1249
January 15, 2014
EncFS Security Audit
This report is the result of a paid 10-hour security audit of EncFS. It has been posted to the EncFS mailing list, so check there for follow-up. I feel that full disclosure is the best approach for disclosing these vulnerabilities, since some of the issues have already been disclosed but haven't been fixed, and by disclosing them, users can immediately re-evaluate their use of EncFS.
Thanks to Igor Sviridov for funding this audit.
Note: This report was updated on February 5, 2014, thanks to feedback from Robert Freudenreich (founder and CTO of boxcryptor), to correct a technical inaccuracy about how initialization vectors are generated and to clarify the conclusion of the report. You can see the old version of the report here.
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EncFS Security Audit
Taylor Hornby
January 14, 2014
(Updated: February 5, 2014)
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1. Introduction
This document describes the results of a 10-hour security audit of
EncFS 1.7.4. The audit was performed on January 13th and 14th of 2014.
1.1. What is EncFS?
EncFS is a user-space encrypted file system. Unlike disk encryption
software like TrueCrypt, EncFS's ciphertext directory structure
mirrors the plaintext's directory structure. This introduces unique
challenges, such as guaranteeing unique IVs for file name and content
encryption, while maintaining performance.
1.2. Audit Results Summary
This audit finds that EncFS is not up to speed with modern
cryptography practices. Several previously known vulnerabilities have
been reported [1, 2], which have not been completely fixed. New issues
were also discovered during the audit.
The next section presents a list of the issues that were discovered.
Each issue is given a severity rating from 1 to 10. Due to lack of
time, most issues have not been confirmed with a proof-of-concept.
2. Issues
2.1. Same Key Used for Encryption and Authentication
Exploitability: Low
Security Impact: Low
EncFS uses the same key for encrypting data and computing MACs. This
is generally considered to be bad practice.
EncFS should use separate keys for encrypting data and computing MACs.
2.2. Stream Cipher Used to Encrypt Last File Block
Exploitability: Unknown
Security Impact: High
As reported in [1], EncFS uses a stream cipher mode to encrypt the
last file block. The change log says that the ability to add random
bytes to a block was added as a workaround for this issue. However, it
does not solve the problem, and is not enabled by default.
EncFS needs to use a block mode to encrypt the last block.
EncFS's stream encryption is unorthodox:
1. Run "Shuffle Bytes" on the plaintext.
N[J+1] = Xor-Sum(i = 0 TO J) { P[i] }
(N = "shuffled" plaintext value, P = plaintext)
2. Encrypt with (setIVec(IV), key) using CFB mode.
3. Run "Flip Bytes" on the ciphertext.
This reverses bytes in 64-byte chunks.
4. Run "Shuffle Bytes" on the ciphertext.
5. Encrypt with (setIVec(IV + 1), key) using CFB mode.
Where setIVec(IV) = HMAC(globalIV || (IV), key), and,
- 'globalIV' is an IV shared across the entire filesystem.
- 'key' is the encryption key.
This should be removed and replaced with something more standard. As
far as I can see, this provides no useful security benefit, however,
it is relied upon to prevent the attacks in [1]. This is security by
obscurity.
2.3. Generating Block IV by XORing Block Number
Exploitability: Low
Security Impact: Medium
Given the File IV (an IV unique to a file), EncFS generates per-block
IVs by XORing the File IV with the Block Number, then passing the
result to setIVec(), which is described in Section 2.2. This is not
a good solution, as it leads to IV re-use when combined with the
last-block stream cipher issue in Section 2.2:
The stream algorithm (see previous section) adds 1 to the IV, which
could *undo* the XOR with the block number, causing the IV to be
re-used. Suppose the file consists of one and a half blocks, and that
the File IV's least significant bit (LSB) is 1. The first block will
be encrypted with the File IV (block number = 0). The second (partial)
block will be encrypted with File IV XOR 1 (since block number = 1),
making the LSB 0, using the stream algorithm. The stream algorithm
adds 1 to the IV, bringing the LSB back to 1, and hence the same IV is
used twice. The IVs are reused with different encryption modes (CBC
and CFB), but CFB mode starts out similar to CBC mode, so this is
worrisome.
EncFS should use a mode like XTS for random-access block encryption.
Correction 12/05/2014: XTS mode is probably not the ideal option, see
Thomas Ptacek's blog post for good reasons why:
http://sockpuppet.org/blog/2014/04/30/you-dont-want-xts/
2.4. File Holes are Not Authenticated
Exploitability: High
Security Impact: Low
File holes allow large files to contain "holes" of all zero bytes,
which are not saved to disk. EncFS supports these, but it determines
if a file block is part of a file hole by checking if it is all
zeroes. If an entire block is zeroes, it passes the zeroes on without
decrypting it or verifying a MAC.
This allows an attacker to insert zero blocks inside a file (or append
zero blocks to the end of the file), without being detected when MAC
headers are enabled.
2.5. MACs Not Compared in Constant Time
Exploitability: Medium
Security Impact: Medium
MACs are not compared in constant time (MACFileIO.cpp, Line 209). This
allows an attacker with write access to the ciphertext to use a timing
attack to compute the MAC of arbitrary values.
A constant-time string comparison should be used.
2.6. 64-bit MACs
Exploitability: Low
Security Impact: Medium
EncFS uses 64-bit MACs. This is not long enough, as they can be forged
in 2^64 time, which is feasible today.
EncFS should use (at least) 128-bit MACs.
2.7. Editing Configuration File Disables MACs
Exploitability: High
Security Impact: Medium
The purpose of MAC headers is to prevent an attacker with read/write
access to the ciphertext from being able to make changes without being
detected. Unfortunately, this feature provides little security, since
it is controlled by an option in the .encfs6.xml configuration file
(part of the ciphertext), so the attacker can just disable it by
setting "blockMACBytes" to 0 and adding 8 to "blockMACRandBytes" (so
that the MAC is not interpreted as data).
EncFS needs to re-evaluate the purpose of MAC headers and come up with
something more robust. As a workaround, EncFS could add a command line
option --require-macs that will trigger an error if the configuration
file does not have MAC headers enabled.
3. Future Work
There were a few potential problems that I didn't have time to
evaluate. This section lists the most important ones. These will be
prioritized in future audits.
3.1. Information Leakage Between Decryption and MAC Check
EncFS uses Mac-then-Encrypt. Therefore it is possible for any
processing done on the decrypted plaintext before the MAC is checked
to leak information about it, in a style similar to a padding oracle
vulnerability. EncFS doesn't use padding, but the MAC code does
iteratively check if the entire block is zero, so the number of
leading zero bytes in the plaintext is leaked by the execution time.
3.2. Chosen Ciphertext Attacks
Since the same key is used to encrypt all files, it may be possible
for an attacker with read/write access to the ciphertext and partial
read access to the plaintext (e.g. to one directory when --public is
used) to perform a chosen ciphertext attack and decrypt ciphertexts
for which they have no plaintext access.
EncFS should consider using XTS mode.
Correction 12/05/2014: XTS mode is probably not the ideal option, see
Thomas Ptacek's blog post for good reasons why:
http://sockpuppet.org/blog/2014/04/30/you-dont-want-xts/
3.3. Possible Out of Bounds Write in StreamNameIO and BlockNameIO
There is a possible buffer overflow in the encodeName method of
StreamNameIO and BlockNameIO. The methods write to the 'encodedName'
argument without checking its length. This may allow an attacker with
control over file names to crash EncFS or execute arbitrary code.
3.4. 64-bit Initialization Vectors
Initialization vectors are only 64 bits, even when using AES instead
of Blowfish. This may lead to vulnerabilities when encrypting large
(or lots of) files.
4. Conclusion
In conclusion, while EncFS is a useful tool, it ignores many standard
best-practices in cryptography. This is most likely due to it's old
age (originally developed before 2005), however, it is still being
used today, and needs to be updated.
The EncFS author says that a 2.0 version is being developed [3]. This
would be a good time to fix the old problems.
EncFS is probably safe as long as the adversary only gets one copy of
the ciphertext and nothing more. EncFS is not safe if the adversary
has the opportunity to see two or more snapshots of the ciphertext at
different times. EncFS attempts to protect files from malicious
modification, but there are serious problems with this feature.
5. References
[1] http://archives.neohapsis.com/archives/fulldisclosure/2010-08/0316.html
[2] http://code.google.com/p/encfs/issues/detail?id=128
[3] https://code.google.com/p/encfs/issues/detail?id=186