Mbed Tls CVE-2026-34877

Attacker must have ability to modify serialized SSL context or session structures in Mbed TLS 2.19.0-3.6.5 or 4.0.0. Requires access to serialized session data (e.g., stored on disk, in transit, or in memory). No authentication or user interaction needed for exploitation if serialized data is accessible.

CVSS vector and score are not provided, limiting standard severity quantification. However, the reported tags (Buffer Overflow, RCE) and description indicate high impact potential. The vulnerability requires an attacker to modify serialized structures in transit or at rest, meaning the attack vector depends on the application's use of serialization-this may be network-based (if sessions are transmitted) or local (if stored files are accessible). Exploitation complexity is likely moderate to high, as the attacker must understand the serialization format and craft valid malformed payloads. EPSS and CVSS metrics are absent, precluding quantitative exploitation probability assessment. The vulnerability's real-world risk is significant for applications that deserialize untrusted SSL session data (e.g., distributed session caches, session persistence layers, TLS proxies), but risk is lower for applications that only use in-memory SSL contexts. No KEV or public exploit availability is documented in provided intelligence.

An attacker intercepts or modifies a serialized SSL session stored in a distributed cache or transmitted between application servers. When the application deserializes the modified session structure to resume the TLS connection, the malformed data triggers a buffer overflow in Mbed TLS's deserialization code, allowing the attacker to overwrite memory and execute arbitrary code with the privileges of the TLS library process. This attack is most practical in environments where sessions are persisted or replicated across multiple servers without integrity protection.