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Fix a buffer overflow in password encryption, and allow arbitrary password length. 

Fixes bug #4033 - Claws Mail crashes [malloc(): memory
corruption] while trying to save account password greater
than 136 chars
This commit is contained in:
Andrej Kacian 2018-06-10 00:38:42 +02:00
parent ad39cec752
commit 8f139f2a1c
2 changed files with 29 additions and 12 deletions
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15
doc/src/password_encryption.txt
View File

@ -12,9 +12,18 @@ IV (initialization vector) for the cipher is filled with random bytes.
Encryption
----------
We prepare a buffer 128+blocksize bytes long, with one block of random
data at the beginning, followed by the password we want to encrypt (in
UTF-8), rest is padded with zero bytes.
We prepare a buffer long enough to fit the NULL-terminated password string
plus one cipher block in it, with one block of random data at the beginning,
followed by the password we want to encrypt (in UTF-8), rest is padded
with zero bytes.
The minimal buffer size is 128+blocksize, and if the password (including
the trailing NULL byte) is longer than 128 bytes, the size is increased by
another 128 bytes until it is long enough to fit the password plus one
cipher block. This is to make it harder to guess the password length from
length of the encrypted string. So for example, if the password (again,
including the trailing NULL byte) is 129 characters long, our buffer will
be 256+blocksize bytes long.
We encrypt the buffer using the encryption key and IV mentioned above,
resulting in ciphertext of the same length as the buffer.

26
src/password.c
View File

@ -319,7 +319,7 @@ gchar *password_encrypt_gnutls(const gchar *password,
gnutls_cipher_algorithm_t algo = GNUTLS_CIPHER_AES_256_CBC;
gnutls_cipher_hd_t handle;
gnutls_datum_t key, iv;
int keylen, blocklen, ret;
int keylen, blocklen, ret, len, i;
unsigned char *buf, *encbuf, *base, *output;
guint rounds = prefs_common_get_prefs()->master_passphrase_pbkdf2_rounds;
@ -353,10 +353,18 @@ gchar *password_encrypt_gnutls(const gchar *password,
return NULL;
}
/* Find out how big buffer (in multiples of BUFSIZE)
* we need to store the password. */
i = 1;
len = strlen(password);
while(len >= i * BUFSIZE)
i++;
len = i * BUFSIZE;
/* Fill buf with one block of random data, our password, pad the
* rest with zero bytes. */
buf = malloc(BUFSIZE + blocklen);
memset(buf, 0, BUFSIZE + blocklen);
buf = malloc(len + blocklen);
memset(buf, 0, len + blocklen);
if (!get_random_bytes(buf, blocklen)) {
g_free(buf);
g_free(key.data);
@ -368,10 +376,10 @@ gchar *password_encrypt_gnutls(const gchar *password,
memcpy(buf + blocklen, password, strlen(password));
/* Encrypt into encbuf */
encbuf = malloc(BUFSIZE + blocklen);
memset(encbuf, 0, BUFSIZE + blocklen);
ret = gnutls_cipher_encrypt2(handle, buf, BUFSIZE + blocklen,
encbuf, BUFSIZE + blocklen);
encbuf = malloc(len + blocklen);
memset(encbuf, 0, len + blocklen);
ret = gnutls_cipher_encrypt2(handle, buf, len + blocklen,
encbuf, len + blocklen);
if (ret < 0) {
g_free(key.data);
g_free(iv.data);
@ -389,7 +397,7 @@ gchar *password_encrypt_gnutls(const gchar *password,
/* And finally prepare the resulting string:
* "{algorithm,rounds}base64encodedciphertext" */
base = g_base64_encode(encbuf, BUFSIZE + blocklen);
base = g_base64_encode(encbuf, len + blocklen);
g_free(encbuf);
output = g_strdup_printf("{%s,%d}%s",
gnutls_cipher_get_name(algo), rounds, base);
@ -515,7 +523,7 @@ gchar *password_decrypt_gnutls(const gchar *password,
/* 'buf+blocklen' should now be pointing to the plaintext
* password string. The first block contains random data from the IV. */
tmp = g_strndup(buf + blocklen, MIN(strlen(buf + blocklen), BUFSIZE));
tmp = g_strndup(buf + blocklen, strlen(buf + blocklen));
memset(buf, 0, len);
g_free(buf);