CVE-2026-28388
Published: 07 April 2026
Description
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…
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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 X509_V_FLAG_USE_DELTAS flag to be enabled in the verification context, the certificate being verified to contain a freshestCRL extension or the base CRL to have the EXFLAG_FRESHEST 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.
Mitigating Controls (NIST 800-53 r5)AI
Timely remediation through application of OpenSSL patches directly eliminates the NULL pointer dereference in delta CRL processing.
Secure error handling ensures that NULL pointer conditions during malformed delta CRL verification do not result in application crashes or denial of service.
Input validation of CRL structures verifies the presence of required extensions like CRL Number before processing delta CRLs with Delta CRL Indicator.
Security SummaryAI
CVE-2026-28388 is a NULL pointer dereference vulnerability in the OpenSSL cryptographic library during X.509 certificate verification. It occurs when processing a delta Certificate Revocation List (CRL) that includes a Delta CRL Indicator extension but lacks the required CRL Number extension. The issue arises specifically when CRL processing and delta CRL processing are enabled via the X509_V_FLAG_USE_DELTAS flag in the verification context, combined with a freshestCRL extension in the certificate or the EXFLAG_FRESHEST flag set on the base CRL.
An attacker can exploit this vulnerability by supplying a malformed delta CRL to a vulnerable OpenSSL-based application that performs certificate verification under the specified conditions. Exploitation is network-based with low complexity, requiring no privileges or user interaction (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H). Successful exploitation triggers a crash, resulting in a denial-of-service condition for the application, but cannot lead to code execution or memory disclosure.
Mitigation involves applying patches from OpenSSL, available in the following GitHub commits: 59c3b3158553ab53275bbbccca5cb305d591cf2e, 5a0b4930779cd2408880979db765db919da55139, 602542f2c0c2d5edb47128f93eac10b62aeeefb3, a9d187dd1000130100fa7ab915f8513532cb3bb8, and d3a901e8d9f021f3e67d6cfbc12e768129862726. The FIPS modules in OpenSSL versions 3.6, 3.5, 3.4, 3.3, and 3.0 are unaffected, as the vulnerable code lies outside the FIPS module boundary. The issue is rated low severity per OpenSSL's Security Policy despite the CVSS score of 7.5 (CWE-476).
Details
- CWE(s)
Affected Products
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
NULL dereference in OpenSSL X.509/CRL processing enables remote unauthenticated attackers to crash applications (DoS) by supplying a malformed delta CRL during certificate verification; directly maps to exploitation of public-facing apps and application exploitation for endpoint DoS.