CVE-2021-46520Public exploit

Out-of-bounds read via crafted JSON serialization

Published Jan 27, 2022 · Updated Aug 4, 2024

Heap buffer overflow in Cesanta MJS 2.20.0 allows local users to trigger an out-of-bounds read by executing crafted JavaScript. The mjs_jprintf formatter reads one byte past its allocated heap buffer while printing JSON.stringify output containing Object.create in an array. Reaching the flaw requires execution of crafted JavaScript; the published sanitizer trace demonstrates a one-byte over-read beyond a 107-byte allocation, not code execution.

CVSS severity7.8
High
EPSS probability0.73%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap buffer overflow in Cesanta MJS 2.20.0 allows local users to trigger an out-of-bounds read by executing crafted JavaScript. The mjs_jprintf formatter reads one byte past its allocated heap buffer while printing JSON.stringify output containing Object.create in an array. Reaching the flaw requires execution of crafted JavaScript; the published sanitizer trace demonstrates a one-byte over-read beyond a 107-byte allocation, not code execution.

The record

CVE
CVE-2021-46520
Published
Jan 27, 2022
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1203
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 27, 2022CVE 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 vectorlocal
Required privilegesunauthenticated

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
7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/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)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1203Exploitation for Client Execution

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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