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Response snapshot Oct 26, 2026, 23:04 UTC

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VulnerabilityCVE-2026-113799

Slack Morphism verifier errors disclose valid HMACs in the logs

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`.

Published Updated

At a glance

Exploited
Not known
Severity
5.3 Medium (CVSS 3.1)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: slack-morphism: 2.28.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-26w4-8w32-4x3v (opens a new tab)

What happened

### 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.

Continue reading

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:

```text src/signature_verifier.rs:46-59 ```

The error type is defined around:

```text 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:

```text 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:

```text 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:

```text >= 2.26.0,

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:

```text SlackEventWrongSignatureError { ... generated_hash: "v0= " } ```

where ` ` 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:

```text X-Slack-Signature: v0= ```

#### Result

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

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

```text 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:

```text WrongSignatureError(... generated_hash: "v0= ") ```

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 w

Affected package ranges reported by GitHub: slack-morphism: >= 1.0.0, <= 2.27.0

Scores and how they were measured

CVSS base severity

5.3

Version 3.1As of

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.4%

Scored Percentile 32.2%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-26w4-8w32-4x3v (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-26w4-8w32-4x3v, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-113799, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 21). Slack Morphism verifier errors disclose valid HMACs in the logs (CVE-2026-113799). https://kevwire.com/record/CVE-2026-113799

MLA

"Slack Morphism verifier errors disclose valid HMACs in the logs (CVE-2026-113799)." KEVWire, 21 Oct. 2026, kevwire.com/record/CVE-2026-113799.

Chicago

KEVWire. "Slack Morphism verifier errors disclose valid HMACs in the logs (CVE-2026-113799)." Last modified October 21, 2026. https://kevwire.com/record/CVE-2026-113799.

Evidence summary (plain text)

Slack Morphism verifier errors disclose valid HMACs in the logs (Oct 21, 2026).
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.
CVSS 5.3 (version 3.1; evidence as of Oct 21, 2026; https://github.com/advisories/GHSA-26w4-8w32-4x3v).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-113799&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-26w4-8w32-4x3v: https://github.com/advisories/GHSA-26w4-8w32-4x3v
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNTo1Mjo1NlrOAAfxiA%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNDowNjoxNFrOAAfxZw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQwODoyNzoyM1rOAAYp9A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozNTo1OFrOAAfwCQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozMDoxM1rOAAfv2A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyODowOVrOAAfv1g%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyNDoyMFrOAAfv1Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDo0NTowN1rOAAfvzQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDozMDoyNlrOAAfvxw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDowMzo1N1rOAAfvxQ%3D%3D
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-113799&date=2026-10-26
Record ID: CVE-2026-113799.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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