Remote exploitation of a buffer overflow vulnerability in the xpdf PDF viewer included in multiple UNIX and Linux distributions could allow for arbitrary code execution as the user viewing a PDF file.
The vulnerability specifically exists due to insufficient bounds checking while processing a PDF file that provides malicious values in the /Encrypt /Length tag.
Vulnerable Code:
The offending code can be found in the Decrypt::makeFileKey2 function in the source file xpdf/Decrypt.cc. GBool Decrypt::makeFileKey2(int encVersion, int encRevision,
int keyLength, GString *ownerKey,
GString *userKey, int permissions,
GString *fileID, String *userPassword,
Guchar *fileKey) {
Guchar *buf;
Guchar test[32];
Guchar fState[256];
Guchar tmpKey[16];
Guchar fx, fy;
int len, i, j;
GBool ok;
...
memcpy(test, userKey->getCString(), 32);
for (i = 19; i >= 0; --i) {
for (j = 0; j < keyLength; ++j) {
[overflow] tmpKey[j] = fileKey[j] ^ i;
}
...
}
...
}
In this piece of code, the keyLength value is ultimately supplied by the PDF file. This allows an attacker to specify an arbitrarily large value and overwrite portions of stack memory. As a consequence, arbitrary code execution is possible.
Successful exploitation of this vulnerability leads to arbitrary code execution as the user who opened the malicious file. An attacker would have to convince a target to open the provided file in order to exploit this vulnerability, thus lessening the impact. Exploitation can be performed reliably, especially with knowledge of the target system.
