High-Severity Heap Use-After-Free in OpenSSL PKCS#7 Signature Verification (CVE-2026-45447)
June 15th, 2026
High
_edited.jpg)
Our Cyber Threat Intelligence Unit is monitoring a high-severity vulnerability in OpenSSL, one of the most widely used cryptographic libraries on the internet, tracked as CVE-2026-45447. Disclosed on June 9, 2026, and reported by Thai Duong of Calif.io in collaboration with Claude and Anthropic Research, the vulnerability allows an attacker to send a specially crafted PKCS#7 or S/MIME signed message that causes OpenSSL to mishandle memory, potentially crashing the application or allowing the attacker to execute arbitrary code on the target system. The issue affects any software that uses OpenSSL to verify PKCS#7 or S/MIME digital signatures, a common operation in email clients, mail servers, and document processing tools. OpenSSL released patches across all affected versions as part of a broader batch of 18 security fixes. Organizations using OpenSSL to process externally supplied signed messages or documents should apply the available patches immediately.
Technical Details
CVE ID: CVE-2026-45447
Vulnerability Type: CWE-416: Heap Use-After-Free (Memory Corruption)
Severity: High (per OpenSSL; NVD CVSS score pending enrichment)
Affected Component: PKCS#7 and S/MIME Signature Verification (PKCS7_verify())
Affected Products and Versions:
OpenSSL 4.0 — versions prior to 4.0.1; patch to 4.0.1
OpenSSL 3.6 — versions prior to 3.6.3; patch to 3.6.3
OpenSSL 3.5 — versions prior to 3.5.7; patch to 3.5.7
OpenSSL 3.4 — versions prior to 3.4.6; patch to 3.4.6
OpenSSL 3.0 — versions prior to 3.0.21; patch to 3.0.21
OpenSSL 1.1.1 — versions prior to 1.1.1zh; patch to 1.1.1zh (premium support required)
OpenSSL 1.0.2 — versions prior to 1.0.2zq; patch to 1.0.2zq (premium support required)
Attack Chain Overview:
Initial Access: An attacker delivers a specially crafted PKCS#7 or S/MIME signed message containing an empty SignedData.digestAlgorithms ASN.1 SET to a target application. Primary vectors include S/MIME email clients, mail transfer agents, and any application accepting externally supplied signed documents.
Exploitation: OpenSSL's PKCS7_verify() incorrectly frees a caller-owned BIO object while processing the malformed message, leaving the calling application holding a dangling reference to freed memory.
Code Execution: When the application subsequently uses or frees the same BIO (e.g., via BIO_free()), a use-after-free condition is triggered. Depending on heap allocator behavior and application-specific BIO usage, this may result in heap corruption, a process crash, or remote code execution.
Not Affected: Applications using the CMS APIs for PKCS#7/S/MIME verification are not affected. OpenSSL FIPS modules in 4.0, 3.6, 3.5, 3.4, and 3.0 are not affected, as the vulnerable code falls outside the FIPS boundary.
Required Configuration for Exposure: Affects any application that calls PKCS7_verify() on externally supplied or attacker-controlled PKCS#7 or S/MIME input.
Exploit Status: No known active exploitation at time of writing; no public proof-of-concept exploit confirmed.
Impact
Remote Code Execution: Depending on heap allocator behavior and application-specific BIO usage, successful exploitation may result in process crashes or heap corruption. In some application contexts, this may potentially be leveraged for remote code execution.
Process Crashes and Heap Corruption: Even where full code execution is not achievable, the use-after-free condition can result in heap corruption or application crashes, allowing denial-of-service against dependent services.
S/MIME Email Infrastructure: Email clients and mail transfer agents processing S/MIME signed messages represent a primary exposure category, as any inbound signed message from an attacker can serve as the attack payload.
Broad Application Attack Surface: Document management platforms, code-signing verification pipelines, certificate processing tools, and any application calling PKCS7_verify() on externally sourced input fall within scope.
Detection Method
Security teams should monitor application and network telemetry for indicators consistent with exploitation or probing of CVE-2026-45447:
Monitor mail server and email client logs for inbound S/MIME signed messages containing malformed or anomalous PKCS#7 structures, particularly from unknown or external senders.
Review application logs and crash reports for unexpected process crashes, segmentation faults, or heap corruption errors in services that call PKCS7_verify() or perform PKCS#7/S/MIME signature verification.
Audit running processes and container filesystems to identify applications using statically linked OpenSSL versions predating the patched releases, as system package updates alone will not remediate these instances.
Correlate memory error logs, core dump generation, or service restarts in applications that process externally supplied signed documents or messages, which may indicate failed or partially successful exploitation.
Deploy available vendor or SIEM signatures for CVE-2026-45447 where available, and monitor security advisories from the OpenSSL Project for updated detection guidance.
Indicators of Compromise
There are no Indicators of Compromise (IOCs) for this Advisory.
Recommendations
Apply patches immediately:
Upgrade to the applicable fixed release: OpenSSL 4.0.1, 3.6.3, 3.5.7, 3.4.6, or 3.0.21.
Organizations on end-of-life branches 1.1.1 and 1.0.2 must hold premium support contracts to receive 1.1.1zh and 1.0.2zq, respectively; where that is not available, migration to a supported branch is strongly recommended.
Audit statically linked OpenSSL deployments:
System package updates will not remediate applications that bundle their own OpenSSL copy.
Conduct a comprehensive inventory of all binaries and containers to identify statically linked instances requiring separate remediation.
Restrict processing of externally supplied signed messages: Where operationally feasible, restrict or disable S/MIME processing on edge mail servers and email clients until patching is confirmed.
Monitor for exploitation activity: Implement the detection guidance in Section 4.0 and review mail server and application logs for anomalous PKCS#7 input patterns.
Track NVD enrichment: A NVD CVSS score for CVE-2026-45447 is pending enrichment at the time of publication. Organizations should revisit the NVD entry to incorporate the published score into risk prioritization once available.
Conclusion
The recently disclosed CVE-2026-45447 exposes significant risks in processing untrusted PKCS#7 and S/MIME content within widely used cryptographic infrastructure. Since a single unauthenticated, malformed message can exploit this vulnerability, S/MIME-capable mail infrastructure and document-processing applications should be prioritized for remediation. Organizations should coordinate across security, infrastructure, application, and operations teams to identify affected systems, deploy updates, verify remediation for statically linked deployments, and implement monitoring to detect exploitation attempts during patching.