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CVE Record

CVE-2020-21686: A stack-use-after-scope issue discovered in expand_mmac_params function in preproc.c in nasm before 2.15.04...

A stack-use-after-scope issue discovered in expand_mmac_params function in preproc.c in nasm before 2.15.04 allows remote attackers to cause a denial of service via crafted asm file.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2020-21686 is a NASM assembler flaw that can crash processing when NASM handles a specially crafted assembly file. Business impact is most relevant where build systems, CI jobs, or developer workflows process assembly from outside trusted sources.

Executive priority

Handle through normal vulnerability management unless NASM processes untrusted assembly in production or shared CI. Prioritize build-system inventory and version confirmation before emergency response.

Technical view

The issue is described as a stack-use-after-scope flaw in expand_mmac_params in preproc.c in NASM before 2.15.04. The stated impact is denial of service through a crafted asm file. The provided record has no CVSS score and sparse affected-product metadata.

Likely exposure

Exposure is likely limited to environments using NASM before 2.15.04, especially automated build pipelines or services that process untrusted or third-party assembly files. The bundled affected-product fields are incomplete.

Exploitation context

The source bundle does not show CISA KEV listing or cited active exploitation. Exploitation requires getting NASM to process a crafted asm file, so practical risk depends on source-trust boundaries in build workflows.

Researcher notes

Evidence supports a memory lifetime bug leading to denial of service in NASM preprocessing. The public bundle does not provide CVSS, exploit evidence, detailed affected CPEs, or explicit remediation text beyond the version boundary.

Mitigation direction

  • Inventory NASM versions across developer machines, CI images, and build containers.
  • Update NASM to 2.15.04 or later where vendor guidance confirms applicability.
  • Avoid processing untrusted assembly files in privileged or shared build environments.
  • Run assembly processing inside isolated, disposable build workers.
  • Monitor NASM and distribution advisories for package-specific guidance.

Validation and detection

  • Confirm whether NASM is present in build tooling, containers, and developer base images.
  • Check whether any workflow processes externally supplied assembly files.
  • Verify NASM versions are not earlier than 2.15.04 where NASM is required.
  • Review build logs for unexplained NASM crashes on assembly preprocessing.
  • Confirm vendor or distribution advisories for the deployed NASM package.
Prepared
Confidence
medium
Sources
3

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2020-21686 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
2Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

CWE-562

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.