nodejs/node

Runtime-deprecate calling digest() on HMAC more than once

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#62838 opened on Apr 20, 2026

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 (11 comments) (6 reactions) (0 assignees)JavaScript (117,218 stars) (35,535 forks)batch import
cryptogood first issuesecurity

Description

(Upd: detected by the scanner behind @deepview-autofix)

Hash behavior is reasonable:

> hash = require('crypto').createHash('sha256').update('data')
> hash.digest()
<Buffer 3a 6e b0 79 0f 39 ac 87 c9 4f 38 56 b2 dd 2c 5d 11 0e 68 11 60 22 61 a9 a9 23 d3 bb 23 ad c8 b7>
> hash.digest()
Uncaught Error [ERR_CRYPTO_HASH_FINALIZED]: Digest already called
    at Hash.digest (node:internal/crypto/hash:155:11) {
  code: 'ERR_CRYPTO_HASH_FINALIZED'
}

But HMAC, on the other hand, returns empty buffers on further .digest() calls, likely for compat reasons:

> hmac = require('crypto').createHmac('sha256', 'key').update('data')
> hmac.digest()
<Buffer 50 31 fe 3d 98 9c 6d 15 37 a0 13 fa 6e 73 9d a2 34 63 fd ae c3 b7 01 37 d8 28 e3 6a ce 22 1b d0>
> hmac.digest()
<Buffer >

This is a footgun with potential security risks, and should be first runtime-deprecated, then removed if no breakage is detected.

Contributor guide

Tech stack
javascriptnodejs
Domain
backendsecurity
Issue type
security
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Node.js core development basicscrypto module internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Examine the HMAC class implementation in Node.js crypto module (likely in lib/internal/crypto/hmac.js). Understand the current behavior where calling digest() multiple times returns an empty buffer. Then implement a runtime deprecation warning using process.emitWarning when digest() is called more than once, following Node.js deprecation guidelines (see doc/api/deprecations.md). Also add tests for the deprecation warning.