Node.js Web Cryptography HMAC timing oracle in signature verification
CVE-2026-21713 is a medium-severity timing side-channel vulnerability in Node.js HMAC verification affecting the 20.x, 22.x, 24.x, and 25.x release lines. The flaw is caused by use of a non-constant-time comparison, described in the supporting content as memcmp(), when validating user-provided HMAC signatures in Node.js Web Cryptography/HMAC verification code, including crypto_hmac.cc. Because the comparison exits based on the number of matching bytes, response timing can vary proportionally to the length of the correct prefix. Under threat models where an attacker can obtain sufficiently precise timing measurements, this creates a timing oracle that can be used to infer the correct HMAC value byte-by-byte. The issue is characterized in the content as an implementation oversight, since Node.js already includes timing-safe comparison primitives used elsewhere in the codebase.
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Impact, mitigation & remediation
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Impact
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Mitigation
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Remediation
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Exploits
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Affected products & vendors
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Recent activity
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A Node.js timing side-channel vulnerability in HMAC verification caused by use of memcmp() rather than a constant-time comparison, potentially leaking timing data.
A medium-severity timing side-channel in Node.js Web Cryptography HMAC verification that can act as a timing oracle and potentially enable HMAC inference or MAC forgery.
The version that knows your environment.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.