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Public proof of conceptFix available

CVE-2021-23841

A public proof of concept exists; no exploitation attested.

  • Published2021-02-16
  • WeaknessCWE-476

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

The OpenSSL public API function X509_issuer_and_serial_hash() attempts to create a unique hash value based on the issuer and serial number data contained within an X509 certificate. However it fails to correctly handle any errors that may occur while parsing the issuer field (which might occur if the issuer field is maliciously constructed). This may subsequently result in a NULL pointer deref and a crash leading to a potential denial of service attack. The function X509_issuer_and_serial_hash() is never directly called by OpenSSL itself so applications are only vulnerable if they use this function directly and they use it on certificates that may have been obtained from untrusted sources. OpenSSL versions 1.1.1i and below are affected by this issue. Users of these versions should upgrade to OpenSSL 1.1.1j. OpenSSL versions 1.0.2x and below are affected by this issue. However OpenSSL 1.0.2 is out of support and no longer receiving public updates. Premium support customers of OpenSSL 1.0.2 should upgrade to 1.0.2y. Other users should upgrade to 1.1.1j. Fixed in OpenSSL 1.1.1j (Affected 1.1.1-1.1.1i). Fixed in OpenSSL 1.0.2y (Affected 1.0.2-1.0.2x).

Timeline

dates in UTC
  1. 2021-02-1617:15CVE publishedNVD
  2. 2026-10-2319:23First public proof of conceptPoC-in-GitHub

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-23841?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-23841?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Tenable nessus network monitor
BranchAffectedFixed in
5.11= 5.11.0no fix published
5.11= 5.11.1no fix published
5.12= 5.12.0no fix published
5.12= 5.12.1no fix published
5.13= 5.13.0no fix published
Oracle business intelligence
BranchAffectedFixed in
5.5= 5.5.0.0.0no fix published
5.9= 5.9.0.0.0no fix published
12.2= 12.2.1.3.0no fix published
12.2= 12.2.1.4.0no fix published
Oracle graalvm
BranchAffectedFixed in
19.3= 19.3.5no fix published
20.3= 20.3.1.2no fix published
21.0= 21.0.0.2no fix published

21 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • edk2-ovmfAlmaLinux:8all versions · fixed in 20210527gite1999b264f1f-3.el8

Full record on NVD

Fix

Advisories and patches

Detection

read 2026-10-23 19:23 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
2 repositories, unverifiedTrinadh465/external_boringssl_openssl_1.1.0g_CVE-2021-23841Satheesh575555/Openssl_1_1_0_CVE-2021-23841
PoC-in-GitHub

Sources

  • CISA-ADPSSVC decisionnone2024-06-26
  • OpenCVESSVC decisionnone—
  • NVDCVSS 3.15.9read 2026-10-23
  • NVDCVSS 2.04.3read 2026-10-23
  • NVDweakness (CWE)CWE-476read 2026-10-23
  • FIRST EPSSEPSS score7.4 %2026-10-23
  • PoC-in-GitHubproof of concept2read 2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2021-02-16
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-23841?

The OpenSSL public API function X509_issuer_and_serial_hash() attempts to create a unique hash value based on the issuer and serial number data contained within an X509 certificate. However it fails to correctly handle any errors that may occur while parsing the issuer field (which might occur if the issuer field is maliciously constructed). This may subsequently result in a NULL pointer deref and a crash leading to a potential denial of service attack. The function X509_issuer_and_serial_hash() is never directly called by OpenSSL itself so applications are only vulnerable if they use this function directly and they use it on certificates that may have been obtained from untrusted sources. OpenSSL versions 1.1.1i and below are affected by this issue. Users of these versions should upgrade to OpenSSL 1.1.1j. OpenSSL versions 1.0.2x and below are affected by this issue. However OpenSSL 1.0.2 is out of support and no longer receiving public updates. Premium support customers of OpenSSL 1.0.2 should upgrade to 1.0.2y. Other users should upgrade to 1.1.1j. Fixed in OpenSSL 1.1.1j (Affected 1.1.1-1.1.1i). Fixed in OpenSSL 1.0.2y (Affected 1.0.2-1.0.2x).

Is CVE-2021-23841 actively exploited?

Active exploitation of CVE-2021-23841 has not been confirmed. Its EPSS score was 7.4% on 2026-10-23, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-23841?

CVE-2021-23841 has a CVSS v3 base score of 5.9 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.

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