Insecure Use of RC4 in LSrunasE and Supercrypt (CVE-2007-6340)

2008.02.06
Risk: Medium
Local: No
Remote: No
CWE: N/A

############################################################# # # COMPASS SECURITY ADVISORY http://www.csnc.ch/ # ############################################################# # # Product: LSrunasE, Supercrypt # Vendor: Geert Moernaut # Type: Flawed Encryption # Risk: Medium # Author: Daniel Roethlisberger # Date: 2008-01-29 # CVE Name: CVE-2007-6340 # ############################################################# Introduction ------------ LSrunasE [1] and Supercrypt [2] are utilities used to run commands under a different user account within Windows batch scripts. Passwords are encrypted using strong cryptography. Due to insecure use of the RC4 algorithm, the encryption can be trivially broken. Affected Product ---------------- Vulnerable: * LSrunasE 1.0 * Supercrypt 1.0 Not vulnerable according to vendor: * LSrunasE 2.0 * Supercrypt 2.0 Since versions 2.0 of the utilities are no longer free, Compass Security has not been able to verify the implemented fix. Technical Description --------------------- The following description of the vulnerability in LSrunasE is also applicable to Supercrypt. LSrunasE has two components, lsrunas.exe and lsencrypt.exe. One would encrypt the password to the user account using lsencrypt.exe, yielding an encrypted password. This encrypted password is then used as parameter to lsrunase.exe, for example in a batch script: lsrunase.exe /user:Administrator /password:/kpyg/8D96HYhJ61SsJagmPw /domain:somecorp /command:"example.exe /dosomething" LSrunasE supposedly uses AES-256 to protect the passwords, so an attacker would have to reverse engineer the key out of the binary, right? However, the properties of the encrypted passwords turn out to be very weak, leading to an easy break very quickly. The password encryption is equivalent to a simple XOR with a constant key, leading to the following simple break: 1) Find the XOR keystream by XORing an arbitrary string s with its encrypted form: keystream = unbase64(lsencrypt(s)) XOR s 2) Now you can decrypt any encrypted password: password = unbase64(encrypted_password) XOR keystream This result strongly suggested that AES-256 has no role in the encryption scheme. Digging a little deeper, Compass Security was able to unpack the PECompact packed binary and reverse engineer the cryptographical algorithm used by LSrunasE. The result is interesting, since it turned out that LSrunasE is using strong cryptography, albeit in an insecure way. LSrunasE uses SHA1 and RC4 to encrypt the passwords. The SHA1 hash of a constant string is directly used as 160 bit RC4 key. There is no random IV construction. secret = "(constant string embedded in the binary)" key = sha1(secret) encrypted_password = base64(rc4(key, password)) As a stream cipher, RC4 is only secure as long as you make sure that no two plaintexts are encrypted using the same keystream. When using raw RC4 without some form of unique IV construction, the keystream will be the same for every key. In the case of LSrunasE and Supercrypt, the very same RC4 keystream is being generated for all encrypted passwords, completely compromising the security of RC4. Other Attacks ------------- Note that this advisory is only about the insecure use of the RC4 stream cipher, not about the fact that reverse engineering is possible. Of course, utilities like LSrunasE and Supercrypt cannot be secure against reverse engineering; it will always be possible to reverse engineer the algorithm and find the key within the binary. However, encrypted passwords should not be very easily broken without resorting to more time consuming binary reverse engineering. And there is the obvious attack in LSrunasE of copying the encrypted password and using it to run arbitrary commands, which Supercrypt addresses. Recommendation -------------- Compass Security recommends not to use any utilities which store credentials, especially not with privileged accounts on untrusted systems. When used in logon scripts or within automated staging scripts, privileged accounts used with LSrunasE or similar tools can be compromised, leading to privilege escalation. PoC Code -------- Available on request. Disclosure Timeline ------------------- 2008-01-29: Coordinated public disclosure 2007-12-17: Initial vendor response 2007-12-17: Initial vendor notification 2007-12-14: Assigned CVE-2007-6340 2007-12-05: Discovery by Walter Sprenger, Ivan Buetler, Martin Sueess and Daniel Roethlisberger of Compass Security AG References ---------- [1] http://www.moernaut.com/default.aspx?item=lsrunase [2] http://www.moernaut.com/default.aspx?item=supercrypt [3] http://www.csnc.ch/en/downloads/advisories.html -- Daniel Roethlisberger <daniel.roethlisberger (at) csnc (dot) ch [email concealed]> Compass Security Network Computing AG, Rapperswil, Switzerland http://www.csnc.ch/ - Tel: +41 55 214 41 77 - PGP: 0x69292CBA Security Review - Penetration Testing - Computer Forensics


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