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Date: Thu, 12 Jan 2012 16:56:14 -0700
From: "Zooko Wilcox-O'Hearn" <>
Subject: details about Tahoe-LAFS security problem #1654

---------- Forwarded message ----------
From: Brian Warner <>
Date: Thu, Jan 12, 2012 at 4:35 PM
Subject: [tahoe-dev] details about #1654 security problem
To: Tahoe-LAFS development <>

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._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 ( 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.

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