GHSA-gxhx-2686-5h9g: Signature Verification Bypass in slack-go via Empty SecretsVerifier

The slack-go package provides a Go client for the Slack API. A critical component of this library is the SecretsVerifier, which authenticates incoming HTTP requests from Slack. These requests typically trigger application logic via Webhooks, Slash Commands, or interactive component payloads. The verification mechanism relies on calculating an HMAC-SHA256 hash using a predefined signing secret and comparing it against the signature provided in the X-Slack-Signature HTTP header.

Versions of slack-go prior to 0.23.1 do not implement validation checks on the signing secret provided during the initialization of the SecretsVerifier. If an application provisions the verifier with an empty string, the library silently accepts it and utilizes it as the cryptographic key for all subsequent HMAC operations. This behavior introduces a severe authentication bypass vulnerability classified under CWE-347 (Improper Verification of Cryptographic Signature).

The vulnerability manifests strictly under specific configuration conditions. The application must supply an empty string to the initialization function, a scenario most commonly arising when the application retrieves the secret from an unset or misconfigured environment variable. When this condition is met, the cryptographic control designed to ensure message origin integrity is entirely nullified.

Once the verifier is initialized with an empty key, any external entity can interact with the protected endpoints. The attacker does not need to extract or guess a secret; they simply exploit the predictable mathematical output of an HMAC function keyed with an empty byte array. This exposes internal application logic to unauthenticated external manipulation.

The root cause of this vulnerability lies in the lack of precondition validation within the NewSecretsVerifier initialization function. The function is responsible for preparing the cryptographic structures necessary for request validation. It accepts the raw HTTP headers and the signing secret as arguments.

In vulnerable versions, the function directly casts the provided secret string to a byte array and passes it to hmac.New. The Go standard library's HMAC implementation correctly permits empty keys according to RFC 2104, treating an empty byte array as a valid input. Because slack-go does not enforce a minimum length or non-empty constraint on the secret, the resulting hash object is successfully instantiated.

An HMAC initialized with an empty key produces deterministic and reproducible outputs for any given input. The cryptographic strength of HMAC relies entirely on the secrecy and entropy of the key. When the key is empty, the operation devolves into a predictable hashing function rather than a secure message authentication code.

The application proceeds to run without logging any warnings or errors regarding the missing cryptographic material. This fail-open behavior in the security configuration pipeline ensures that misconfigured applications start successfully but operate in a completely vulnerable state.

The vulnerability exists in the implementation of NewSecretsVerifier. The function takes a string as the secret and utilizes it without performing any sanitization or validation checks. This allows an empty string to be silently accepted and processed.

// Vulnerable Code (Prior to v0.23.1)
func NewSecretsVerifier(header http.Header, secret string) (SecretsVerifier, error) {
    // The raw secret is used to create a new HMAC hash without checking if it is empty.
    hash := hmac.New(sha256.New, []byte(secret))
    
    // ... remaining initialization logic ...
    return SecretsVerifier{ /* ... */ }, nil
}

The fix introduced in version 0.23.1 alters the initialization logic to enforce strict validation. By adding a simple length check, the library ensures that an empty secret actively prevents the creation of the verifier object. This represents a shift to a fail-closed architecture for security configurations.

// Patched Code (v0.23.1)
func NewSecretsVerifier(header http.Header, secret string) (SecretsVerifier, error) {
    if secret == "" {
        return SecretsVerifier{}, errors.New("empty signing secret provided")
    }
    
    hash := hmac.New(sha256.New, []byte(secret))
    // ... remaining initialization logic ...
}

By returning an explicit error, the patched version shifts the responsibility to the developer to handle the misconfiguration. If the application does not handle the initialization error gracefully, it will likely panic or terminate, which is the correct behavior for an application missing critical authentication material.

Exploiting this vulnerability requires the attacker to understand the Slack request signing protocol and identify a target application suffering from the empty-secret misconfiguration. The attacker acts as an unauthenticated external entity interacting over the network with the application's exposed webhook endpoints.

The Slack signing protocol constructs a base string by concatenating the version number (v0), the request timestamp extracted from the X-Slack-Request-Timestamp header, and the raw request body. These components are delimited by colons. The attacker begins by selecting an arbitrary timestamp and crafting a malicious JSON or URL-encoded payload designed to trigger specific application functionality.

Once the base string is formatted, the attacker computes the HMAC-SHA256 hash using an empty byte array as the key. They prefix the resulting hex-encoded hash with v0= to match the expected Slack signature format. The attacker then transmits the HTTP request containing the crafted payload, the chosen timestamp header, and the forged signature header.

Upon receiving the request, the vulnerable slack-go application reconstructs the base string using the incoming data. Because the application's verifier is initialized with an empty key, its internal HMAC calculation yields the exact same hash generated by the attacker. The verification succeeds, and the application processes the malicious payload as a trusted, origin-verified request.

The successful exploitation of this vulnerability completely bypasses the primary authentication mechanism for Slack integrations. An attacker gains the ability to submit arbitrary commands, events, or payloads to the application. The specific impact depends entirely on the business logic exposed by the vulnerable endpoints.

For applications that execute infrastructure commands, database queries, or user management actions based on Slack input, the impact is severe. An attacker can invoke these functions with forged identities, leading to unauthorized data modification, privilege escalation, or resource manipulation. This maps to CWE-306 (Missing Authentication for Critical Function) and CWE-287 (Improper Authentication).

The CVSS v4.0 vector (CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N/E:U) assigns a base score of 7.7. The Attack Complexity is Low, requiring only standard HTTP requests and basic cryptographic calculations. However, the Attack Requirements (AT:P) metric is present, correctly reflecting that the vulnerability is conditionally exploitable based on application misconfiguration.

The Vulnerability Integrity impact is rated High because the core protection mechanism against request forgery is defeated. Confidentiality impact is rated Low or None, assuming the endpoints primarily consume input rather than leak sensitive data in their responses. The lack of required privileges and user interaction further elevates the practical risk for misconfigured deployments.

The definitive remediation for this vulnerability is upgrading the slack-go library to version 0.23.1 or later. This release introduces the necessary precondition checks to prevent the creation of an insecure SecretsVerifier. Developers managing Go modules should update their go.mod file and execute go mod tidy to retrieve the patched version.

Organizations cannot rely solely on the library update; they must also address the underlying misconfiguration that triggers the vulnerability. Application startup routines should explicitly validate the presence and length of the Slack signing secret before passing it to any security functions. Failing to set the environment variable should result in an immediate application halt.

secret := os.Getenv("SLACK_SIGNING_SECRET")
if len(strings.TrimSpace(secret)) == 0 {
    log.Fatalf("CRITICAL: SLACK_SIGNING_SECRET is empty or unset. Application cannot start securely.")
}
 
verifier, err := slack.NewSecretsVerifier(header, secret)
if err != nil {
    log.Fatalf("Failed to initialize verifier: %v", err)
}

For applications that cannot immediately deploy the updated library, implementing the validation logic shown above serves as a complete mitigation. By ensuring the slack-go library never receives an empty string, the vulnerable code path becomes unreachable. Security teams should audit deployment environments to verify that SLACK_SIGNING_SECRET variables are properly injected and accessible by the running application process.