Critical OpenSSL Flaws Allow Remote Code Execution

Key Takeaways

• A critical remote code execution (RCE) vulnerability (CVE-2026-45447) has been identified in OpenSSL.
• The flaw specifically impacts applications that process specially crafted PKCS7 or S/MIME signed messages using OpenSSL’s PKCS7 APIs.
• OpenSSL versions 4.0, 3.6, 3.5, 3.4, 3.0, 1.1.1, and 1.0.2 are all affected.
• Patched versions are available, and immediate upgrades are strongly recommended.

Critical OpenSSL Flaws Uncovered, RCE Threat Looms

A significant security flaw, disclosed on June 9, 2026, presents a critical remote code execution (RCE) risk within the widely used OpenSSL cryptographic library. This severe vulnerability can be triggered when applications handle specially crafted PKCS7 or S/MIME signed messages, potentially allowing attackers to compromise systems.

Designated as CVE-2026-45447, the issue is a heap use-after-free bug located within the PKCS7_verify function. This defect can lead to memory corruption, and in certain operational environments, it could enable malicious actors to execute arbitrary code on affected systems.

The vulnerability manifests when a signed message includes an empty SignedData.digestAlgorithms ASN.1 SET. This specific condition causes OpenSSL to prematurely free a BIO object that is still under the ownership of the calling application, without notifying the application of this action. Consequently, if the application later attempts to reuse or free the same BIO object, it can trigger a use-after-free condition. The outcome ranges from application crashes and heap corruption to controlled exploitation, depending on the memory allocator’s behavior and the application’s management of the BIO object.

Affected Versions and Patch Availability

The critical vulnerability primarily impacts applications that leverage OpenSSL’s PKCS7 APIs for verifying PKCS7 or S/MIME signatures. Importantly, applications utilizing the CMS APIs for similar functionalities are not susceptible to this particular flaw.

The security advisory confirms that numerous OpenSSL versions are vulnerable to CVE-2026-45447, including versions 4.0, 3.6, 3.5, 3.4, 3.0, 1.1.1, and 1.0.2. OpenSSL has promptly released patched versions for each affected branch.

System administrators are urgently advised to upgrade to the following secure versions: OpenSSL 4.0.1, 3.6.3, 3.5.7, 3.4.6, or 3.0.21. For users reliant on legacy lines with extended support, the recommended upgrades are 1.1.1zh or 1.0.2zq. It’s noteworthy that the FIPS modules for versions 4.0, 3.6, 3.5, 3.4, and 3.0 are not affected by this specific issue, as the vulnerable code resides outside the FIPS boundary.

Broader Security Advisory Details Additional Flaws

Beyond the critical PKCS7 bug, the comprehensive security advisory also outlines a multitude of other vulnerabilities. These additional flaws span a range of severities, from high to moderate, and target various components of the OpenSSL codebase.

Highlighted among these are weaknesses in CMS AuthEnvelopedData processing, which could lead to key-equivalent capabilities or integrity bypasses. QUIC logic flaws have also been identified, potentially enabling denial-of-service attacks through memory exhaustion or NULL pointer dereferences. An AES-OCB misuse issue was found where Initialization Vectors (IVs) are silently ignored when using the low-level EVP_Cipher interface, compromising nonce uniqueness and tag authenticity.

Further issues include several ASN.1 parsing bugs, validation problems in PKCS12 PBMAC1, CMS password-based decryption vulnerabilities, and CMP handling flaws. While many of these primarily pose denial-of-service risks, some could facilitate more advanced cryptographic attacks. The specific OpenSSL protocols affected, such as TLS, QUIC, CMS, PKCS7, HPKE, and S/MIME, vary depending on the vulnerability, configuration, and feature usage.

However, some of the most perilous cryptographic weaknesses predominantly impact custom applications that utilize low-level EVP primitives or implement bespoke messaging protocols built upon OpenSSL, especially when these applications fail to enforce stringent input validation or rely on error codes as oracles. The OpenSSL team strongly recommends that organizations not only apply the latest patches but also conduct thorough audits of their PKCS7, CMS, QUIC, AES-OCB, AES-SIV, and PKCS12 workflows to pinpoint any areas of high-risk exposure.

What You Should Do

• Immediately Upgrade: Apply the latest patched versions of OpenSSL as soon as possible. Refer to the specific versions recommended for your deployed OpenSSL branch.
• Audit Custom Applications: Review custom applications that use low-level EVP primitives or implement custom messaging protocols built on OpenSSL, paying close attention to input validation and error handling.
• Review Feature Usage: Audit your use of PKCS7, CMS, QUIC, AES-OCB, AES-SIV, and PKCS12 workflows to identify and mitigate any high-risk exposures.
• Interim Mitigation: If immediate upgrades are not feasible, consider disabling nonessential features like OCSP stapling and vulnerable PKCS7-based paths as a temporary hardening measure.

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