Slack Morphism verifier errors disclose valid HMACs in the logs

Summary

Slack Morphism exposes 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.

I reproduced this end-to-end against both the Axum and Hyper integrations in slack-morphism 2.26.0 and 2.27.0.

Details

The vulnerability is in Slack Morphism's request signature verification error path.

In src/signature_verifier.rs, SlackEventSignatureVerifier::verify computes the expected signature before comparing it with the untrusted X-Slack-Signature supplied by the request.

On a mismatch, the implementation creates a SlackEventWrongSignatureError containing the complete correct signature in its public generated_hash field.

Relevant source path:

src/signature_verifier.rs:46-59

The error type is defined around:

src/signature_verifier.rs:182-199

generated_hash is a public String; the type derives Debug, and its Display implementation also prints the generated signature.

The resulting source-to-sink path is:

1. SlackEventSignatureVerifier::verify computes the expected HMAC for the attacker-controlled timestamp and raw request body.
2. Signature verification fails because the attacker initially sends an invalid signature.
3. Slack Morphism places the correct expected value in SlackEventWrongSignatureError.generated_hash.
4. The error's Debug and Display representations reveal that HMAC.
5. The Axum middleware propagates/formats the error through src/axum_support/slack_events_middleware.rs.
6. The default listener error handler in src/listener.rs logs the error at error level.
7. The attacker reads the generated HMAC from the logs.
8. The attacker resends the identical timestamp and body using the disclosed value as X-Slack-Signature.
9. Slack Morphism accepts the request and executes the protected callback.

The Hyper path is also affected. src/hyper_tokio/hyper_ext.rs propagates the verifier error as a service error, and formatting that error similarly exposes the generated signature.

The disclosed value is exactly the credential required to authenticate the corresponding request:

v0=hex(HMAC-SHA256(signing_secret, "v0:" || timestamp || ":" || raw_body))

There is no weakness in HMAC-SHA256 or the constant-time comparison itself. The vulnerability is that the expected authentication value is disclosed through the verifier's error path.

Affected versions

I reproduced the vulnerability against:

• slack-morphism 2.26.0
• slack-morphism 2.27.0
• current master at audit time: e3b3ecabf99dfcf6c3548a42fba89fbfa510d21c

Git history indicates that the public generated_hash field was introduced in:

9bf623bd51c6a9ef0cd22d842cc707a3311f1f22

and appears to have existed since v0.10.0. Since versions before 2.26.0 are unsupported and were not dynamically tested during this assessment, I am reporting the independently validated supported range as:

>= 2.26.0, <= 2.27.0

No patched release is currently available.

PoC

The PoC uses only localhost and a synthetic signing secret.

Tested environment:

slack-morphism = 2.27.0
Rust 1.98.1
Linux x86_64
Axum and Hyper integrations

Synthetic signing secret:

SLACK_MORPHISM_SYNTHETIC_SIGNING_SECRET_7c91

Start a Slack Morphism Axum callback endpoint using the normal Slack request signature verification middleware and the default listener error handling.

Send a request using:

Body:
{"type":"attacker_chosen","action":"transfer"}

X-Slack-Request-Timestamp:
<current Unix timestamp>

X-Slack-Signature:
v0=0000000000000000000000000000000000000000000000000000000000000000

The all-zero signature is deliberately invalid.

First request

The first request is correctly rejected with HTTP 400.

However, Slack Morphism's default error output contains an entry equivalent to:

SlackEventWrongSignatureError {
    ...
    generated_hash: "v0=<correct-HMAC>"
}

where <correct-HMAC> is the valid Slack request signature for the exact attacker-controlled timestamp and body.

Copy that generated_hash.

Before the five-minute timestamp window expires, resend the identical body and timestamp, changing only the signature header:

X-Slack-Signature: v0=<correct-HMAC-from-error>

Result

The second request returns HTTP 200 and the protected application callback executes.

In my end-to-end Axum reproduction the application observed:

PROTECTED_CALLBACK_EXECUTED body={"type":"attacker_chosen","action":"transfer"}

I independently reproduced the same primitive through the Hyper integration. The invalid request produces a service error containing:

WrongSignatureError(... generated_hash: "v0=<correct-HMAC>")

and replaying that value with the same timestamp and body causes the protected callback to execute.

Expected behavior

An invalid Slack request should be rejected without exposing any verifier-derived value that could authenticate a subsequent request.

Neither verifier error objects nor application logs should contain the expected HMAC.

Actual behavior

The invalid request is rejected, but Slack Morphism exposes the correct HMAC for the attacker-selected timestamp and body through the verifier error.

Reusing that disclosed HMAC within the freshness window causes the previously invalid attacker-controlled request to pass authentication and reach the protected callback.

Impact

An attacker requires:

1. network access to a Slack Morphism Events API, slash-command, interaction, or equivalent signed callback endpoint; and
2. lower-privilege read access to the application's/service's logs containing the verifier error.

The attacker does not need:

• the Slack signing secret;
• a Slack access token;
• a Slack app token;
• access to a Slack workspace; or
• permission to modify the application's configuration.

The demonstrated impact is an authentication-boundary bypass: a lower-trust log reader can generate a valid signature for attacker-chosen request bytes and cause application functionality protected by Slack request authentication to execute.

The downstream consequences depend on the consuming application. A forged event, slash command, or interaction may trigger whatever business operations that application deliberately exposes to authenticated Slack callbacks. I am not claiming that the vulnerability itself grants access to Slack data or provides a specific application-level operation.

This is a Slack Morphism vulnerability rather than an Axum/Hyper or downstream application issue because Slack Morphism itself:

• computes the correct HMAC;
• places it into its own public error type;
• exposes it through Debug and Display;
• and, in the default Axum path, sends it to its own error logger.

Signature verification is enabled and correctly configured in the reproduction.

Suggested remediation

The safest fix is to remove generated_hash from SlackEventWrongSignatureError entirely.

The expected HMAC should not be retained in a public error object because merely changing Display would still leave other disclosure