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A04 — Cryptographic Failures

Cryptographic failures cover weak algorithms, insecure configurations, hardcoded secrets, and disabled security checks. These rules detect the most common cryptographic mistakes in .NET code.

OWASPA04001 — Weak hashing algorithm​

PropertyValue
SeverityWarning
CategoryA04 Cryptographic Failures

What it detects​

Use of MD5 or SHA1 via MD5.Create(), SHA1.Create(), MD5.HashData(), or SHA1.HashData().

Why it matters​

MD5 and SHA1 are cryptographically broken and must not be used for password hashing, digital signatures, or integrity verification. They are trivially reversible via rainbow tables and vulnerable to collision attacks.

❌ Non-compliant​

using var md5 = MD5.Create();
byte[] hash = md5.ComputeHash(data);

✅ Compliant​

byte[] hash = SHA256.HashData(data);
// or for passwords:
string hashed = BCrypt.HashPassword(password);

OWASPA04002 — ECB block cipher mode​

PropertyValue
SeverityWarning
CategoryA04 Cryptographic Failures

What it detects​

Symmetric ciphers (AES, DES, 3DES) configured with CipherMode.ECB.

Why it matters​

ECB mode encrypts each block independently with the same key. Identical plaintext blocks produce identical ciphertext blocks, leaking structural information about the plaintext. The "ECB penguin" attack demonstrates that ECB-encrypted images reveal their content.

❌ Non-compliant​

using var aes = Aes.Create();
aes.Mode = CipherMode.ECB;

✅ Compliant​

using var aes = Aes.Create();
aes.Mode = CipherMode.CBC;  // or GCM via AesGcm
aes.GenerateIV();

OWASPA04003 — System.Random for security purposes​

PropertyValue
SeverityInfo
CategoryA04 Cryptographic Failures

What it detects​

Instantiation of System.Random or new Random() in code paths that may be used for security-sensitive values (tokens, salts, keys).

Why it matters​

System.Random is a pseudo-random number generator seeded with the system clock — it is not cryptographically secure. An attacker who can observe or guess the seed can predict all generated values.

❌ Non-compliant​

var rng = new Random();
string token = rng.Next().ToString();

✅ Compliant​

byte[] token = RandomNumberGenerator.GetBytes(32);
string tokenHex = Convert.ToHexString(token);

OWASPA04004 — Hardcoded cryptographic key or IV​

PropertyValue
SeverityError
CategoryA04 Cryptographic Failures

What it detects​

Hardcoded byte arrays or strings assigned to properties named Key or IV on symmetric cipher objects.

Why it matters​

Hardcoded keys are discoverable by anyone with access to the binary or source code. A compromised key invalidates all encrypted data.

❌ Non-compliant​

using var aes = Aes.Create();
aes.Key = new byte[] { 0x00, 0x01, 0x02, /* ... */ 0x0f };
aes.IV  = new byte[] { 0x10, 0x11, /* ... */ };

✅ Compliant​

using var aes = Aes.Create();
aes.GenerateKey();
aes.GenerateIV();

Or load from a secure configuration source (environment variable, Azure Key Vault, etc.).

OWASPA04005 — Legacy TLS protocol version​

PropertyValue
SeverityWarning
CategoryA04 Cryptographic Failures

What it detects​

Use of SslProtocols.Ssl2, SslProtocols.Ssl3, SslProtocols.Tls (TLS 1.0), or SslProtocols.Tls11 (TLS 1.1).

Why it matters​

SSL 2/3, TLS 1.0, and TLS 1.1 have known vulnerabilities (POODLE, BEAST, etc.) and have been deprecated by major browsers and RFCs. They must not be used in production.

❌ Non-compliant​

var handler = new HttpClientHandler
{
    SslProtocols = SslProtocols.Tls | SslProtocols.Tls11
};

✅ Compliant​

var handler = new HttpClientHandler
{
    SslProtocols = SslProtocols.Tls12 | SslProtocols.Tls13
};

OWASPA04006 — Certificate validation disabled​

PropertyValue
SeverityError
CategoryA04 Cryptographic Failures

What it detects​

ServerCertificateCustomValidationCallback set to a lambda that always returns true, or ServicePointManager.ServerCertificateValidationCallback assigned a delegate that ignores errors.

Why it matters​

Disabling certificate validation makes TLS meaningless — any certificate is accepted, enabling trivial man-in-the-middle attacks.

❌ Non-compliant​

var handler = new HttpClientHandler
{
    ServerCertificateCustomValidationCallback = (_, _, _, _) => true
};

✅ Compliant​

// Use the default handler — it validates certificates properly
var client = new HttpClient();

If you need to trust a custom CA, add it to the system certificate store rather than bypassing validation entirely.

OWASPA04007 — HTTP URL (not HTTPS)​

PropertyValue
SeverityWarning
CategoryA04 Cryptographic Failures

What it detects​

String literals starting with http:// used as URL arguments to HttpClient, WebClient, or HttpWebRequest.

Why it matters​

Plain HTTP transmits data in cleartext. Even if the server redirects to HTTPS, the initial request is unencrypted and the redirect itself can be intercepted (SSL stripping).

❌ Non-compliant​

var response = await client.GetAsync("http://api.example.com/data");

✅ Compliant​

var response = await client.GetAsync("https://api.example.com/data");

OWASPA04008 — Missing HSTS configuration​

PropertyValue
SeverityWarning
CategoryA04 Cryptographic Failures

What it detects​

ASP.NET Core applications with a Configure/ConfigureApp method that never calls app.UseHsts().

Why it matters​

HTTPS redirection alone doesn't prevent the first request from being sent over HTTP. HSTS instructs browsers to always use HTTPS for a domain, preventing protocol downgrade attacks.

❌ Non-compliant​

app.UseHttpsRedirection();
// UseHsts() missing
app.UseRouting();

✅ Compliant​

app.UseHsts();
app.UseHttpsRedirection();
app.UseRouting();