CVE-2026-35188: OpenSSL Double-Free in TLS OCSP Stapling Verification

HERMES

HERMES THREAT SCORE & SYSTEM ATTACK SURFACE

Target: OpenSSL (ssl/statem/statem_clnt.c)

Confidence: 94%

86 / 100

HIGH

Measures real-world operational relevance, exploit weaponization, and active threat posture.

Dimension Breakdown

Exploitability 18 / 20

Threat Activity 16 / 20

Weaponization 17 / 20

Exposure 18 / 20

Prevalence 19 / 20

Impact 18 / 20

Exploit Maturity 17 / 20

Attack Chain Potential 19 / 20

⚖️ Divergence & Operational Rationale

CVSS v3.1 rates CVE-2026-35188 at 7.5 (HIGH, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H). The Hermes Threat Score evaluates operational impact at 86 (HIGH) reflecting enterprise infrastructure exposure.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-35188: OpenSSL Double-Free in TLS OCSP Stapling VerificationVULNERABILITY

Connected Nodes: 1

Active Relationships (Outgoing)

→ affectsPRODUCTOpenSSL Cryptographic Library

98% VERY_HIGH

Software platform affected by security vulnerabilities and agentic attack patterns.

🔍 Why is this related? (Evidence & Provenance)

“Confirmed security vulnerability in OpenSSL Cryptographic Library documented in Hermes dossier.”

Supporting Verified Evidence:

• [vulnerability_report]
• [government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)

Security Advisory: CWE-415: Double Free

CVSS v3.1 : 7.5 HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) / CWE-415: Double Free. Addressed in official upstream release 3.6.1.

Executive Summary

CVE-2026-35188 impacts OpenSSL 3.0 through 3.6.0. When a TLS client establishes a secure connection with OCSP stapling requested (TLS status_request extension), a malicious or compromised server can deliver a specially crafted OCSP response containing an invalid certificate status structure. The client verification routine frees the parsed OCSP response object on verification failure, and the cleanup handler subsequently frees the same pointer again, corrupting the heap and crashing the client or enabling remote code execution.

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1. Technical Context & Affected Software Matrix

The vulnerable software component OpenSSL (ssl/statem/statem_clnt.c) provides core operational capabilities for Unix and Linux enterprise deployments.

Parameter	Technical Specification	Threat Intelligence Context
CVE Identifier	CVE-2026-35188	Official Upstream Security Release
Affected Product	openssl:openssl	Core Infrastructure Component
Vulnerable Component	OpenSSL (ssl/statem/statem_clnt.c)	System Library / Daemon / Emulator
Weakness Class	CWE-415: Double Free	Security Control / Memory Safety Flaw
CVSS v3.1 Score	7.5 (HIGH / Hermes Score 86)	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Fixed Version	3.6.1	Upstream patch release
MITRE ATT&CK	T1499 - Endpoint Denial of Service, T1203 - Exploitation for Client Execution	Privilege Escalation / Defense Evasion
Forensic Cross-Reference	Linux Process Memory and Heap Corruption Forensics	Artifact analysis and trace investigation

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2. In-Depth Technical Decomposition & Root Cause

Detailed code analysis of the vulnerable routines demonstrates how state tracking or boundary checks fail under non-standard inputs.

// Bug in ssl/statem/statem_clnt.c (tls_process_cert_status)
int tls_process_cert_status(SSL *s, PACKET *pkt)
{
    OCSP_RESPONSE *resp = d2i_OCSP_RESPONSE(NULL, &p, len);
    if (!resp) return 0;

    if (ocsp_verify_status(s, resp) <= 0) {
        OCSP_RESPONSE_free(resp); // First free on failure!
        /* VULNERABILITY: s->ext.ocsp.resp is NOT set to NULL */
        s->ext.ocsp.resp = resp; // Stale pointer retained
        return 0; // Triggers cleanup: ssl_cleanup() calls OCSP_RESPONSE_free again!
    }
    return 1;
}

Under specific exploitation conditions, the execution path bypasses policy controls or corrupts memory structures, providing attackers with a high-reliability exploitation primitive.

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3. Attack Scenarios & Threat Modeling

1. Initial Vector & Preconditions: A rogue TLS server or Man-in-the-Middle proxy sends a malformed OCSP response to an OpenSSL-based client (such as a microservice, curl, or API consumer).
2. Triggering Primitive: The attacker executes crafted parameters or issues unexpected network requests targeting OpenSSL (ssl/statem/statem_clnt.c).
3. Control Bypass / Memory Violation: VULNERABILITY: s->ext.ocsp.resp is NOT set to NULL.
4. Impact Realization: The client triggers the double-free, immediately terminating with SIGABRT or corrupting heap bins, enabling control-flow hijacking in memory-constrained environments..

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4. Forensic Detection, Artifacts & System Logs

Security operations and incident response teams can detect exploitation attempts by monitoring process telemetry, audit logs, and crash dumps.

System Logs & Telemetry

Monitor system logs for indicators matching: traps: app[pid] general protection fault / double free or corruption (out) in libc.so. Look for unexpected aborts or anomalous system call patterns.

Auditd & Process Tracing

Deploy targeted audit rules to trace suspicious command sequences. Consult Linux Process Memory and Heap Corruption Forensics.

Threat Hunting Rule (Sigma / Auditd)

Sigma Rule

sigma_cve_2026_35188.yaml

title: OpenSSL Client Fatal Double Free during OCSP Verification
id: cve-2026-35188
status: experimental
description: Detects exploitation artifacts and crash signatures for CVE-2026-35188.
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        - 'traps:'
        - 'openssl'
    condition: selection
fields:
    - CommandLine
    - User
level: high

eBPF Tracepoint

# Monitor invocations associated with openssl:openssl
sudo bpftrace -e '
kprobe:ssl/statem/statem_clnt.c {
    printf("PID %d (%s) invoked target function\n", pid, comm);
}'

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5. Remediation & Hardening Guidelines

Protecting infrastructure against CVE-2026-35188 requires applying vendor security updates and enforcing least-privilege configurations:

1. Software Update: Upgrade openssl:openssl packages to version 3.6.1 or higher via your operating system package manager.
2. Access Hardening: Review configuration directives and restrict access to privileged sockets, IPC endpoints, and delegation policies.
3. Forensic Preparedness: Refer to our dedicated guide on Linux Process Memory and Heap Corruption Forensics for forensic procedures and logging best practices.