OpenSSL up to 3.6.2/4.0.0 TLS OCSP Stapling double free

Summaryinfo

A vulnerability classified as problematic has been found in OpenSSL up to 3.6.2/4.0.0. This issue affects some unknown processing of the component TLS OCSP Stapling. This manipulation causes double free. This vulnerability is handled as CVE-2026-35188. The attack can be initiated remotely. There is not any exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability has been found in OpenSSL up to 3.6.2/4.0.0 and classified as problematic. Affected by this vulnerability is some unknown processing of the component TLS OCSP Stapling. The manipulation with an unknown input leads to a double free vulnerability. The CWE definition for the vulnerability is CWE-415. The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

Issue summary: A malicious server can exploit TLS OCSP stapling by delivering a crafted response through the status_request extension, triggering a double-free in the client's certificate verification path. Impact summary: Successful exploitation allows an attacker to corrupt heap memory via a double-free, potentially leading to a Denial of Service or possibly an attacker controlled code execution or other undefined behavior. If OCSP stapling is enabled and the TLS client connects to a malicious server, a crafted OCSP stapled response can trigger a double free in the TLS client when the stapled response is checked. The OCSP stapling is not enabled by default. Reliable code execution through a double-free is technically complex and highly environment-dependent but the Denial of Service impact is straightforward to achieve, warranting Moderate severity. No FIPS modules are affected by this issue as the affected code is outside the OpenSSL FIPS module boundary.

It is possible to read the advisory at openssl-library.org. This vulnerability is known as CVE-2026-35188 since 04/01/2026. The exploitation appears to be difficult. The attack can be launched remotely. The exploitation doesn't need any form of authentication. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $5k-$25k at the moment (estimation calculated on 06/30/2026).

The vulnerability scanner Nessus provides a plugin with the ID 320349 (Linux Distros Unpatched Vulnerability : CVE-2026-35188), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.6.3 or 4.0.1 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (320349) and CERT Bund (WID-SEC-2026-1852). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Affected

• Debian Linux
• Amazon Linux 2
• FreeBSD Project FreeBSD OS
• Red Hat Enterprise Linux
• Fedora Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• NetApp FAS
• NetApp AFF
• SUSE openSUSE
• RESF Rocky Linux
• Open Source OpenSSL

Productinfo

Type

• Network Encryption Software

Name

• OpenSSL

Version

• 3.6.0
• 3.6.1
• 3.6.2
• 4.0

License

• open-source

Website

• Product: https://www.openssl.org/

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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