Published: April 10, 2019 Identifiers: - CVE-2019-9495 (cache attack against EAP-pwd) Latest version available from: https://w1.fi/security/2019-2/ Vulnerability Number of potential side channel attacks were recently discovered in the SAE implementations used by both hostapd and wpa_supplicant (see security advisory 2019-1 and VU#871675). EAP-pwd uses a similar design for deriving PWE from the password and while a specific attack against EAP-pwd is not yet known to be tested, there is no reason to believe that the EAP-pwd implementation would be immune against the type of cache attack that was identified for the SAE implementation. Since the EAP-pwd implementation in hostapd (EAP server) and wpa_supplicant (EAP peer) does not support MODP groups, the timing attack described against SAE is not applicable for the EAP-pwd implementation. A novel cache-based attack against SAE handshake would likely be applicable against the EAP-pwd implementation. Even though the wpa_supplicant/hostapd PWE derivation iteration for EAP-pwd has protections against timing attacks, this new cache-based attack might enable an attacker to determine which code branch is taken in the iteration if the attacker is able to run unprivileged code on the victim machine (e.g., an app installed on a smart phone or potentially a JavaScript code on a web site loaded by a web browser). This depends on the used CPU not providing sufficient protection to prevent unprivileged applications from observing memory access patterns through the shared cache (which is the most likely case with today's designs). The attacker could use information about the selected branch to learn information about the password and combine this information from number of handshake instances with an offline dictionary attack. With sufficient number of handshakes and sufficiently weak password, this might result in full recovery of the used password if that password is not strong enough to protect against dictionary attacks. This attack requires the attacker to be able to run a program on the target device. This is not commonly the case on an authentication server (EAP server), so the most likely target for this would be a client device using EAP-pwd. The commits listed in the end of this advisory change the EAP-pwd implementation shared by hostapd and wpa_supplicant to perform the PWE derivation loop using operations that use constant time and memory access pattern to minimize the externally observable differences from operations that depend on the password even for the case where the attacker might be able to run unprivileged code on the same device. Vulnerable versions/configurations All wpa_supplicant and hostapd versions with EAP-pwd support (CONFIG_EAP_PWD=y in the build configuration and EAP-pwd being enabled in the runtime configuration). It should also be noted that older versions of wpa_supplicant/hostapd prior to v2.7 did not include additional protection against certain timing differences. The definition of the EAP-pwd (RFC 5931) does not describe such protection, but the same issue that was addressed in SAE earlier can be applicable against EAP-pwd as well and as such, that implementation specific extra protection (commit 22ac3dfebf7b, "EAP-pwd: Mask timing of PWE derivation") is needed to avoid showing externally visible timing differences that could leak information about the password. Any uses of older wpa_supplicant/hostapd versions with EAP-pwd are recommended to update to v2.7 or newer in addition to the mitigation steps listed below for the more recently discovered issue. Possible mitigation steps - Merge the following commits to wpa_supplicant/hostapd and rebuild: OpenSSL: Use constant time operations for private bignums Add helper functions for constant time operations OpenSSL: Use constant time selection for crypto_bignum_legendre() EAP-pwd: Use constant time and memory access for finding the PWE These patches are available from https://w1.fi/security/2019-2/ - Update to wpa_supplicant/hostapd v2.8 or newer, once available - Use strong passwords to prevent dictionary attacks -- Jouni Malinen PGP id EFC895FA _______________________________________________ Hostap mailing list Hostap@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/hostap
