Use After Free in F5 Dos
CVE-2026-40701
NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_ssl_module module when the ssl_verify_client directive is set to "on" or "optional," and the ssl_ocsp directive is set to "on" or the leaf parameters are configured with a resolver. With this configuration, an unauthenticated attacker can send requests along with conditions beyond its control that may cause a heap-use-after-free error in the NGINX worker process. This vulnerability may result in limited modification of data or the NGINX worker process restarting. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.
Vulnerability class: Use-After-Free
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L.
EPSS exploit prediction
EPSS: 0.007 (51.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 52nd percentile — 51.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000
Affected products
- F5 Dos — versions 4.8.0
- F5 nginx Gateway Fabric
- F5 nginx Ingress Controller
- F5 nginx Instance Manager
- F5 nginx Open Source — versions 1.31.0, 1.19.0
- F5 nginx Open Source
- F5 nginx Plus — versions R37, R36, R32
- F5 nginx Plus
- F5 Waf
Weakness classification (CWE)
Public proof-of-concept exploits
References
- my.f5.com/manage/s/article/K000161021 (vendor-advisory, Mitigation, patch, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-40701?
- CVE-2026-40701 is a medium-severity vulnerability in F5 Dos, classified under Use After Free. CVSS score: 4.8/10. Published 2026-05-13.
- How severe is CVE-2026-40701?
- Medium severity. CVSS v3 base score is 4.8 out of 10.
- Is CVE-2026-40701 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.