CVE-2026-109399 in SWUpdate
Summary
by MITRE • 10/12/2026
SWUpdate through 2026.05.1 contains an improper certificate validation vulnerability in crypto/swupdate_cms_verify_openssl.c that trusts untrusted X.509 certificates embedded in CMS SignedData for chain building. Attackers holding a signing key from a sibling intermediate under the same root can embed that intermediate in the signature to install malicious updates.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified within SWUpdate versions up to 2026.05.1 represents a critical failure in cryptographic validation logic, specifically located in the file crypto/swupdate_cms_verify_openssl.c. This flaw stems from an improper certificate validation mechanism that incorrectly trusts untrusted X.509 certificates embedded directly within CMS SignedData structures during the chain-building process. In standard secure update workflows, SWUpdate is designed to verify digital signatures against a predefined set of trusted root or intermediate certificates stored in its trust store. However, this implementation flaw allows an attacker who possesses a signing key from a sibling intermediate certificate under the same root authority to inject that specific intermediate certificate into the CMS payload itself. By doing so, the vulnerable SWUpdate instance accepts this injected certificate as part of the valid certification path, effectively bypassing the intended restriction where only certificates explicitly listed in the local trust store should be considered for validation.
From a technical perspective, this issue exploits the flexibility inherent in Cryptographic Message Syntax (CMS) standards, which allow signers to include auxiliary certificates within their signatures to assist recipients who may not have access to all necessary CA certificates. While this feature is intended to facilitate interoperability and ease of deployment, it introduces significant security risks if the recipient does not strictly validate that included certificates are already trusted locally or belong to a pre-approved set of issuers. The vulnerability arises because SWUpdate fails to cross-reference the embedded intermediate certificate against its local trust anchor list before accepting it as part of the chain of trust. Consequently, any entity holding a valid signing key from an intermediate CA under the same root can generate a malicious update package that appears cryptographically authentic and trusted by the vulnerable system. This undermines the fundamental integrity guarantees provided by digital signatures in embedded systems management.
The operational impact of this vulnerability is severe, particularly for IoT devices and embedded systems relying on SWUpdate for firmware or software upgrades. An attacker with access to signing keys from a sibling intermediate CA can craft malicious update images that will be successfully verified and installed by the target device. This capability enables remote code execution, data exfiltration, or complete system compromise without detection. Since these updates are often applied automatically or require minimal user intervention, the attack vector is highly effective in environments where physical access to devices is limited but network connectivity exists. The ability to install arbitrary code under the guise of a legitimate update breaks the chain of trust essential for secure boot and safe OTA (Over-The-Air) processes, potentially allowing attackers to persist within the device ecosystem or manipulate critical system functions.
This vulnerability aligns with CWE-295 Improper Certificate Validation, as it involves failing to properly validate certificates against expected trusted authorities during the verification process. Furthermore, from a threat modeling perspective consistent with MITRE ATT&CK frameworks, this behavior facilitates techniques related to Defense Evasion and Persistence through malicious software injection via legitimate update channels. The exploitation method resembles aspects of CWE-347 Improper Verification of Cryptographic Signature, where the signature is technically valid but relies on an untrusted component within the validation chain. To mitigate this risk, developers must enforce strict policy checks that reject any certificates included in CMS SignedData unless they are explicitly present in the local trust store or meet predefined criteria regarding issuer identity and validity periods. Additionally, implementing certificate pinning for update sources and ensuring that SWUpdate configurations do not allow dynamic addition of trusted intermediates from unverified payloads will significantly reduce the attack surface. Organizations should also audit their PKI hierarchy to ensure that sibling intermediate CAs are isolated sufficiently so that compromise of one does not grant signing authority over updates intended for systems trusting another sibling under the same root.