Date: Wed, 14 Nov 2012 10:43:22 +0200 From: George Kargiotakis <kargig@...d.gr> To: oss-security@...ts.openwall.com Subject: Linux kernel handling of IPv6 temporary addresses Hello all, Due to the way the Linux kernel handles the creation of IPv6 temporary addresses a malicious LAN user can remotely disable them altogether which may lead to privacy violations and information disclosure. By default the Linux kernel uses the 'ipv6.max_addresses' option to specify how many IPv6 addresses an interface may have. The 'ipv6.regen_max_retry' option specifies how many times the kernel will try to create a new address. Currently, in net/ipv6/addrconf.c,lines 898-910, there is no distinction between the events of reaching max_addresses for an interface and failing to generate a new address. Upon reaching any of the above conditions the following error is emitted by the kernel times 'regen_max_retry' (default value 3): [183.793393] ipv6_create_tempaddr(): retry temporary address regeneration [183.793405] ipv6_create_tempaddr(): retry temporary address regeneration [183.793411] ipv6_create_tempaddr(): retry temporary address regeneration After 'regen_max_retry' is reached the kernel completely disables temporary address generation for that interface. [183.793413] ipv6_create_tempaddr(): regeneration time exceeded - disabled temporary address support RFC4941 3.3.7 specifies that disabling temp_addresses MUST happen upon failure to create non-unique addresses which is not the above case. Addresses would have been created if the kernel had a higher 'ipv6.max_addresses' limit. A malicious LAN user can send a limited amount of RA prefixes and thus disable IPv6 temporary address creation for any Linux host. Recent distributions which enable the IPv6 Privacy extensions by default, like Ubuntu 12.04 and 12.10, are vulnerable to such attacks. Due to the kernel's default values for valid (604800) and preferred (86400) lifetimes, this scenario may even occur under normal usage when a Router sends both a public and a ULA prefix, which is not an uncommon scenario for IPv6. 16 addresses are not enough with the current default timers when more than 1 prefix is advertised. The kernel should at least differentiate between the two cases of reaching max_addresses and being unable to create new addresses, due to DAD conflicts for example. Best regards, -- George Kargiotakis https://void.gr GPG KeyID: 0xE4F4FFE6 GPG Fingerprint: 9EB8 31BE C618 07CE 1B51 818D 4A0A 1BC8 E4F4 FFE6
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