Scanning images for vulnerabilities in Docker - Mini Project: Build & Apply

Scanning Docker Images for Vulnerabilities

📖 Scenario: You work in a team that builds software using Docker containers. Before deploying containers to production, you need to check if the Docker images have any security problems. This helps keep the software safe and reliable.

🎯 Goal: Learn how to scan a Docker image for vulnerabilities using a simple command-line tool and understand the scan results.

📋 What You'll Learn

Have Docker installed on your computer

Use the docker command line

Use the docker scan command to check images

💡 Why This Matters

🌍 Real World

Scanning Docker images helps teams find security problems before software is released. This keeps users safe and avoids costly fixes later.

💼 Career

DevOps engineers and developers use image scanning to maintain secure container environments in production.

1

Create a Docker image to scan

Create a Dockerfile with the exact content:
FROM alpine:3.18
This will be the image you scan for vulnerabilities.

Docker

# Create a Dockerfile with the line: FROM alpine:3.18

Hint

The Dockerfile must start with FROM alpine:3.18 exactly.

2

Build the Docker image

Build the Docker image with the tag my-alpine using the command:
docker build -t my-alpine .

Docker

FROM alpine:3.18
# Run the command: docker build -t my-alpine . in your terminal

Hint

Use the terminal to run the build command exactly as shown.

3

Scan the Docker image for vulnerabilities

Use the command docker scan my-alpine to scan the image you built for security issues.

Docker

FROM alpine:3.18
# Run the command: docker scan my-alpine in your terminal

Hint

Run the scan command in your terminal to see the vulnerability report.

4

Display the scan results

Show the output of docker scan my-alpine in your terminal. It should list vulnerabilities or say no vulnerabilities found.

Docker

FROM alpine:3.18
# Paste the output of docker scan my-alpine here

Hint

The scan output usually ends with a message like 'No vulnerabilities found' if the image is safe.

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