Crlf Injection in Strapi with Dynamodb

Crlf Injection occurs when untrusted input containing carriage return (CR, \r) and line feed (\n) characters is reflected into HTTP headers or logs without proper sanitization. In Strapi, a headless CMS that can use DynamoDB as a persistence layer, this risk arises when user-controlled data (e.g., query parameters, request headers, or form fields) is written to DynamoDB and later used in contexts such as redirects, Set-Cookie headers, or log entries.

When Strapi stores user input directly into a DynamoDB item and later reads that item to construct an HTTP response, the stored \r\n sequences can break header parsing. For example, an attacker might supply a username like alice\r\nSet-Cookie: session=hijacked. If Strapi saves this value in a DynamoDB attribute and later includes it in a redirect or response header, the injected control characters can cause the client to interpret a new header, potentially leading to session fixation or response splitting. DynamoDB itself does not interpret these sequences—it stores them as-is—so the injection occurs at the application layer when Strapi uses the retrieved value in header construction or logging.

Additionally, DynamoDB Streams or exported data can propagate unsafe values to downstream systems or logs. If Strapi emits these values into log lines that are later parsed by tools expecting newline-delimited entries, an attacker-supplied \r\n can forge log entries or facilitate log injection. This is particularly relevant when using DynamoDB as the primary datastore for user profiles or session metadata, as the integrity of logs and redirects becomes dependent on input validation at the Strapi layer.

To mitigate Crlf Injection when using DynamoDB with Strapi, sanitize and validate any user-controlled data before storing it in DynamoDB and before using it in HTTP headers or logs. Below are concrete, DynamoDB-aware remediation steps and code examples.

1. Input validation and normalization
Strip or encode CR and LF characters in user input before saving to DynamoDB. Use a consistent allowlist approach for text fields.

// strapi/services/user.js
const sanitizeCrlf = (str) => {
  if (typeof str !== 'string') return str;
  // Remove CR and LF characters; consider replacing with a space if needed
  return str.replace(/[\r\n]/g, '');
};

module.exports = {
  sanitizeAndSave: async (userData) => {
    const cleanData = {
      ...userData,
      username: sanitizeCrlf(userData.username),
      email: sanitizeCrlf(userData.email),
    };
    // Proceed to DynamoDB save via Strapi entity service
    return strapi.entityService.create('api::user.user', { data: cleanData });
  },
};

2. Safe DynamoDB put/query patterns in Strapi
When interacting with DynamoDB directly (e.g., using the AWS SDK), ensure retrieved values are sanitized before header usage. Below is an example of retrieving a user from DynamoDB and safely using a field in a redirect.

// strapi/services/redirect.js
const { DynamoDBClient, GetCommand } = require('@aws-sdk/client-dynamodb');
const client = new DynamoDBClient({ region: 'us-east-1' });

const getUserFromDynamo = async (userId) => {
  const command = new GetCommand({
    TableName: process.env.DYNAMO_TABLE,
    Key: { userId: { S: userId } },
  });
  const response = await client.send(command);
  return response.Item;
};

module.exports = async (userId, res) => {
  const item = await getUserFromDynamo(userId);
  const username = item.username.S.replace(/[\r\n]/g, '');
  // Safe: username no longer contains CR/LF
  res.set('X-User-Name', username);
  res.redirect(302, `/dashboard/${encodeURIComponent(username)}`);
};

3. Header construction and logging safeguards
When generating HTTP responses or logs, avoid directly embedding DynamoDB-stored strings into headers. If embedding is necessary, validate with a strict regex and encode where appropriate. For logs, ensure newline characters are escaped or replaced to prevent log injection.

// Example: safe header and log handling
const formatLogMessage = (input) => {
  return input.replace(/[\r\n]/g, ' ').replace(/\s+/g, ' ').trim();
};

const logUserAction = (user) => {
  const safeUser = formatLogMessage(user.username.S);
  console.log(`[action] user=${safeUser} action=login`);
};

// In an authenticated controller
const updateProfile = async (ctx) => {
  const { bio } = ctx.request.body;
  const safeBio = bio.replace(/[\r\n]/g, ' ');
  await strapi.entityService.update('api::profile.profile', ctx.state.user.id, {
    data: { bio: safeBio },
  });
  ctx.body = { message: 'Profile updated' };
};

4. Use Strapi middleware for early validation
Leverage Strapi middleware to reject requests containing CR or LF in headers and URL paths before they reach DynamoDB operations.

// config/middlewares.js
module.exports = [
  {
    name: 'crlf-validation',
    config: {
      paths: ['/users/:id/*'],
      async handler(ctx, next) {
        const combined = [ctx.params, ctx.query, ctx.body].join('\n');
        if (/[\r\n]/.test(combined)) {
          ctx.throw(400, 'Invalid characters in request');
        }
        await next();
      },
    },
  },
];

These DynamoDB-aware practices reduce the risk of Crlf Injection by ensuring that data stored in and retrieved from DynamoDB is controlled before it influences headers, redirects, or logs. Combining input sanitization, safe encoding, and middleware checks aligns with secure patterns for CMS-driven applications using DynamoDB as a backend.