CVE-2026-28388: 
OpenSSL vulnerability analysis and mitigation

CVE-2026-28388 is a NULL pointer dereference vulnerability in OpenSSL's delta Certificate Revocation List (CRL) processing that can cause a Denial of Service (DoS). When a delta CRL containing a Delta CRL Indicator extension is processed without the required CRL Number extension, the check_delta_base() function in crypto/x509/x509_vfy.c dereferences a NULL pointer, causing an application crash. Affected versions include OpenSSL 1.0.2 before 1.0.2zp, 1.1.1 before 1.1.1zg, 3.0.x before 3.0.20, 3.3.x before 3.3.7, 3.4.x before 3.4.5, 3.5.x before 3.5.6, and 3.6.x before 3.6.2. The vulnerability was disclosed on April 7, 2026, and carries a CVSS v3.1 base score of 7.5 (High) (OpenSSL Advisory, GitHub Advisory).

The root cause is a missing NULL check (CWE-476: NULL Pointer Dereference) in the check_delta_base() function within crypto/x509/x509_vfy.c. When delta CRL processing is invoked, the code calls ASN1_INTEGER_cmp(delta->crl_number, base->crl_number) without first verifying that delta->crl_number is non-NULL — a condition that arises when a malformed delta CRL omits the mandatory CRL Number extension. The fix, applied across all affected branches, adds a two-line guard: if (delta->crl_number == NULL) return 0; before the comparison (OpenSSL Commit 602542f, OpenSSL Commit d3a901e). Exploitation requires three preconditions: the X509_V_FLAG_USE_DELTAS flag must be enabled in the verification context, the certificate being verified must contain a freshestCRL extension or the base CRL must have the EXFLAG_FRESHEST flag set, and an attacker must be able to supply a malformed delta CRL to the application. The FIPS modules in OpenSSL 3.0, 3.3, 3.4, 3.5, and 3.6 are not affected, as the vulnerable code lies outside the FIPS module boundary (OpenSSL Advisory).

Successful exploitation results exclusively in a Denial of Service — the affected application crashes due to the NULL pointer dereference. OpenSSL has confirmed the vulnerability cannot be escalated to achieve remote code execution or memory disclosure, limiting the impact to availability (OpenSSL Advisory). Any application that performs X.509 certificate verification with delta CRL processing enabled is at risk, which can include TLS servers and clients, PKI validation services, and certificate management tools. The network-accessible attack vector (no authentication or user interaction required) means a remote attacker can trigger the crash by presenting a malformed delta CRL during a certificate verification workflow (GitHub Advisory).

1. Identify a target application: Find an application that uses a vulnerable OpenSSL version (1.0.2 < 1.0.2zp, 1.1.1 < 1.1.1zg, 3.0.x < 3.0.20, 3.3.x < 3.3.7, 3.4.x < 3.4.5, 3.5.x < 3.5.6, or 3.6.x < 3.6.2) and has delta CRL processing enabled via the X509_V_FLAG_USE_DELTAS flag.
2. Confirm delta CRL processing is active: Verify that the target application's certificate verification context uses delta CRLs — either the certificate chain includes a freshestCRL extension or the base CRL has the EXFLAG_FRESHEST flag set.
3. Craft a malformed delta CRL: Create a delta CRL file that contains the Delta CRL Indicator extension but deliberately omits the required CRL Number extension, leaving delta->crl_number as NULL.
4. Deliver the malformed CRL: Supply the crafted delta CRL to the target application through any mechanism it uses to fetch CRLs (e.g., via a controlled LDAP/HTTP CRL distribution point, a man-in-the-middle position, or direct file injection if the application reads CRLs from disk).
5. Trigger the crash: When the application processes the malformed delta CRL during X.509 certificate verification, the NULL pointer dereference in check_delta_base() causes the process to crash, resulting in a DoS (OpenSSL Advisory, OpenSSL Commit 602542f).

• Logs: Application crash logs or core dumps originating from the OpenSSL check_delta_base() function in crypto/x509/x509_vfy.c; segmentation fault signals (SIGSEGV) in process logs associated with certificate verification operations.
• Network: Unusual or unexpected CRL fetch requests (HTTP/LDAP) to unfamiliar CRL distribution point URLs, particularly for delta CRLs; repeated connection attempts to CRL distribution points followed by application crashes.
• File System: Presence of unexpected or externally sourced .crl files in directories used by the application for CRL caching or storage; core dump files generated by the application process.
• Process: Unexpected termination of services that perform TLS/certificate verification (e.g., web servers, VPN daemons, PKI tools) with exit codes indicating a crash rather than a clean shutdown.

Upgrade to a patched OpenSSL version immediately: 1.0.2zp, 1.1.1zg, 3.0.20, 3.3.7, 3.4.5, 3.5.6, or 3.6.2, released on April 7–8, 2026 (OpenSSL Advisory). As a workaround for systems that cannot be patched immediately, disable delta CRL processing by ensuring the X509_V_FLAG_USE_DELTAS flag is not set in any certificate verification context. Downstream distributions including Ubuntu (USN-8155-1/2), Debian, SUSE, openSUSE, Slackware, Amazon Linux 2/2023, and Mageia have also released updated packages (Ubuntu Advisory). Microsoft has released updates for affected products including azl3_openssl_3.3.5-4 (Microsoft MSRC).

The OpenSSL project assessed the vulnerability as Low severity under its own Security Policy despite the network-accessible attack vector, citing the specific preconditions and DoS-only impact (OpenSSL Advisory). Coverage appeared across Linux security news outlets including 9to5Linux, Linuxiac, HelpNet Security, and CyberSecurityNews shortly after disclosure. A Reddit thread in r/Intune noted the CVE appearing in Microsoft vulnerability scans for Windows systems, reflecting broad ecosystem reach (Reddit). Downstream vendors including Splunk, Puppet, Portainer, FreeBSD, and Huawei subsequently issued their own advisories or release notes addressing the vulnerability.