Format String in Buffalo with Jwt Tokens

A format string vulnerability in a Buffalo application becomes high risk when JWT tokens are handled improperly, for example when a token value is passed directly to a function that performs unchecked string formatting. In Go, using functions like fmt.Sprintf with user-controlled token data can lead to reading stack memory or causing crashes if the token contains format verbs such as %s or %x. Because JWTs are often logged or echoed for debugging, a token like eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9 could include sequences that are misinterpreted as format specifiers when combined with unsafe formatting calls.

In Buffalo, a typical pattern that is unsafe looks like:

token := c.Param("token")
message := fmt.Sprintf("Token received: %s", token)
c.Logger().Info(message)

If token contains format verbs, fmt.Sprintf may consume additional stack arguments, leading to information disclosure or instability. This is especially relevant when tokens are logged or included in HTTP responses, because an attacker can supply crafted input to probe memory. Because JWTs carry identity and authorization information, exposing parts of a token through format string reads can aid further attacks such as token tampering or replay. The interaction between Buffalo route handling and Go’s formatting functions means that any place where token strings are merged into format strings without proper sanitization or structured logging increases the attack surface.

Middleware that decodes and validates JWTs before they reach business logic reduces the window, but does not eliminate the risk if token values are later formatted into strings for logging or error reporting. An attacker could submit a token with repeated or malicious format verbs to observe behavior changes, making format string bugs a practical concern in API security testing. Tools like middleBrick can detect such patterns by scanning unauthenticated endpoints and flagging unsafe usage of formatting functions near token handling code, supporting checks aligned with OWASP API Top 10 and relevant compliance mappings.

To remediate format string issues with JWTs in Buffalo, avoid passing token strings directly to formatting verbs. Use structured logging and explicit string concatenation or fmt.Sprintf with properly escaped or positional arguments that do not interpret user input as format verbs. The safest approach is to log token metadata without including the raw token, or to use a logging function that does not perform formatting interpretation on the token itself.

Safe example using structured logging without injecting the token into a format verb chain:

token := c.Param("token")
c.Logger().Info("token_received", "alg", "HS256")

If you must include the token in output, use explicit concatenation or positional arguments to ensure verbs in the token are not interpreted:

token := c.Param("token")
message := fmt.Sprintf("Token received: %s", token)
// Prefer concatenation when logging tokens:
c.Logger().Info("Token received: " + token)

For validation and decoding, rely on a JWT library and do not construct messages from the raw token. Example using a typical JWT parsing flow:

import (
    "github.com/golang-jwt/jwt/v5"
)

raw := c.Param("token")
_, err := jwt.Parse(raw, func(token *jwt.Token) (interface{}, error) {
    if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
        return nil, fmt.Errorf("unexpected signing method")
    }
    return []byte("your-secret"), nil
})
if err != nil {
    c.Logger().Error("jwt_parse_failed", "error", err.Error())
    c.Render(401, r.JSON, map[string]string{"error": "invalid_token"})
    return
}

These changes ensure that token values never act as format specifiers, reducing risk of memory disclosure. By combining safe logging patterns and strict validation, Buffalo applications can handle JWTs securely while remaining observable for legitimate debugging needs.