SB2020090123 - Multiple vulnerabilities in Mbed TLS

Description

This security bulletin contains information about 2 secuirty vulnerabilities.

1) Side channel attack on classical CBC decryption in (D)TLS (CVE-ID: CVE-2020-16150)

The vulnerability allows a local user to perform a side-channel attack.

The vulnerability is caused due to mbed TLS used dummy rounds of the compression function associated with the hash used for HMAC in order to hide the length of the padding to remote attackers when ecrypting/authenticating (D)TLS record in a connection using a CBC ciphersuite without the Encrypt-then-Mac extension. A local user  who is able to observe the state of the cache could monitor the presence of mbedtls_md_process() in the cache in order to determine when the actual computation ends and when the dummy rounds start. This is a reliable target as it's always called at least once.

Successful exploitation of the vulnerability may allow an attacker with access to enough information about the state of the cache (including, but not limited to, an untrusted operating system attacking a secure enclave such as SGX or the TrustZone secure world) to recover portions of the plaintext of a (D)TLS record.

2) Side channel attack on RSA and static Diffie-Hellman (CVE-ID: N/A)

The vulnerability allows a local user to gain access to sensitive information.

The vulnerability exists due to mbed TLS is using the GCD functio, which is prone to a single-trace side-channel attack. An attacker with access to precise enough timing and memory access information (typically an untrusted operating system attacking a secure enclave such as SGX or the TrustZone secure world) can recover the private keys used in RSA or static (finite-field) Diffie-Hellman operations.

Remediation

Install update from vendor's website.

References

• https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2020-09-1
• https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2020-09-2