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
– Dykam, Commented Feb 18, 2012 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
– HABO, Commented Feb 18, 2012 at 18:29

Community · Accepted Answer · 2022-01-07 09:22:48Z

63

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)

edited Jan 7, 2022 at 9:22

CommunityBot

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answered Feb 18, 2012 at 18:05

CodesInChaos

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Lasse Espeholt · Accepted Answer · 2012-02-18 18:09:08Z

26

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, 2012 at 18:09

answered Feb 18, 2012 at 18:03

Lasse Espeholt

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2 Comments

Eoin Campbell Over a year ago

+1 for providing an example and mentioning the compile check.

Mike Marynowski Over a year ago

This is somewhat incorrect - stackalloc will not allow you to buffer overflow.

ziesemer · Accepted Answer · 2012-02-18 18:01:42Z

7

Only if you use the unsafe keyword.

edited Feb 18, 2012 at 18:01

ziesemer

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answered Feb 18, 2012 at 18:00

Chris

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Comments

seand · Accepted Answer · 2012-02-18 18:02:12Z

6

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.

answered Feb 18, 2012 at 18:02

seand

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