Format String in Buffalo with Bearer Tokens

A format string vulnerability occurs when user-controlled input is passed directly to a formatting function such as Sprintf or Printf without a explicit format specifier. In the Buffalo web framework, this typically arises when constructing HTTP headers, log messages, or response bodies using untrusted data. When combined with Bearer Tokens, the risk is compounded because tokens are often handled as strings and may be interpolated into logs, error messages, or redirect URLs.

Consider a route that receives an Authorization header containing a Bearer Token and uses it to build a debug response. If the developer writes fmt.Sprintf("Token: %s", token), this is safe because %s is explicit. However, writing fmt.Sprintf(fmtStr, token) where fmtStr comes from user input or configuration allows the token’s content to be interpreted as format verbs. An attacker can supply a token like %s %s %s to read adjacent memory, or %x to leak stack contents that may include other secrets or session material.

In Buffalo, middleware that logs incoming requests commonly captures the Authorization header. If the logging call uses an unchecked format pattern, the token can be exfiltrated through log injection or log forging. For example, a token containing newline sequences can split log lines, enabling log injection that obscures the true source of requests. Additionally, if a crafted token is used in an HTTP redirect location, format verbs can cause erratic behavior or information disclosure before the redirect is issued.

The combination is particularly dangerous when the token is reflected in HTML or JSON responses. If a developer writes response.Write(fmt.Sprintf(userFormat, token)) and userFormat is supplied from query parameters, the token can break out of the intended context. In JSON, this might lead to injection of new fields or strings that confuse parsers. In HTML, it can lead to injection points for cross-site scripting, especially when the token is later used in a JavaScript context.

Real-world attack patterns mirror classic CVEs seen in other frameworks: format string reads (CVE-2016-10072-like behavior) and writes that can corrupt memory or leak contents. Although Buffalo applications are typically compiled Go, unsafe use of formatting functions still leads to information disclosure and logic confusion. The scanner’s Input Validation checks flag suspicious uses of Sprintf with dynamic format strings, while the Data Exposure checks look for tokens reflected in logs or responses without proper sanitization.

middleBrick detects these issues by tracing how Bearer Tokens flow from request headers into formatting routines and output streams. It correlates findings across the 12 checks, highlighting cases where format verbs in tokens or format strings create unexpected behavior. The tool provides remediation guidance that emphasizes strict format specifiers and avoiding reflection of untrusted data into formatting functions.

To remediate format string issues involving Bearer Tokens in Buffalo, ensure tokens are treated strictly as data and never as format specifiers. Use explicit format verbs and avoid passing untrusted strings as the format string itself. Below are concrete code examples demonstrating insecure patterns and their secure alternatives.

• Insecure logging with token interpolation:

// DO NOT DO THIS
logEntry := fmt.Sprintf(fmtFromUser, authToken) // format string from user input

• Secure alternative with explicit specifier:

// Use a fixed format and treat the token as a value
logEntry := fmt.Sprintf("Authorization: %s", authToken)

• Insecure redirect using token in location header:

// DO NOT DO THIS
redirectLocation := fmt.Sprintf(userSuppliedFormat, authToken)
http.Redirect(w, r, redirectLocation, http.StatusFound)

• Secure alternative with hardcoded location or safe concatenation:

// Validate and use a known-safe base, then append token as a query parameter
base := "/callback"
token := r.Header.Get("Authorization")
if len(token) > 7 && strings.HasPrefix(token, "Bearer ") {
    token = token[7:]
}
redirectLocation := base + "?token=" + url.QueryEscape(token)
http.Redirect(w, r, redirectLocation, http.StatusFound)

• Insecure use in JSON response construction:

// DO NOT DO THIS
output := fmt.Sprintf(jsonFormatFromUser, authToken)
w.Header().Set("Content-Type", "application/json")
w.Write([]byte(output))

• Secure alternative using encoding/json:

import "encoding/json"

type Resp struct {
    Token string `json:"token"`
}
resp := Resp{Token: authToken}
jsonBytes, _ := json.Marshal(resp)
w.Header().Set("Content-Type", "application/json")
w.Write(jsonBytes)

• General middleware advice: When extracting Bearer Tokens, normalize them (trim prefix) and store them in structured types rather than raw strings that could be reused in formatting contexts. Validate length and character set to reduce the risk of injected format verbs.