Date: Thu, 12 Jan 2012 16:56:14 -0700 From: "Zooko Wilcox-O'Hearn" <zooko@...ko.com> To: oss-security@...ts.openwall.com Subject: details about Tahoe-LAFS security problem #1654 ---------- Forwarded message ---------- From: Brian Warner <warner@...har.com> Date: Thu, Jan 12, 2012 at 4:35 PM Subject: [tahoe-dev] details about #1654 security problem To: Tahoe-LAFS development <tahoe-dev@...oe-lafs.org> Dear Tahoe-LAFS Users: On 08-Jan-2012, Tahoe-LAFS core member Kevan Carstensen (author of the MDMF code) discovered a serious bug in v1.9.0 (the current stable release) that allows attackers to corrupt downloads of mutable files in certain cases. We've released Tahoe-LAFS v1.9.1 which removes this vulnerability. All users are encouraged to upgrade immediately to v1.9.1, or to downgrade to v1.8.3. v1.9.0 was released about two months ago. As far as we know, ArchLinux is the only distribution to have packaged v1.9.0 (the others are still on v1.8.3, which is safe). So if you get your Tahoe-LAFS through a non-ArchLinux package, you're probably fine. If you build it yourself, you should upgrade. In Tahoe, files are encrypted, and then encoded into multiple redundant shares. Integrity-checking information (Merkle hash trees) are included in the shares to detect corruption. When downloading, these hashes are checked before combining the shares in the decoder, which generates ciphertext that can be decrypted into the original file. Mutable files have two sets of hash trees, the "share hash tree" (which covers all shares), and the "block hash trees" (which sit under the share-hash-tree and cover the individual blocks that make up each share, one block per 128KiB segment of the original file). The new mutable downloader released in v1.9.0, which supports both the old-style SDMF format and the new MDMF format, has a bug in which the share-hash-tree check is accidentaly bypassed when the Merkle hash tree is already fully populated. This doesn't normally occur, but shares can contain additional hash-tree nodes beyond the ones they strictly need. An attacker could modify one share to include the entire tree, then change the block data in the remaining shares. They would need to update the block-hash-trees in those doctored shares, but because of the bug, these tree roots will not be compared against the share hash tree. The attacker is thus able to control the input to the ZFEC decoder for all but the first share received (which must have valid block data). This gives them the ability to flip bits of the plaintext without triggering the CorruptShareError exceptions that share corruption would normally produce, causing corrupted plaintext to be delivered to an unwitting client. To exploit this bug, the attacker must be able to deliver multiple modified shares to your client, in a particular order: this means they must control one or more of your storage servers. Note that this does not directly reveal the plaintext to the attacker (this is an integrity failure, not a confidentiality failure). However, "encryption without authentication" is never a safe state of affairs, and can frequently be exploited to reveal information about the plaintext (perhaps by inducing observable failures by flipping bits in messages of a known format). In addition, clients which read corrupted data as part of a read-modify-write operation (such as directory modifications) may then write the corrupted data back out to the file, making the corruption persist even after the client has been fixed. v1.9.1 fixes this by removing the accidental "if" clause, making the share-hash-tree check unconditional. The specific bug is in src/allmydata/mutable/retrieve.py, Retrieve._validate_block, around the call to share_hash_tree.set_hashes(), and was introduced in git revisionid ac3b2647dd2c45cd1ddbf5b130ee5a780c66c73b with the MDMF-capable downloader rewrite around 01-Aug-2011. The bug was first present in shipping code in Tahoe-LAFS-1.9.0, on 30-Oct-2011. It was fixed in commit 9b4b03a474a2c9050c8347459ab6698839be7288, shipped in Tahoe-LAFS-1.9.1 on 12-Jan-2012. We are continuing to audit the 1.9.x mutable downloader code to search for other potential bugs. Bug #1654 (https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1654) was created to track this problem, and is now closed. The same fix was applied to trunk a few minutes ago, so trunk is now safe too. sorry! -Brian _______________________________________________ tahoe-dev mailing list tahoe-dev@...oe-lafs.org http://tahoe-lafs.org/cgi-bin/mailman/listinfo/tahoe-dev
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