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.

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.

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.

We classify attacks based on the capabilities of the attacker.

In this section, we consider an attacker who 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.

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 being implemented. (For example Mbed TLS alone won't guarantee that the messages will arrive without delay, as the TLS protocol doesn't guarantee that either.)

Warning! Block ciphers do not yet achieve full protection against attackers who can measure the timing of packets with sufficient precision. For details and workarounds see the Block Ciphers section.

In this section, we consider an attacker who can run software on the same machine. The attacker has insufficient privileges to directly access Mbed TLS assets such as memory and files.

The attacker is able to observe the timing of instructions executed by Mbed TLS by leveraging shared hardware that both Mbed TLS and the attacker have access to. Typical attack vectors include cache timings, memory bus contention and branch prediction.

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 attack techniques.

As attacks keep improving, so does Mbed TLS's protection. Mbed TLS is moving towards a model of fully timing-invariant code, but has not reached this point yet.

Remark: Timing information can be observed over the network or through physical side channels as well. Remote and physical timing attacks are covered in the Remote attacks and Physical attacks sections respectively.

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

The attacker code running on the platform has access to some sensor capable of picking up information on the physical state of the hardware while Mbed TLS is running. This could for example be an analogue-to-digital converter on the platform that is located unfortunately enough to pick up the CPU noise.

Mbed TLS doesn't make any security guarantees against local non-timing-based side channel attacks. If local non-timing attacks are present in a use case or a user application's threat model, they need to be mitigated by the platform.

Software running on the same hardware can affect the physical state of the device and introduce faults.

Mbed TLS doesn't make any security guarantees against local fault injection attacks. If local fault injection attacks are present in a use case or a user application's threat model, they need to be mitigated by the platform.

In this section, we consider an attacker who has access to physical information about the hardware Mbed TLS is running on and/or can alter the physical state of the hardware (e.g. power analysis, radio emissions or fault injection).

Mbed TLS doesn't make any security guarantees against physical attacks. If physical attacks are present in a use case or a user application's threat model, they need to be mitigated by physical countermeasures.

Mbed TLS has evolved organically and a well defined threat model hasn't always been present. Therefore, Mbed TLS might have countermeasures against attacks outside the above defined threat model.

The presence of such countermeasures don't mean that Mbed TLS provides protection against a class of attacks outside of the above described threat model. Neither does it mean that the failure of such a countermeasure is considered a vulnerability.

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.

These timing attacks can be physical, local or depending on network latency even a remote. The attacks can result in key recovery.

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 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.

The HACL* implementation of X25519 taken from the Everest project only protects against remote timing attacks. (See their Security Policy.)

The Everest variant is only used when MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED configuration option is defined. This option is off by default.