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Fencer Vulns
medium 5.5

CVE-2020-21686 json

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 caus…

published 2023-08-22 3 y ago modified 2026-07-25 3 mo ago also EUVD-2020-14454 , JLSEC-2026-201

cvss
5.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 38%
public exploits
0
none known

Summary

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.

Patch / fix

  • julia NASM_jll 2.16.1+0
  • nasm netwide_assembler 2.15.04
  • nasm Debian 14, Debian 13, Debian 12 2.15.04-1

debian advisories DEBIAN-CVE-2020-21686

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2020-21686 updated cvss 5.5

nasm: stack-use-after-scope in expand_mmac_params() in preproc.c

  • Red Hat Enterprise Linux 6 nasm, nasm-rdoff Red Hat Enterprise Linux 7 nasm, nasm-rdoff
    affected: no fix planned “Out of support scope”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
2 y ago

Exploit prediction

0.44% chance of exploitation in the next 30 days, higher than 38% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • bugzilla.nasm.us https://bugzilla.nasm.us/show_bug.cgi?id=3392643 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
OSV 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
julia NASM_jll
<2.16.1+0
2.16.1+0

Classification

Weakness
  • CWE-562 Return of Stack Variable Address

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.