CVE-2026-28388: NULL Pointer Dereference When Processing a Delta CRL
Issue summary: When a delta CRL that contains a Delta CRL Indicator extension is processed a NULL pointer dereference might happen if the required CRL Number extension is missing.
Impact summary: A NULL pointer dereference can trigger a crash which leads to a Denial of Service for an application.
When CRL processing and delta CRL processing is enabled during X.509 certificate verification, the delta CRL processing does not check whether the CRL Number extension is NULL before dereferencing it. When a malformed delta CRL file is being processed, this parameter can be NULL, causing a NULL pointer dereference.
Exploiting this issue requires the X509VFLAGUSEDELTAS flag to be enabled in the verification context, the certificate being verified to contain a freshestCRL extension or the base CRL to have the EXFLAGFRESHEST flag set, and an attacker to provide a malformed CRL to an application that processes it.
The vulnerability is limited to Denial of Service and cannot be escalated to achieve code execution or memory disclosure. For that reason the issue was assessed as Low severity according to our Security Policy.
The FIPS modules in 3.6, 3.5, 3.4, 3.3 and 3.0 are not affected by this issue, as the affected code is outside the OpenSSL FIPS module boundary.
Other sources
NULL Pointer Dereference When Processing a Delta CRL
— Microsoft
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Disable delta CRL processing that requires the X509_V_FLAG_USE_DELTAS flag to be enabled, to prevent NULL pointer dereference during malformed delta CRL processing (DoS).
OpenSSL X.509 delta CRL processing (CRL verification context) X509_V_FLAG_USE_DELTAS (and related delta CRL processing enabled) = disable/don’t enable
Event History
Frequently Asked Questions
What is the severity of CVE-2026-28388?
CVE-2026-28388 has a critical severity rating due to the potential for a NULL pointer dereference leading to application crashes.
How do I fix CVE-2026-28388?
To mitigate CVE-2026-28388, ensure that your software is updated to incorporate the required CRL Number extension in delta CRL processing.
Which software is affected by CVE-2026-28388?
CVE-2026-28388 affects Microsoft azl3 openssl 3.3.5-4 and potentially other software versions using similar libraries.
What happens if CVE-2026-28388 is exploited?
Exploiting CVE-2026-28388 can cause the affected application to crash, leading to denial of service.
When was CVE-2026-28388 disclosed?
CVE-2026-28388 was disclosed on April 7, 2026, as part of a security advisory.