jstedfast/MailKit · error · ArgumentOutOfRangeException

count

Error message

count

What it means

ComputeHash throws ArgumentOutOfRangeException("count") when count is negative or offset+count extends past the buffer (the guard compares offset > buffer.Length - count). A wrong offset can therefore also raise this error.

Solutions

1. Ensure count >= 0 and offset + count <= buffer.Length before calling
2. Recompute count from the actual data length
3. Catch ArgumentOutOfRangeException and validate the length source (e.g. header parse)

Example fix

// before
hmac.ComputeHash(buf, 0, declaredLength); // untrusted length
// after
int count = Math.Min(declaredLength, buf.Length);
if (count >= 0)
	hmac.ComputeHash(buf, 0, count);

if (count < 0 || offset + count > buffer.Length) throw new ArgumentException("count out of bounds");

try { hmac.ComputeHash(buf, off, cnt); } catch (ArgumentOutOfRangeException ex) when (ex.ParamName == "count") { /* recompute count */ }

When it happens

Trigger: Negative counts, counts exceeding buffer.Length - offset, or count computed against a stale offset.

Common situations: Hashing a sub-range of an NTLM message with length fields taken from an incorrect header parse.

Related errors

• offset
• ArgumentOutOfRangeException
• No hash value computed.
• value
• buffer

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/cd850c79f1218ef4. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Security/Ntlm/HMACMD5.cs:109

		{
			hash.Init (new KeyParameter (Key));
		}

		public void Clear ()
		{
			Dispose (false);
		}

		public byte[] ComputeHash (byte[] buffer, int offset, int count)
		{
			if (buffer == null)
				throw new ArgumentNullException (nameof (buffer));

			if (offset < 0 || offset > buffer.Length)
				throw new ArgumentOutOfRangeException (nameof (offset));

			if (count < 0 || offset > buffer.Length - count)
				throw new ArgumentOutOfRangeException (nameof (count));

			if (disposed)
				throw new ObjectDisposedException ("HashAlgorithm");

			HashCore (buffer, offset, count);
			hashValue = HashFinal ();

			return hashValue;
		}

		public byte[] ComputeHash (byte[] buffer)
		{
			if (buffer == null)
				throw new ArgumentNullException (nameof (buffer));

			return ComputeHash (buffer, 0, buffer.Length);
		}

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