Junglewise Threat Intelligence

CVE-2026-28388: OpenSSL NULL pointer dereference in delta CRL processing

CVE-2026-28388 · Severity: high · CVSS 7.5 · Published 2026-04-07

Technologies: Siemens SIMATIC S7-1500 TM MFP - GNU/Linux subsystem, OpenSSL, Siemens SIMATIC CN 4100. Vendors: Siemens, OpenSSL.

Executive brief

OpenSSL is a widely used security library that handles encrypted communications and digital certificates. A flaw in how it processes specific certificate revocation lists (CRLs) could allow an attacker to crash an application by providing a specially crafted file. This results in a denial-of-service, potentially disrupting secure connections or services that rely on certificate verification.

Technical details

A NULL pointer dereference exists in OpenSSL's X.509 certificate verification logic, specifically within the check_delta_base function in crypto/x509/x509_vfy.c. When delta CRL processing is enabled, the code fails to verify if the CRL Number extension is present before dereferencing it. An attacker can trigger this by providing a malformed delta CRL that lacks the required CRL Number extension. Exploitation requires the X509_V_FLAG_USE_DELTAS flag to be enabled and the certificate or base CRL to indicate the use of delta CRLs. The vulnerability results in an application crash (Denial of Service) but does not allow for code execution or data leakage. Patches have been released for OpenSSL versions 3.0 through 3.6.

Affected products

  • OpenSSL OpenSSL 3.6.0 to 3.6.1, 3.5.x, 3.4.x, 3.3.x, 3.0.x
  • Siemens SIMATIC CN 4100 before V5.0
  • Siemens SIMATIC S7-1500 TM MFP - GNU/Linux subsystem all versions

Timeline

  • 2026-04-06: patched: Fixes committed to OpenSSL repository
  • 2026-04-07: advisory: OpenSSL Security Advisory published
  • 2026-04-07: disclosed: CVE-2026-28388 published

References

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