CVE-2026-40701
Description
A flaw was found in the ngx_http_ssl_module module of NGINX. When the ssl_verify_client directive is set to "on" or "optional" and the ssl_ocsp directive is enabled or its leaf parameters are configured with a resolver, an unauthenticated attacker can send crafted requests to cause a use-after-free issue in the worker process, resulting in a limited modification of memory data or a denial of service by forcing the process to restart.
Statement
To exploit this flaw, the ssl_verify_client directive must be set to "on" or "optional" and the ssl_ocsp directive must be enabled or its leaf parameters configured with a resolver, limiting its exposure as this is not the default configuration. This issue allows an attacker to have limited control to modify memory data from the worker process or cause a denial of service by forcing the process to restart, but it cannot cause a complete system denial of service. Due to these reasons, this flaw has been rated with a moderate severity.
Mitigation
To mitigate this issue, specifically set the OCSP responder using the ssl_ocsp_responder directive or switch from live OCSP validation to static CRL files using the ssl_crl directive. If neither configuration is possible, using a local DNS server to cache and quickly resolve OCSP responder names can reduce the probability of exploitation.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 4.8 | N/A | 4.8 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | Low |
| Integrity Impact | Low | N/A | None |
| Availability Impact | Low | N/A | Low |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L
Understanding the Weakness (CWE)
Integrity
Technical Impact: Modify Memory
The use of previously freed memory may corrupt valid data, if the memory area in question has been allocated and used properly elsewhere.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
If chunk consolidation occurs after the use of previously freed data, the process may crash when invalid data is used as chunk information.
Confidentiality
Technical Impact: Read Memory
Read operations on freed memory can sometimes leak sensitive information instead of causing a crash
Integrity,Confidentiality,Availability
Technical Impact: Execute Unauthorized Code or Commands
If malicious data is entered before chunk consolidation can take place, it may be possible to take advantage of a write-what-where primitive to execute arbitrary code. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.
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