Are buffer overflow exploits possible in C#?

Question

Assuming that a C# program uses only managed .NET code, is it possible to have a buffer overflow security vulnerability within that program? If so, how would such vulnerability be possible?

Buffer overflow in the classical sense, or just any kind of buffer overflow exploit? — Dykam, Feb 18 '12 at 17:58
While .NET may make it a challenge to create a traditional buffer overflow vulnerability, i.e. overwriting data or code, it does not guarantee correct handling of exceptions. For example, if a user enters a 30 character username that the application hands off to a stored procedure that accepts a 16 character field an exception should be raised. (Nobody falls for silent truncation, right?) It's still up to the application to Do The Right Thing at that point, not have a catch-all handler dismiss the exception and fall through to code that assumes the user was authenticated. — HABO, Feb 18 '12 at 18:29

score 61 · Accepted Answer · edited Jan 07 '22 at 09:22

Yes, but they are much harder to produce. You can only get buffer overflows if you use certain unsafe constructs, not with "normal" C# code. Memory corrupting code shouldn't be possible at all, when your code is running with lowered trust.

A few possibilities for buffer overflows:

Using the unsafe keyword, which allows pointers. Unsafe code is just as easy to get wrong, as pointer based code in C or C++.
Using unsafe APIs, such as the methods from the Marshal class
(Mono only) You can disable array range checking (safety vs. performance trade-off)

There are also a few other ways to corrupt memory apart from buffer overflows.

StructLayoutKind.Explicit
Wrong native interop signatures

(The runtime itself is written in C++, so a bug in the runtime can also corrupt memory or overflow a buffer, but I consider that out of scope for this question)

Lasse Espeholt · Answer 2 · 2012-02-18T18:09:08.163

25

Yes, in unsafe environments:

unsafe void bufferOverflow(string s)
{
    char* ptr = stackalloc char[10];

    foreach (var c in s)
    {
        *ptr++ = c; // Bufferoverflow if s.Length > 10
    }
}

"Allow unsafe code" has to be checked for this to compile.

You can't a traditional buffer-overflow with an array. It will do bounds-checking before accessing an array unless it (CLR) can guarantee it is safe.

edited Feb 18 '12 at 18:09

answered Feb 18 '12 at 18:03

Lasse Espeholt

17,622
5
63
99

4

+1 for providing an example and mentioning the compile check. – Eoin Campbell Feb 18 '12 at 18:12
1

This is somewhat incorrect - stackalloc will not allow you to buffer overflow. – Mike Marynowski Oct 18 '17 at 02:20

score 7 · Answer 3 · edited Feb 18 '12 at 18:01

7

Only if you use the unsafe keyword.

edited Feb 18 '12 at 18:01

ziesemer

27,712
8
86
94

answered Feb 18 '12 at 18:00

Chris

27,596
25
124
225

score 6 · Answer 4 · answered Feb 18 '12 at 18:02

In an absolute sense, yes a buffer exploit is possible due to bugs in the .NET runtime. However .NET prevents most end user code (except 'unsafe' usage) from these sorts of problems so in real life it's less risky.

In real life, most problems like this will occur from native calls (COM dlls etc) invoked from managed code.

Are buffer overflow exploits possible in C#?

4 Answers4

Linked