InvalidSignatureException

What Does Invalid Signature Mean?

InvalidSignatureException signals a "Syntax Error" in your authentication logic. Unlike SignatureDoesNotMatch (which implies a correct format but wrong Secret Key), this error means AWS cannot even parse your signature. It is commonly triggered by missing headers in the "SignedHeaders" list, an incorrect "Credential Scope," or using the deprecated Signature Version 2 in a region that requires Version 4.

Common Causes

• -Malformed Authorization Header: Missing spaces, incorrect commas, or missing key-value pairs like SignedHeaders or Signature.
• -Credential Scope Mismatch: Signing a request for s3 in us-east-1 but sending it to a dynamodb endpoint in eu-west-1.
• -ISO 8601 Date Errors: The X-Amz-Date header format is incorrect or does not match the date used in the credential scope.
• -Custom Signing Bugs: Manual implementations of the SigV4 algorithm failing to correctly canonicalize the request (e.g., wrong sorting of query parameters).

How to Fix Invalid Signature

1. 1Use Official SDKs: The easiest fix is to let the AWS SDK handle signing. It eliminates manual header construction entirely.
2. 2Validate Scope: Ensure your credential scope follows the format: YYYYMMDD/<region>/<service>/aws4_request.
3. 3Check Algorithm: Ensure you are using AWS4-HMAC-SHA256. Older algorithms like HmacSHA1 are no longer supported for SigV4.
4. 4Verify SignedHeaders: Every header listed in SignedHeaders must be present in the actual HTTP request headers.

Step-by-Step Diagnosis for Invalid Signature

1. 1Inspect the Authorization header: Verify it starts with AWS4-HMAC-SHA256.
2. 2Compare dates: The date in the Credential scope must match the X-Amz-Date header exactly.
3. 3Check URL Encoding: Ensure special characters in the URI are encoded according to AWS SigV4 rules (sorting is critical).
4. 4Use a Proxy: Capture the request with Burp Suite or Charles Proxy to see exactly what is being sent over the wire.

import { SignatureV4 } from "@aws-sdk/signature-v4";
import { Sha256 } from "@aws-crypto/sha256-js";

const signer = new SignatureV4({
  credentials: { accessKeyId: "AKIA...", secretAccessKey: "..." },
  region: "us-east-1",
  service: "execute-api",
  sha256: Sha256,
});

// The SDK handles all the canonicalization and header formatting
const signed = await signer.sign(request);

curl -v -H "Authorization: AWS4-HMAC-SHA256 Credential=AKIA.../20260321/us-east-1/s3/aws4_request, SignedHeaders=host;x-amz-date, Signature=..." \
     https://my-bucket.s3.amazonaws.com

How to Verify the Fix

• -Test with aws sts get-caller-identity using the same signing logic to see if AWS recognizes the identity.
• -For custom integrations, use the AWS SigV4 Test Suite to validate your algorithm against known good inputs.
• -Confirm that the HTTP 403 response is replaced by a 200 OK after fixing the header structure.

How to Prevent Recurrence

• -Prefer SDK Signers: Use the SignatureV4 class from the @aws-sdk/signature-v4 package instead of writing manual HMAC logic.
• -Environment Awareness: Never hardcode us-east-1 in your signing logic; derive the region dynamically from the endpoint URL.
• -Automated Auth Tests: Include a "Smoke Test" in your pipeline that performs a simple HEAD request to verify signing integrity before deployment.
• -Pro-tip: When debugging custom signers, always print the "Canonical Request" and "String to Sign" to your logs. Comparing these against AWS examples is the fastest way to find off-by-one errors.

Related Errors

Decision Support

Official References

• -SigV4: Canonical Request Creation
• -Troubleshooting SigV4 Errors
• -AWS Signature Version 4 (SigV4) Signing Process
• -AWS Official Documentation