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](https://developer.trustedfirmware.org/w/mbed-tls/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`](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.

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.)

### Local attacks

The attacker is capable of running code on the same hardware as Mbed TLS, but
there is still a security boundary between them (ie. the attacker can't for
example read secrets from Mbed TLS' memory directly).

#### Timing attacks

The attacker can gain information about the time taken by certain sets of
instructions in Mbed TLS operations. (See for example the [Flush+Reload
paper](https://eprint.iacr.org/2013/448.pdf).)

(Technically, timing information can be observed over the network or through
physical side channels as well. Network timing attacks are less powerful than
local and countermeasures protecting against local attacks prevent network
attacks as well. If the timing information is gained through physical side
channels, we consider them physical attacks and as such they are out of scope.)

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.

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](docs/architecture/alternative-implementations.md) 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.

#### Local non-timing side channels

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 can for example be any analogue to digital converter on the
platform that is located unfortunately enough to pick up the CPU noise. (See
for example the [Leaky Noise
paper](https://tches.iacr.org/index.php/TCHES/article/view/8297).)

Mbed TLS doesn't offer 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, it needs to be mitigated by the platform.

#### Local fault injection attacks

Software running on the same hardware can affect the physical state of the
device and introduce faults. (See for example the [Row Hammer
paper](https://users.ece.cmu.edu/~yoonguk/papers/kim-isca14.pdf).)

Mbed TLS doesn't offer 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, it needs to be mitigated by the platform.

### 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 (eg. power
analysis, radio emissions or fault injection).

Mbed TLS doesn't offer any security guarantees against 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.

### Caveats

#### Out of scope 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.