CVE-2021-46521Public exploit

Process crash via formatter length underflow

Published Jan 27, 2022 · Updated Aug 4, 2024

Global buffer overflow in Cesanta mJS 2.20.0 allows context-dependent attackers to crash the process via crafted JavaScript. In c_vsnprintf, the output count can exceed the destination size before a numeric conversion passes buf+i and buf_size-i to c_itoa, turning the remaining length into a large unsigned value and writing beyond the global buffer. Exploitation requires a user or consuming application to execute attacker-supplied JavaScript that reaches the affected error-formatting path; the public proof of concept demonstrates an AddressSanitizer abort, while stronger consequences are not established.

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

Global buffer overflow in Cesanta mJS 2.20.0 allows context-dependent attackers to crash the process via crafted JavaScript. In c_vsnprintf, the output count can exceed the destination size before a numeric conversion passes buf+i and buf_size-i to c_itoa, turning the remaining length into a large unsigned value and writing beyond the global buffer. Exploitation requires a user or consuming application to execute attacker-supplied JavaScript that reaches the affected error-formatting path; the public proof of concept demonstrates an AddressSanitizer abort, while stronger consequences are not established.

The record

CVE
CVE-2021-46521
Published
Jan 27, 2022
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-120, T1204.002
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

7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P

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.

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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-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
T1204.002User Execution: Malicious File

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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