Format String in Fiber with Basic Auth

A format string vulnerability occurs when user-controlled input is passed directly to a formatting function such as fmt.Sprintf without a explicit format specifier. In the Fiber web framework, this risk is amplified when Basic Auth handlers are used to gate routes. Basic Auth credentials are typically extracted from the Authorization header on each request. If the application uses these credentials in a logging, tracing, or error-reporting path that involves formatted strings, an attacker can control the format verbs and cause memory disclosure or crashes.

Consider a Fiber route that uses Basic Auth and then logs the username using a format string built from user input:

// Vulnerable example
app.Get('/profile', func(c *fiber.Ctx) error {
    user, pass, _ := c.Request().AuthUser()
    // user or pass may come from external sources and be reused downstream
    message := fmt.Sprintf(user) // user controls the format verb
    c.Logger().Info(message)
    return c.SendString('ok')
})

Here, user is taken directly from the Basic Auth identity and passed to fmt.Sprintf with no format specifier. An attacker can supply a payload like %s%s%s%s%s%s%s%s to leak adjacent stack memory, or use %x to dump hex bytes. Because Basic Auth credentials are often reused across multiple internal functions (e.g., constructing audit logs, metrics labels, or error contexts), a single format string bug can expose multiple data flows.

Another common pattern is building error messages or HTTP responses by concatenating formatted strings with credentials:

// Also vulnerable
app.Get('/login', func(c *fiber.Ctx) error {
    user, _, _ := c.Request().AuthUser()
    resp := fmt.Sprintf("Login attempt for %s", user)
    return c.SendString(resp)
})

If user contains format verbs, the output can be coerced into revealing internal state. Because Fiber does not sanitize Basic Auth identities, developers must treat these values as untrusted input. The interaction between authentication context and unchecked format verbs creates a chain where an attacker can probe memory contents or induce denial of service through repeated malformed requests.

OpenAPI specifications that describe Basic Auth security schemes must also be reviewed. If spec definitions reference user identity fields that later participate in formatted outputs, cross-referencing runtime findings against the spec can highlight mismatches between declared authentication usage and actual implementation, helping to surface latent format string risks.

Remediation focuses on ensuring that any data derived from Basic Auth is never used as a format string. Always use a static format string and pass user data as arguments, or avoid formatting functions entirely when constructing logs or responses.

Safe logging with explicit format specifiers:

// Safe: format string is a literal, user data is an argument
app.Get('/profile', func(c *fiber.Ctx) error {
    user, _, _ := c.Request().AuthUser()
    c.Logger().Info("profile accessed by %s", user)
    return c.SendString('ok')
})

Avoiding formatting when not required:

// Safe: no formatting, direct output
app.Get('/login', func(c *fiber.Ctx) error {
    user, _, _ := c.Request().AuthUser()
    // Construct response without fmt.Sprintf
    response := "Login attempt for " + user
    return c.SendString(response)
})

Validation and normalization of identity values:

Because Basic Auth usernames and passwords can contain a wide range of byte sequences, treat them as opaque strings. Do not attempt to reinterpret them as format templates. If you need to sanitize or normalize credentials for internal use, apply canonicalization before any logging or routing logic.

When using the middleBrick CLI to scan endpoints that rely on Basic Auth, you can validate that your handlers avoid format string misuse. The scanner checks for dangerous formatting patterns and maps findings to frameworks such as OWASP API Top 10. For teams that require continuous assurance, the middleBrick Pro plan provides ongoing monitoring and can integrate into CI/CD pipelines to fail builds if risky patterns are detected.

Finally, ensure that any generated documentation or OpenAPI specs accurately reflects how Basic Auth identities are consumed. This alignment reduces the chance that format string vulnerabilities are introduced during refactoring or when extending the API surface.