---
applyTo: '**/*'
description: Certificate Best Practices
version: 1.2.0
---
rule_id: codeguard-1-digital-certificates
When you encounter data that appears to be an X.509 certificate—whether embedded as a string or loaded from a file—you must flag the certificate for verification and ensure the following security properties are validated, reporting any concerns with clear explanations and recommended actions.
### 1. How to Identify Certificate Data
Actively scan for certificate data using the following heuristics:
- PEM-Encoded Strings: Identify multi-line string literals or constants that begin with `-----BEGIN CERTIFICATE-----` and end with `-----END CERTIFICATE-----`.
- File Operations: Pay close attention to file read operations on files with common certificate extensions, such as `.pem`, `.crt`, `.cer`, and `.der`.
- Library Function Calls: Recognize the usage of functions from cryptographic libraries used to load or parse certificates (e.g., OpenSSL's `PEM_read_X509`, Python's `cryptography.x509.load_pem_x509_certificate`, Java's `CertificateFactory`).
### 2. Mandatory Sanity Checks
Once certificate data is identified, flag it for verification. The following properties must be validated to ensure the certificate meets security requirements:
#### Verification Guidance
To inspect certificate properties, recommend running:
```
openssl x509 -text -noout -in <certificate_file>
```
This command displays expiration dates, key algorithm and size, signature algorithm, and issuer/subject information needed for the checks below.
#### Check 1: Expiration Status
- Condition: The certificate's `notAfter` (expiration) date is in the past.
- Severity: CRITICAL VULNERABILITY
- Report Message: `This certificate expired on [YYYY-MM-DD]. It is no longer valid and will be rejected by clients, causing connection failures. It must be renewed and replaced immediately.`
- Condition: The certificate's `notBefore` (validity start) date is in the future.
- Severity: Warning
- Report Message: `This certificate is not yet valid. Its validity period begins on [YYYY-MM-DD].`
#### Check 2: Public Key Strength
- Condition: The public key algorithm or size is weak.
- Weak Keys: RSA keys with a modulus smaller than 2048 bits. Elliptic Curve (EC) keys using curves with less than a 256-bit prime modulus (e.g., `secp192r1`, `P-192`, `P-224`).
- Severity: High-Priority Warning
- Report Message: `The certificate's public key is cryptographically weak ([Algorithm], [Key Size]). Keys of this strength are vulnerable to factorization or discrete logarithm attacks. The certificate should be re-issued using at least an RSA 2048-bit key or an ECDSA key on a P-256 (or higher) curve.`
#### Check 3: Signature Algorithm
- Condition: The algorithm used to sign the certificate is insecure.
- Insecure Algorithms: Any signature algorithm using MD5 or SHA-1 (e.g., `md5WithRSAEncryption`, `sha1WithRSAEncryption`).
- Severity: High-Priority Warning
- Report Message: `The certificate is signed with the insecure algorithm '[Algorithm]'. This makes it vulnerable to collision attacks, potentially allowing for certificate forgery. It must be re-issued using a signature based on the SHA-2 family (e.g., sha256WithRSAEncryption).`
#### Check 4: Issuer Type (Self-Signed Check)
- Condition: The certificate's `Issuer` and `Subject` fields are identical.
- Severity: Informational
- Report Message: `This is a self-signed certificate. Ensure this is intentional and only used for development, testing, or internal services where trust is explicitly configured. Self-signed certificates should never be used for public-facing production systems as they will not be trusted by browsers or standard clients.`
### 3. Actionable Examples
Your feedback should be direct and easy to understand.
Example 1: Flagging a Hardcoded Certificate
- Code Snippet:
```
# Certificate for connecting to legacy_service
LEGACY_CERT = """
-----BEGIN CERTIFICATE-----
MIIC... (certificate data) ...
-----END CERTIFICATE-----
"""
```
- Your Analysis and Report:
> Security Review Required: A certificate is hardcoded in the `LEGACY_CERT` variable. Unverified certificates may contain critical vulnerabilities.
>
> - Concern: Hardcoded certificates cannot be easily rotated and may contain security issues that are not immediately visible in the code.
>
> - Required Verification: This certificate must be verified for:
> - Expiration: Certificate must not be expired or not yet valid
> - Key Strength: RSA keys must be at least 2048-bit; EC keys must use P-256 or higher curve
> - Signature Algorithm: Must use SHA-2 family (not MD5 or SHA-1)
> - Self-Signed: If self-signed, must only be used for development, testing, or internal services
>
> - Action: Verify this certificate meets the above requirements before deployment. Consider loading certificates from external files or a certificate store for easier rotation.
>
Example 2: Flagging a Certificate Loaded from File
- Code Snippet:
```
// Load the server certificate from disk
cert, err := tls.LoadX509KeyPair("server.crt", "server.key")
```
- Your Analysis and Report:
> Security Review Required: Certificate loaded from `server.crt`. Unverified certificates may contain critical vulnerabilities.
>
> - Required Verification: Ensure this certificate meets security requirements:
> - Expiration: Certificate must not be expired or not yet valid
> - Key Strength: RSA keys must be at least 2048-bit; EC keys must use P-256 or higher curve
> - Signature Algorithm: Must use SHA-2 family (not MD5 or SHA-1)
> - Self-Signed: If self-signed, must only be used for development, testing, or internal services
>
> - Action: Verify this certificate meets the above requirements before deployment.
You must always explain how this rule was applied and why it was applied.
