mbedtls/SECURITY.md

Reporting Vulnerabilities

If you think you have found an Mbed TLS security vulnerability, then please send an email to the security team at mbed-tls-security@lists.trustedfirmware.org.

Security Incident Handling Process

Our security process is detailed in our security center.

Its primary goal is to ensure fixes are ready to be deployed when the issue goes public.

Maintained branches

Only the maintained branches, as listed in BRANCHES.md, get security fixes. Users are urged to always use the latest version of a maintained branch.

Threat model

We use the following classification of attacks:

• Remote Attacks: The attacker can observe and modify data sent over the network. This includes observing the content and timing of individual packets, as well as suppressing or delaying legitimate messages, and injecting messages.
• Timing Attacks: The attacker can gain information about the time taken by certain sets of instructions in Mbed TLS operations.
• Physical Attacks: The attacker has access to physical information about the hardware Mbed TLS is running on and/or can alter the physical state of the hardware.

Remote attacks

Mbed TLS aims to fully protect against remote attacks and to enable the user application in providing full protection against remote attacks. Said protection is limited to providing security guarantees offered by the protocol in question. (For example Mbed TLS alone won't guarantee that the messages will arrive without delay, as the TLS protocol doesn't guarantee that either.)

Timing attacks

Mbed TLS provides limited protection against timing attacks. The cost of protecting against timing attacks widely varies depending on the granularity of the measurements and the noise present. Therefore the protection in Mbed TLS is limited. We are only aiming to provide protection against publicly documented attacks, and this protection is not currently complete.

Warning! Block ciphers do not yet achieve full protection. For details and workarounds see the section below.

Block Ciphers

Currently there are four block ciphers in Mbed TLS: AES, CAMELLIA, ARIA and DES. The pure software implementation in Mbed TLS implementation uses lookup tables, which are vulnerable to timing attacks.

Workarounds:

• Turn on hardware acceleration for AES. This is supported only on selected architectures and currently only available for AES. See configuration options MBEDTLS_AESCE_C, MBEDTLS_AESNI_C and MBEDTLS_PADLOCK_C for details.
• Add a secure alternative implementation (typically hardware acceleration) for the vulnerable cipher. See the Alternative Implementations Guide for more information.
• Use cryptographic mechanisms that are not based on block ciphers. In particular, for authenticated encryption, use ChaCha20/Poly1305 instead of block cipher modes. For random generation, use HMAC_DRBG instead of CTR_DRBG.

Physical attacks

Physical attacks are out of scope (eg. power analysis or radio emissions). Any attack using information about or influencing the physical state of the hardware is considered physical, independently of the attack vector. (For example Row Hammer and Screaming Channels are considered physical attacks.) If physical attacks are present in a use case or a user application's threat model, it needs to be mitigated by physical countermeasures.