CVE-2020-21686

Process crash via macro-parameter stack lifetime error

Published Aug 22, 2023 · Updated Oct 7, 2024

Stack-use-after-scope in NASM before 2.15.04 allows context-dependent attackers to crash the assembler via a crafted asm file. The expand_mmac_params function in asm/preproc.c uses a stack-backed object after its scope ends during macro-parameter expansion. Processing an attacker-supplied assembly file requires a user or build process to invoke NASM; successful triggering terminates that process.

CVSS severity5.5
Medium
EPSS probability0.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stack-use-after-scope in NASM before 2.15.04 allows context-dependent attackers to crash the assembler via a crafted asm file. The expand_mmac_params function in asm/preproc.c uses a stack-backed object after its scope ends during macro-parameter expansion. Processing an attacker-supplied assembly file requires a user or build process to invoke NASM; successful triggering terminates that process.

The record

CVE
CVE-2020-21686
Published
Aug 22, 2023
Updated
Oct 7, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-562, T1204.002
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 22, 2023CVE publishedPublication date reported by the CVE source.
  2. Oct 7, 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

5.5NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/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.

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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 exploitUnconfirmed

Weakness, pattern, technique

CWE-562Return of Stack Variable Address
T1204.002Malicious File

Public exploit references

No public exploit references are available in this record.

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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