CVE-2026-113799 in slack-morphism-rust
Summary
by MITRE • 10/22/2026
Slack Morphism is a client library for Slack Web/Events API/Socket Mode and Block Kit. Versions 1.0.0 through 2.27.0 expose the correct Slack request HMAC when signature verification fails. For an invalid request, `slack-morphism` computes the correct signature for the attacker-controlled timestamp and request body and stores it in `SlackEventWrongSignatureError.generated_hash`. The error's `Debug` and `Display` representations expose that value, and the default Axum listener error handler logs it at error level. An attacker who can both reach the application's Slack callback endpoint and read its application/service logs can therefore use the verifier as a chosen-message HMAC oracle. The attacker can send an arbitrary request with an invalid signature, recover the correct signature from the resulting log entry, and resend the same request within the five-minute freshness window. The second request passes Slack Morphism's signature verification and reaches the protected application callback without the attacker knowing the Slack signing secret. Version 2.28.0 patches the issue.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified in Slack Morphism, a client library for interacting with the Slack Web API, Events API, Socket Mode, and Block Kit, represents a critical information disclosure flaw that undermines the integrity of HMAC-based signature verification mechanisms. Versions ranging from 1.0.0 through 2.27.0 are affected by this issue, which stems from how the library handles failed authentication attempts during request validation. Specifically, when an incoming Slack event or webhook fails signature verification due to a mismatched or invalid signature, the internal error handling logic computes what it believes is the correct HMAC for the provided timestamp and request body. This computed value is then stored within the generated_hash field of the SlackEventWrongSignatureError object. While this behavior might seem like standard debugging practice intended to help developers identify why their signing secret was incorrect or misconfigured, it creates a severe security exposure when combined with specific logging configurations.
The core technical flaw lies in the interaction between the error generation process and the default Axum listener error handler used by many applications integrating Slack Morphism. When an invalid request is received, the library generates the correct HMAC for that specific attacker-controlled payload and timestamp. The Debug and Display implementations of this error object expose this generated hash value. Furthermore, if the application utilizes a standard logging setup where error-level logs are recorded to accessible files or centralized log management systems, this sensitive cryptographic material is written to disk. This effectively transforms the Slack Morphism verifier into a chosen-message HMAC oracle. An attacker who can send requests to the application's Slack callback endpoint and has read access to the application service logs can exploit this behavior to recover valid signatures for arbitrary payloads without ever knowing the actual Slack signing secret.
The operational impact of this vulnerability is significant, as it allows an unauthenticated actor to bypass authentication controls entirely under specific conditions. By sending a request with an intentionally invalid signature, the attacker triggers the error condition that leaks the correct HMAC in the logs. Once the valid signature is extracted from these logs, the attacker can resend the exact same request within Slack's five-minute freshness window for timestamps. Because the library will now compute a matching hash using the leaked secret logic (or rather, because the attacker has obtained the pre-computed correct hash), the verification process succeeds, and the malicious payload reaches the protected application callback endpoint. This bypasses security controls designed to ensure that only legitimate Slack events are processed by the backend services, potentially leading to unauthorized actions, data exfiltration, or further exploitation depending on what those callbacks perform.
This vulnerability aligns with CWE-209: Generation of Error Message Containing Sensitive Information and CWE-359: Exposure of Private Key (or Secret) in an Error Message. It also relates to ATT&CK technique T1608: Link Account, as it allows an attacker to impersonate a trusted source like Slack, thereby gaining unauthorized access to resources that rely on this authentication mechanism for trust validation. The flaw highlights the danger of exposing internal state or computed cryptographic values in error responses and logs, especially when those values can be used directly to authenticate subsequent requests.
To mitigate this vulnerability, organizations must immediately upgrade to version 2.28.0 or later of Slack Morphism, which patches the issue by preventing the exposure of the generated hash in error contexts. For applications that cannot immediately update due to dependency constraints, it is crucial to review and harden logging configurations. Developers should ensure that sensitive data such as cryptographic hashes, secrets, or internal debugging information are never written to log files accessible to unauthorized users. Implementing custom error handlers that sanitize output before logging can prevent the leakage of these values. Additionally, restricting access to application logs through proper file permissions and centralized log management security controls adds a necessary layer of defense in depth against this type of oracle attack.