Format String in Aspnet with Hmac Signatures

A format string vulnerability in an ASP.NET application becomes particularly risky when HMAC signatures are involved. In this context, an attacker can supply a payload that includes format specifiers (e.g., %s, %x, %n) within the data that is included in or influences the signed payload. If the application uses insecure string formatting functions such as string.Format or Console.WriteLine on user-controlled input that is later incorporated into the data that is HMAC-verified, the runtime behavior can be altered. This occurs because the format string is processed before or during the construction of the message, potentially reading from the stack or writing arbitrary data via %n.

Consider a scenario where an API endpoint accepts a JSON object with a message field and an HMAC signature in a header. The server recomputes the HMAC over the raw message. If the server-side code then logs or echoes the message using a vulnerable formatter, an attacker can supply { "message": "Auth token: %s %s %s" }. Even though the HMAC covers the original attacker-controlled string, the format string causes unintended reads of adjacent stack memory, potentially leaking parts of the HMAC key or other sensitive variables used during signature generation. This does not break the HMAC cryptographically, but it undermines confidentiality and can aid further exploitation.

Moreover, if the application uses composite formatting to build a canonical string for signing, and the format provider or culture is influenced by user input, the resulting canonical representation can change unpredictably. This leads to signature mismatch or, worse, to a situation where an attacker can subtly alter the interpreted fields while keeping the signature valid due to how the formatted string is constructed. The vulnerability is not in the HMAC algorithm itself, but in how the message content is constructed and handled before signing and verification, allowing an attacker to manipulate runtime behavior through format injection.

To remediate format string issues in the context of HMAC signatures in ASP.NET, ensure that all user-controlled data is handled as inert data and never passed to formatting functions that interpret format specifiers. Use invariant culture and explicit, safe string handling to ensure the canonical representation used for signing remains stable and predictable.

Example of a vulnerable pattern that mixes formatting with signing:

// Vulnerable: userMessage may contain format specifiers
string userMessage = request.Form["message"];
string logEntry = string.Format(userMessage, "extra"); // Risk: format string attack
string dataToSign = $"{userMessage}|{timestamp}";
var hmac = new HMACSHA256(key);
byte[] hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(dataToSign));
string signature = Convert.ToBase64String(hash);

Secure alternative using explicit placeholders and string.Create or simple concatenation with escaped data:

// Secure: treat user input as data, not format string
string userMessage = request.Form["message"];
// Option A: use concatenation with invariant culture
string dataToSign = string.Concat(CultureInfo.InvariantCulture, userMessage, "|", timestamp);
// Option B: if you need templating, use a placeholder and replace safely
string template = "{0}|{1}";
string dataToSign = string.Format(CultureInfo.InvariantCulture, template, userMessage, timestamp);
var hmac = new HMACSHA256(key);
byte[] hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(dataToSign));
string signature = Convert.ToBase64String(hash);

When logging or displaying user-supplied messages, avoid passing them directly to formatters that interpret format strings:

// Instead of this:
// Logger.Log(userSuppliedFormat);
// Use this:
Logger.Log("{UserMessage}", userMessage); // structured logging with placeholders
// Or simply:
Console.WriteLine(userMessage); // output as plain data

For HMAC verification, always normalize inputs (e.g., trim, enforce UTF-8) and use the same canonicalization method on both sides. Do not allow any part of the signed string to be built using uncontrolled format strings. If you use JSON, serialize with deterministic settings (e.g., JsonSerializerOptions with PropertyNamingPolicy and Encoder set to invariant defaults) to ensure byte-for-byte consistency before computing or validating the signature.