2 known security problems.

2. Step 2: Understand scan purpose

   Docker scan reports vulnerabilities, not just outdated packages or failures.

3. Final Answer:

   The Alpine image has 2 known security issues to fix -> Option C

4. Quick Check:

   Scan output = found vulnerabilities [OK]

Hint: Vulnerabilities count means issues found [OK]

Common Mistakes:

• Thinking vulnerabilities count means no issues
• Confusing scan failure with zero vulnerabilities
• Assuming scan only checks outdated packages

4. You tried docker scan myimage but got an error: ERROR: No such image: myimage. What is the likely cause?

medium

A. The image name must include a tag like ':latest'

B. The docker scan command is misspelled

C. Docker daemon is not running

D. The image 'myimage' is not present locally or in the registry

Solution

1. Step 1: Analyze error message

   The error says the image 'myimage' does not exist locally or remotely.

2. Step 2: Check common causes

   Misspelling or daemon issues cause different errors; tag is optional if 'latest' is default.

3. Final Answer:

   The image 'myimage' is not present locally or in the registry -> Option D

4. Quick Check:

   Image missing = error [OK]

Hint: Check image exists locally or remotely before scanning [OK]

Common Mistakes:

• Assuming scan command typo causes this error
• Thinking docker daemon stopped causes this error
• Believing tag is always required

5. You want to automate vulnerability scanning in your CI pipeline for image webapp:prod. Which approach is best?

hard

A. Add a step to run docker scan webapp:prod and fail if vulnerabilities found

B. Only scan images manually before deployment

C. Use docker build with a scan flag to auto-scan

D. Ignore scanning and rely on OS firewall for security

Solution

1. Step 1: Understand automation needs

   Automating scanning in CI means running docker scan during build or test steps.

2. Step 2: Evaluate options

   Manual scanning misses automation benefits; docker build has no scan flag; ignoring scanning is unsafe.

3. Final Answer:

   Add a step to run docker scan webapp:prod and fail if vulnerabilities found -> Option A

4. Quick Check:

   Automate scan + fail on issues = best practice [OK]

Hint: Automate scans in CI, fail on vulnerabilities [OK]

Common Mistakes:

• Skipping automation and scanning manually
• Expecting docker build to scan automatically
• Ignoring scanning for security