Format String in Fiber with Jwt Tokens

A format string vulnerability occurs when user-controlled input is passed directly to a formatting function such as fmt.Sprintf without proper sanitization. In the Fiber web framework, this becomes high risk when JWT token strings supplied by a client are incorporated into log messages, error responses, or internal routing logic using unprotected format verbs. For example, if a developer writes fmt.Sprintf("token: %s", token), this is safe because %s is a standard verb. However, using untrusted JWT data in a format string like fmt.Sprintf(token) or fmt.Printf(userToken) can cause the application to interpret characters within the token as format specifiers such as %v, %x, or %n. This can lead to reading stack memory (information disclosure) or, in more complex runtime environments, to arbitrary memory writes.

When JWT tokens are involved, the impact is amplified because tokens often contain sensitive metadata in their payload sections, such as user roles or session identifiers. An attacker can craft a JWT with format specifiers embedded in its claims or signature portion and send it to endpoints that log or echo the token. A vulnerable Fiber route might look like this:

c.Get("/debug", func(c *fiber.Ctx) error {
	token := c.Params("token")
	// Dangerous: user-controlled token used directly in a format verb
	log.Printf(token)
	return c.SendString("ok")
})

If the token contains %s sequences, log.Printf will attempt to read additional arguments from the stack, potentially exposing private variables or causing a panic. In a worst-case scenario with a maliciously crafted token containing %n, an attacker might manipulate memory writes, leading to unpredictable application behavior. Because JWT tokens are often base64-encoded strings that can include a wide range of byte values, they are a realistic source of format specifiers. The combination of a high-performance framework like Fiber and the widespread use of JWTs for authentication increases the likelihood that such patterns appear in legacy or hastily written code.

Another vector involves response formatting where JSON or text output is constructed using format verbs. For instance, returning user-controlled data in an error message built with fmt.Sprintf can expose sensitive fields if the developer inadvertently includes format verbs in the template. This can be part of an information disclosure chain that reveals internal structure alongside the JWT content. Because the attack requires no authentication and can be triggered with a single request, it fits naturally into the unauthenticated attack surface that middleBrick scans for.

Detection of this issue relies on static analysis of route handlers and logging statements, cross-referenced with the presence of JWT extraction from headers or parameters. middleBrick flags instances where token-like parameters are passed to formatting functions without explicit sanitization, providing precise locations and remediation steps in the scan report. Understanding this specific interaction helps developers recognize that even well-known libraries like Fiber do not automatically protect against classic programming errors when handling structured security tokens.

Remediation centers on ensuring that JWT token values are treated strictly as data and never as format strings. The safest approach is to avoid passing JWT variables directly to formatting functions. Instead, use explicit placeholders and pass the token as an argument. For example, change dangerous code like this:

// Dangerous
log.Printf(token)

to a safe pattern:

// Safe
log.Printf("token=%s", token)

This ensures that token is interpreted only as a string value, regardless of its contents. In Fiber handlers, you should apply the same principle when constructing responses or logging authentication events:

c.Get("/login", func(c *fiber.Ctx) error {
	token := c.Get("Authorization")
	// Safe: explicit format verb with argument
	c.Logger().Infof("auth attempt: token=%s", token)
	return c.SendString("handled")
})

When using structured logging or JSON output, prefer concatenation or dedicated serialization libraries rather than manual format verbs. For example, using log.Printf("error processing token: %s", token) is acceptable, but constructing dynamic format strings from token content is strictly prohibited. Developers should also validate and sanitize JWT inputs at the edge, rejecting tokens that contain unexpected characters or patterns that could interfere with formatting functions.

For existing codebases, a systematic search for patterns like log.Printf( or fmt.Sprintf( with variables derived from c.Params or c.Get() helps identify vulnerable routes. Automated tools like middleBrick can highlight these locations and provide prioritized findings with severity levels and step-by-step remediation guidance. By combining static analysis with disciplined coding practices—such as never using user-controlled strings as the format argument—teams can eliminate format string risks while continuing to leverage JWTs for secure session management in Fiber applications.

Additionally, ensure that any third-party middleware that logs or traces requests is configured to treat JWTs as opaque strings. This prevents indirect exposure through library code that might also use unsafe formatting. The goal is not to avoid JWTs, but to handle them in a way that preserves their security properties and prevents memory-related side effects.