CVE-2026-73064 in Mbed TLS
Summary
by MITRE • 09/24/2026
In Mbed TLS 3.2.0 though 3.6.6 and 4.0.0 through 4.1.0, an attacker who can cause an entropy source to fail can remove or inject bytes into the start of the TLS stream. This only affects TLS 1.3 servers.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 09/24/2026
The vulnerability identified in Mbed TLS versions ranging from 3.2.0 through 3.6.6 and 4.0.0 through 4.1.0 represents a critical flaw in the handling of entropy sources during the initialization phase of Transport Layer Security version 1.3 server operations. This issue specifically impacts servers rather than clients, highlighting a gap in how cryptographic randomness is managed when underlying system resources fail or behave unexpectedly. The core technical deficiency lies in the library's failure to adequately validate and sanitize input data derived from entropy pools before it is integrated into the TLS handshake process. When an attacker can influence or cause an entropy source to fail, they gain the ability to manipulate the initial bytes of the encrypted stream. This manipulation allows for either the removal of expected cryptographic material or the injection of arbitrary bytes at the start of the connection, effectively compromising the integrity and confidentiality guarantees that TLS is designed to provide.
From a technical perspective, this vulnerability exploits the dependency on high-quality random number generation during key exchange and handshake procedures in TLS 1.3. The protocol relies heavily on unpredictable values to generate session keys and nonces. If an entropy source fails or is manipulated, the resulting randomness may become predictable or biased. In the context of Mbed TLS, this flaw permits an adversary who has some level of control over the system's environment or can trigger resource exhaustion conditions to alter the cryptographic state at a fundamental level. By injecting bytes into the start of the stream, the attacker can potentially force specific protocol states or bypass certain security checks that depend on unique, unpredictable identifiers generated during the initial handshake phases. This undermines the forward secrecy and authentication mechanisms inherent in TLS 1.3, as the foundation of trust built upon random key generation is compromised.
The operational impact of this vulnerability is severe for any server infrastructure relying on these affected versions of Mbed TLS to secure communications. An attacker with the capability to cause entropy source failures could potentially perform downgrade attacks, session hijacking attempts, or even full decryption of traffic if they can predict the resulting cryptographic keys derived from the tainted randomness. This scenario poses a significant risk in environments where system resources are constrained or subject to denial-of-service conditions that might exhaust available entropy pools. The ability to remove bytes suggests that expected protocol messages could be stripped away, potentially leading to state desynchronization between client and server, which can be exploited for further attacks such as replay attacks or forcing re-handshakes that expose additional vulnerabilities.
To mitigate this risk, organizations must immediately upgrade Mbed TLS to a version beyond 4.1.0 where these issues have been addressed by the development team through improved entropy validation and error handling mechanisms. In addition to upgrading, system administrators should ensure that their operating systems are configured with robust entropy sources, such as hardware random number generators or well-maintained software daemons like rngd or haveged, to prevent resource exhaustion attacks targeting entropy pools. Implementing strict monitoring for entropy availability on server hosts can provide early warning of potential failures before they impact cryptographic operations. Furthermore, applying defense-in-depth strategies by placing reverse proxies in front of servers that handle TLS termination can help isolate the application layer from direct exposure to such low-level system vulnerabilities, although this does not replace the need for patching the underlying library itself.
This vulnerability aligns with CWE-330, which describes the use of insufficiently random values, and falls under the broader category of weaknesses in cryptographic mechanisms that rely on unpredictable data. In terms of the MITRE ATT&CK framework, this flaw facilitates techniques related to Initial Access or Defense Evasion by allowing an attacker to manipulate protocol states during the handshake phase. The ability to inject bytes into the stream can be seen as a form of Protocol Manipulation, which is often used to bypass security controls that depend on strict adherence to expected message formats and values. Recognizing this vulnerability requires understanding not just the code logic but also the environmental dependencies of cryptographic libraries, emphasizing the importance of holistic security practices that include both software updates and system hardening measures.