mbed TLS Security Advisory 2018-01

This Security Advisory describes two vulnerabilities, their impact and fixes for each possible attack.

Title	Risk of remote code execution when truncated HMAC is enabled
CVE	CVE-2018-0488
Date	1st February 2018 ( Updated on 5th February 2018 )
Affects	All versions of Mbed TLS from version 1.3.0 and up, including all 2.1 and later releases
Not affected	Mbed TLS 1.2.19 and earlier
Impact	Enabling truncated HMAC can lead to remote code execution
Severity	High

Vulnerability

When the truncated HMAC extension is enabled and CBC is used, sending a malicious application packet can be used to selectively corrupt 6 bytes on the peer’s heap, potentially leading to a crash or remote code execution. This can be triggered remotely from either side in both TLS and DTLS.

If the truncated HMAC extension, which can be set by the compile time option MBEDTLS_SSL_TRUNCATED_HMAC in config.h, is disabled when compiling the library, then the vulnerability is not present. The truncated HMAC extension is enabled in the default configuration.

The vulnerability is only present if

The compile-time option MBEDTLS_SSL_TRUNCATED_HMAC is set in config.h. (It is set by default) AND
The truncated HMAC extension is explicitly offered by calling mbedtls_ssl_conf_truncated_hmac(). (It is not offered by default)

Impact

Depending on the platform, an attack exploiting this vulnerability could lead to an application crash or allow remote code execution.

Resolution

Affected users should upgrade to Mbed TLS 1.3.22, Mbed TLS 2.1.10 or Mbed TLS 2.7.0.

Workaround

Users should wherever possible upgrade to the newer version of Mbed TLS. Where this is not practical, users should consider disabling the truncated HMAC extension by removing any call to mbedtls_ssl_conf_truncated_hmac() in their application, and the option MBEDTLS_SSL_TRUNCATED_HMAC in the Mbed TLS configuration is practical for their application.

Title	Risk of remote code execution when verifying RSASSA-PSS signatures
CVE	CVE-2018-0487
Date	1st February 2018 ( Updated on 5th February 2018 )
Affects	All versions of Mbed TLS from version 1.3.8 and up, including all 2.1 and later releases
Not affected	Mbed TLS 1.3.7 and earlier
Impact	Verifying a maliciously constructed RSASSA-PSS signature can lead to remote code execution
Severity	High

Vulnerability

When RSASSA-PSS signature verification is enabled, sending a maliciously constructed certificate chain can be used to cause a buffer overflow on the peer’s stack, potentially leading to crash or remote code execution. This can be triggered remotely from either side in both TLS and DTLS.

RSASSA-PSS is the part of PKCS #1 v2.1 standard and can be enabled by the compile time option MBEDTLS_PKCS1_V21 in config.h. If MBEDTLS_PKCS1_V21 is disabled when compiling the library, then the vulnerability is not present. RSASSA-PSS signatures are enabled in the default configuration.

Impact

Depending on the platform, an attack exploiting this vulnerability could lead to an application crash or remote code execution.

Resolution

Affected users should upgrade to Mbed TLS 1.3.22, Mbed TLS 2.1.10 or Mbed TLS 2.7.0.

Workaround

Users should wherever possible upgrade to the newer version of Mbed TLS. Where this is not practical, users should consider if disabling the option MBEDTLS_PKCS1_V21 in the Mbed TLS configuration is practical for their application. Disabling RSASSA-PSS signatures in the verification profile at runtime is not a sufficient countermeasure.