CVE-2021-42139Public exploit

Arbitrary code execution via YAML function reconstruction

Published Oct 11, 2021 · Updated Aug 4, 2024

Code injection in Deno Standard Modules before 0.107.0 allows context-dependent attackers to execute JavaScript through crafted YAML. The encoding/yaml parser's nondefault EXTENDED_SCHEMA passes !!js/function tag content to reconstructFunction, which evaluates it with the Function constructor. Exploitation requires an application to enable EXTENDED_SCHEMA and parse attacker-controlled YAML, after which injected code runs with that Deno process's permissions.

CVSS severity9.8
Critical
EPSS probability2.07%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Code injection in Deno Standard Modules before 0.107.0 allows context-dependent attackers to execute JavaScript through crafted YAML. The encoding/yaml parser's nondefault EXTENDED_SCHEMA passes !!js/function tag content to reconstructFunction, which evaluates it with the Function constructor. Exploitation requires an application to enable EXTENDED_SCHEMA and parse attacker-controlled YAML, after which injected code runs with that Deno process's permissions.

The record

CVE
CVE-2021-42139
Published
Oct 11, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-94
Attack vector
Unavailable
Privileges
Unavailable

Timeline

How it unfolded

  1. Oct 11, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectorUnavailable in this record.
Required privilegesUnavailable in this record.

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
VectorUnavailable
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-94Improper Control of Generation of Code ('Code Injection')

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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