Format String in Fiber with Hmac Signatures

A format string vulnerability occurs when user-controlled input is passed directly to a function that formats output, such as fmt.Sprintf in Go. In the Fiber web framework, this becomes high risk when the formatted output is used to construct an HMAC signature or when the signature is generated over user-controlled data that includes format verbs. If the data being signed is built using an unvalidated format string, an attacker can inject format specifiers (e.g., %s, %x, %n) that cause memory reads or writes. This can lead to information disclosure or, in extreme cases, code execution when the resulting signed payload is processed further.

Consider a scenario where a Fiber route builds a message string using user input and then signs it with HMAC. If the message format string is derived from or influenced by attacker-controlled data, the signature operation may process unintended format verbs. For example:

message := fmt.Sprintf(userSuppliedFormat, arg1, arg2) // dangerous if userSuppliedFormat contains format verbs
mac := hmac.New(sha256.New, []byte(secret))
mac.Write([]byte(message))
signature := hex.EncodeToString(mac.Sum(nil))

An attacker could supply userSuppliedFormat as "username=%s secret=%x", causing the formatted output to include parts of memory (due to %x) and altering the signed content in unpredictable ways. This breaks the integrity guarantees of the HMAC because the same logical input can produce different byte representations depending on injected format verbs.

Additionally, if the HMAC signature itself is presented to the client and later verified by formatting or parsing user-controlled data into a verification string, a format string bug in the verification path can allow an attacker to forge signatures. For example, if verification uses another fmt.Sprintf call with unchecked format strings, the attacker may manipulate the parsed values to match a valid signature without knowing the secret.

In the context of the 12 security checks run by middleBrick, this pattern falls under Input Validation and Data Exposure. The scanner tests whether format strings are hardcoded or properly sanitized and whether HMAC operations are performed over canonical, validated data. An unauthenticated scan can detect unsafe usage patterns where user input directly influences formatting before signing, flagging the endpoint for review.

To remediate format string issues when using HMAC signatures in Fiber, ensure that format strings are static and not influenced by user input. Always validate and sanitize any data that will be included in the message before signing, and use explicit, controlled formatting.

Instead of using user-controlled format strings, use fixed templates or concatenate trusted values safely. For example:

// Safe: format string is static, values are provided explicitly
username := getUserInputUsername() // validated elsewhere
userId := getUserInputID()         // validated elsewhere
message := fmt.Sprintf("user:%s:id:%d", username, userId)
mac := hmac.New(sha256.New, []byte(secret))
mac.Write([]byte(message))
signature := hex.EncodeToString(mac.Sum(nil))

If you must include dynamic components, use structured serialization (such as JSON with a canonical ordering) and sign the serialized bytes directly, avoiding fmt.Sprintf for message construction:

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "encoding/json"
)

type Payload struct {
    Username string `json:"username"
    UserID   int    `json:"user_id"
}

payload := Payload{
    Username: sanitizeUsername(getUserInputUsername()),
    UserID:   sanitizeUserID(getUserInputID()),
}
body, _ := json.Marshal(payload)
mac := hmac.New(sha256.New, []byte(secret))
mac.Write(body)
signature := hex.EncodeToString(mac.Sum(nil))

On the verification side, ensure that the data used to recompute the HMAC follows the exact same canonical form. Avoid formatting or reparsing user input in a way that could introduce variability. With middleBrick’s checks for Input Validation and Property Authorization, endpoints using HMAC can be assessed for whether signature generation depends on validated, static format patterns.

For teams using the CLI, scanning with middlebrick scan <url> can highlight endpoints where format strings interact with signing logic. Pro plan users can enable continuous monitoring so changes that reintroduce format string risks are flagged before deployment, and the GitHub Action can fail builds when such patterns are detected in CI/CD pipelines.