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CVE Record

CVE-2021-23841: Null pointer deref in X509_issuer_and_serial_hash()

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).

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This OpenSSL issue can let a maliciously crafted certificate crash an application, causing denial of service. Exposure is narrower than typical OpenSSL flaws because OpenSSL does not call the vulnerable function internally; applications are affected only if they directly use that API on untrusted certificates.

Executive priority

Address through normal vulnerability remediation, with faster handling for exposed certificate-processing services. Business urgency is lower than broad remote-code-execution OpenSSL flaws, but unsupported OpenSSL 1.0.2 systems should be escalated as technical debt.

Technical view

X509_issuer_and_serial_hash() mishandles issuer parsing errors and can dereference a NULL pointer. Affected releases include OpenSSL 1.1.1 through 1.1.1i and 1.0.2 through 1.0.2x. Fixed versions are 1.1.1j and 1.0.2y for premium-support 1.0.2 customers.

Likely exposure

Most organizations should treat exposure as targeted and application-specific. Risk exists where software directly calls X509_issuer_and_serial_hash() and processes certificates from untrusted sources. Products bundling affected OpenSSL may also need vendor-specific updates.

Exploitation context

The source bundle does not show CISA KEV listing or cited active exploitation. The described impact is application crash and potential denial of service, not code execution. Practical exploitability depends on reachable certificate-processing paths using this specific API.

Researcher notes

The key scoping question is direct API use. OpenSSL’s advisory says the function is not directly called by OpenSSL itself. Validate reachability before assuming exposure, and avoid broad claims about all OpenSSL-linked applications.

Mitigation direction

  • Upgrade OpenSSL 1.1.1 deployments to 1.1.1j or later.
  • For OpenSSL 1.0.2, use 1.0.2y only if eligible for premium support.
  • Otherwise migrate unsupported OpenSSL 1.0.2 systems to a supported branch.
  • Apply operating system or product vendor updates where OpenSSL is bundled.
  • Prioritize internet-facing services that parse untrusted certificates.

Validation and detection

  • Inventory OpenSSL versions in hosts, containers, appliances, and embedded products.
  • Check application code and dependencies for X509_issuer_and_serial_hash() usage.
  • Confirm whether affected paths process certificates from untrusted sources.
  • Verify distro or vendor package changelogs include the CVE fix.
  • Review crash telemetry for certificate-processing denial-of-service symptoms.
Prepared
Confidence
high
Sources
8

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

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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
20Source 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
OpenSSLOpenSSLFixed 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)Listed
Weakness

CWE details

No CWE listed

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