CWE-789: Memory Allocation with Excessive Size Value
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 DescriptionThe product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. Alternate Terms RelationshipsThe table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Research Concepts" (CWE-1000)
Modes Of IntroductionThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Applicable PlatformsThe listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance. Languages C (Undetermined Prevalence) C++ (Undetermined Prevalence) Class: Language-Independent (Undetermined Prevalence) Common ConsequencesThe table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Demonstrative ExamplesExample 1 Consider the following code, which accepts an untrusted size value and allocates a buffer to contain a string of the given size. (bad code) Example Language: C unsigned int size = GetUntrustedInt();
/* ignore integer overflow (CWE-190) for this example */ unsigned int totBytes = size * sizeof(char); char *string = (char *)malloc(totBytes); InitializeString(string); Suppose an attacker provides a size value of:
12345678
This will cause 305,419,896 bytes (over 291 megabytes) to be allocated for the string. Example 2 Consider the following code, which accepts an untrusted size value and uses the size as an initial capacity for a HashMap. (bad code) Example Language: Java unsigned int size = GetUntrustedInt();
HashMap list = new HashMap(size); The HashMap constructor will verify that the initial capacity is not negative, however there is no check in place to verify that sufficient memory is present. If the attacker provides a large enough value, the application will run into an OutOfMemoryError. Example 3 This code performs a stack allocation based on a length calculation. (bad code) Example Language: C
int a = 5, b = 6;
}
size_t len = a - b; char buf[len]; // Just blows up the stack Since a and b are declared as signed ints, the "a - b" subtraction gives a negative result (-1). However, since len is declared to be unsigned, len is cast to an extremely large positive number (on 32-bit systems - 4294967295). As a result, the buffer buf[len] declaration uses an extremely large size to allocate on the stack, very likely more than the entire computer's memory space. Miscalculations usually will not be so obvious. The calculation will either be complicated or the result of an attacker's input to attain the negative value. Example 4 This example shows a typical attempt to parse a string with an error resulting from a difference in assumptions between the caller to a function and the function's action. (bad code) Example Language: C
int proc_msg(char *s, int msg_len) {
// Note space at the end of the string - assume all strings have preamble with space
}int pre_len = sizeof("preamble: "); char buf[pre_len - msg_len]; ... Do processing here if we get this far char *s = "preamble: message\n"; char *sl = strchr(s, ':'); // Number of characters up to ':' (not including space) int jnklen = sl == NULL ? 0 : sl - s; // If undefined pointer, use zero length int ret_val = proc_msg ("s", jnklen); // Violate assumption of preamble length, end up with negative value, blow out stack The buffer length ends up being -1, resulting in a blown out stack. The space character after the colon is included in the function calculation, but not in the caller's calculation. This, unfortunately, is not usually so obvious but exists in an obtuse series of calculations. Example 5 The following code obtains an untrusted number that is used as an index into an array of messages. (bad code) Example Language: Perl my $num = GetUntrustedNumber();
my @messages = (); $messages[$num] = "Hello World"; The index is not validated at all (CWE-129), so it might be possible for an attacker to modify an element in @messages that was not intended. If an index is used that is larger than the current size of the array, the Perl interpreter automatically expands the array so that the large index works. If $num is a large value such as 2147483648 (1<<31), then the assignment to $messages[$num] would attempt to create a very large array, then eventually produce an error message such as: Out of memory during array extend This memory exhaustion will cause the Perl program to exit, possibly a denial of service. In addition, the lack of memory could also prevent many other programs from successfully running on the system. Example 6 This example shows a typical attempt to parse a string with an error resulting from a difference in assumptions between the caller to a function and the function's action. The buffer length ends up being -1 resulting in a blown out stack. The space character after the colon is included in the function calculation, but not in the caller's calculation. This, unfortunately, is not usually so obvious but exists in an obtuse series of calculations. (bad code) Example Language: C
int proc_msg(char *s, int msg_len) { int pre_len = sizeof("preamble: "); // Note space at the end of the string - assume all strings have preamble with space
char buf[pre_len - msg_len];
... Do processing here and set status
return status;
}
char *s = "preamble: message\n"; char *sl = strchr(s, ':'); // Number of characters up to ':' (not including space) int jnklen = sl == NULL ? 0 : sl - s; // If undefined pointer, use zero length int ret_val = proc_msg ("s", jnklen); // Violate assumption of preamble length, end up with negative value, blow out stack (good code) Example Language: C
int proc_msg(char *s, int msg_len) { int pre_len = sizeof("preamble: "); // Note space at the end of the string - assume all strings have preamble with space
if (pre_len <= msg_len) { // Log error; return error_code; }
char buf[pre_len - msg_len];
... Do processing here and set status
return status;
}
char *s = "preamble: message\n"; char *sl = strchr(s, ':'); // Number of characters up to ':' (not including space) int jnklen = sl == NULL ? 0 : sl - s; // If undefined pointer, use zero length int ret_val = proc_msg ("s", jnklen); // Violate assumption of preamble length, end up with negative value, blow out stack Observed Examples
Potential Mitigations
Weakness Ordinalities
MembershipsThis MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NotesApplicable Platform Uncontrolled memory allocation is possible in many languages, such as dynamic array allocation in perl or initial size parameters in Collections in Java. However, languages like C and C++ where programmers have the power to more directly control memory management will be more susceptible. Relationship This weakness can be closely associated with integer overflows (CWE-190). Integer overflow attacks would concentrate on providing an extremely large number that triggers an overflow that causes less memory to be allocated than expected. By providing a large value that does not trigger an integer overflow, the attacker could still cause excessive amounts of memory to be allocated. Taxonomy Mappings
References
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