CVE-2025-52497 identifies a heap-based buffer underflow vulnerability caused by an off-by-one error (CWE-193) in the Mbed TLS library versions before 3.6.4. The vulnerability exists in the PEM parsing routines, specifically within the mbedtls_pem_read_buffer and two mbedtls_pk_parse functions, which process PEM-encoded cryptographic keys and certificates. When untrusted PEM input is parsed, the off-by-one error leads to a one-byte underflow on the heap, potentially corrupting adjacent memory. This memory corruption can cause application instability or limited information disclosure, primarily impacting the confidentiality and availability of the affected system. The vulnerability is remotely exploitable over the network without requiring authentication or user interaction, but the attack complexity is high due to the need for carefully crafted PEM input to trigger the underflow. The CVSS 3.1 base score is 4.8 (medium), reflecting low confidentiality impact, no integrity impact, and low availability impact. No public exploits or active exploitation have been reported to date. Mbed TLS is widely used in embedded systems, IoT devices, and network appliances for TLS/SSL communications, making this vulnerability relevant for devices that parse PEM certificates from untrusted sources. The issue was reserved in mid-June 2025 and published in early July 2025, with no official patch links provided yet, indicating that affected users should monitor vendor updates closely.

For European organizations, the vulnerability poses a risk primarily to embedded and IoT devices that utilize Mbed TLS for secure communications and certificate handling. Successful exploitation could lead to denial of service conditions via application crashes or limited information leakage, potentially exposing sensitive cryptographic material or causing service disruptions. This could impact critical infrastructure sectors such as manufacturing, automotive, telecommunications, and healthcare, where embedded devices are prevalent. The medium severity and high attack complexity reduce the likelihood of widespread exploitation, but targeted attacks against high-value assets remain a concern. The confidentiality impact, although low, is significant in environments where cryptographic key material is highly sensitive. Availability impacts could disrupt services relying on affected devices, leading to operational downtime. Given the increasing deployment of IoT and embedded systems across Europe, especially in industrial and smart city applications, the vulnerability could have cascading effects if exploited in critical systems.

European organizations should immediately plan to upgrade Mbed TLS to version 3.6.4 or later once available, as this version addresses the off-by-one buffer underflow. Until patches are deployed, organizations should implement strict input validation and sanitization for PEM files, rejecting untrusted or malformed certificate data before parsing. Network-level controls such as firewall rules and intrusion detection systems should monitor and restrict access to services that accept PEM input from untrusted sources. Employ runtime protections like memory safety tools or address sanitizers during development and testing phases to detect similar memory corruption issues early. For embedded and IoT devices where patching is delayed or difficult, consider isolating affected devices from critical networks and applying compensating controls such as limiting certificate update mechanisms. Regularly audit device firmware and software versions to identify vulnerable instances of Mbed TLS. Additionally, maintain vigilant monitoring for anomalous application crashes or unusual certificate parsing errors that could indicate exploitation attempts.