CWE-1285: Improper Validation of Specified Index, Position, or Offset in Input

How it's found

Improper Validation of Specified Index, Position, or Offset in Input is typically found by tracing untrusted input from where it enters the system to the point where it is used without the check or neutralization this weakness describes, combining manual code review with dynamic testing.

Often, indexable resources such as memory buffers or files can be accessed using a specific position, index, or offset, such as an index for an array or a position for a file. When untrusted input is not properly validated before it is used as an index, attackers could access (or attempt to access) unauthorized portions of these resources. This could be used to cause buffer overflows, excessive resource allocation, or trigger unexpected failures.

Vulnerable vs. safe

/* capture the sizes of all messages */
// read values from socket and added to sizes array
// continue read from socket until buf only contains '.'break;

sizes[num - 1] = size;if (DOTLINE(buf))else if (sscanf(buf, "%d %d", &num, &size) == 2)

......char buf[BUFFER_SIZE];int ok;int num, size;while ((ok = gen_recv(sock, buf, sizeof(buf))) == 0){}int getsizes(int sock, int count, int *sizes) {}

/* capture the sizes of all messages */
// read values from socket and added to sizes array
// continue read from socket until buf only contains '.'break;

sizes[num - 1] = size;

/* warn about possible attempt to induce buffer overflow */report(stderr, "Warning: ignoring bogus data for message sizes returned by server.\n");
if (num > 0 && num <= (unsigned)count)elseif (DOTLINE(buf))else if (sscanf(buf, "%d %d", &num, &size) == 2) {}

......char buf[BUFFER_SIZE];int ok;int num, size;while ((ok = gen_recv(sock, buf, sizeof(buf))) == 0){}int getsizes(int sock, int count, int *sizes) {}

Consequences

• Varies by Context

Mitigations

• Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Where this fits in a TurboPentest engagement

This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.

Frequently asked questions

Related CWEs

• Class weakness
  CWE-20: Improper Input Validation