A deeper dive into CVE-2021-39137 – a Golang security bug that Rust would have prevented

This blog post discusses two erroneous computation patterns in Golang. By erroneous computation we mean simply that given certain input, a computer program with certain state returns incorrect output or enters an incorrect state. While clearly there are no limits on how erroneous computations can happen in general, there are language usage patterns which make erroneous computation more likely. In blockchain, erroneous computation is a problem as the ledger can end up in an unexpected state or the blockchain may get wedged at a certain corrupt endpoint. In addition, if erroneous computation happens in only a subset of nodes on the network, a netsplit occurs, which may result in double-spend attacks.

The first erroneous computation example is CVE-2021-39137 which is an interesting go-ethereum bug identified by Guido Vranken. The bug caused a netsplit in the Ethereum network and essentially results from the ability to have a mutable and non-mutable slice referencing the same chunk of memory. The second erroneous computation pattern example is extracted from the Go Programming book by Donovan and Kerninghan and concerns deferred functions’ access to parent-scope variables. This pattern could happen in any language that supports parent-scope variables in a similar way Golang does.

This blog post is motivated by NCC Group’s reviews of Cosmos blockchain implementations. Cosmos is a blockchain building framework which offers an out-of-the-box consensus protocol and an SDK that provides tools backing common state transition code and, as such aims to be the "Ruby on Rails of blockchain". It allows connecting to other Cosmos blockchains via IBC (Inter Blockchain Communication Protocol) and allows drawing the consensus security from parent chains (Inter-blockchain security). The developer mainly implements the state-transition logic. It is worth noting that:

  • Panics are recovered from by the Cosmos framework and treated as errors. This renders a whole class of Golang Denial of Service bugs unimportant. This includes eg. nil pointer dereferences, out of bounds array/slice reads and writes, calling methods on nil interfaces, etc.
  • Often times, Cosmos SDK blockchain apps are fairly simple in terms of the Golang constructs they use. They may be conservative in that they don’t use Go routines and channels, known as another common source of bugs in Golang. In addition, severity of eventual race conditions may be low due to the fact that the condition needs to happen on a large portion of nodes at once.

Given the setting described above, other generic erroneous computation patterns in Golang are worth discussing (unrelated to Denial of Service via panic or Golang’s concurrency primitives). Let’s discuss the first bug, for which we speculate would not happen if Rust was used since Rust does not allow simultaneously having a mutable and an immutable reference pointing to the same memory.

CVE-2021-39137: Erroneous computation in go-ethereum

Given a specially crafted contract, go-ethereum nodes would fork off the network, as their contract evaluation result would be different than the rest of the network. Erroneous computation in blockchain clients is a serious issue, since it results in network forks and can lead to double-spend attacks.

The post-mortem which explains this bug involves several EVM details with which the reader may not be familiar. We start with a code snippet that removes all the EVM details, is very simple, and makes the bug obvious. It should be noted that the code below is not present in the go-ethereum implementation, it just mimics the essence of the issue when all of the EVM details are removed:

func returnAndCopy(mem []byte, n int, copyTo int) []byte {
	// boundary checks omitted

	ret := mem[0:n]

	copy(mem[copyTo:copyTo+n], mem[0:n])    // copy(dst, src)

	return ret

The returnAndCopy function returns the first n bytes of a slice. Before that, it copies those same n bytes to a different location in the slice. For example:

 slice := []int{0,1,2,3,4,5,6,7,8,9}
 fmt.Println(returnAndCopy(slice, 3, 5))    // 0,1,2

In this case, the returnAndCopy function returns correct values:

  0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
  ----------         -----------  
   return  	        copyTo

  0 | 1 | 2 | 3 | 4 | 0 | 1 | 2 | 8 | 9
  ----------         -----------  
   return	        copyTo

  return: 0 | 1 | 2      // correct

In this example, the return interval and copy interval do not intersect. If the intervals have shared (common) locations in the array, but do not point to the exact same subarray, returnAndCopy fails to produce the correct result:

 slice := []int{0,1,2,3,4,5,6,7,8,9}
 fmt.Println(returnAndCopy(slice, 3, 1))

In more detail:

  0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

  0 | 0 | 1 | 2 | 4 | 5 | 6 | 7 | 8 | 9

  return: 0 | 0 | 1      // incorrect

The issue is that ret and mem are pointing to the same backing memory. A modification to mem affects the ret slice in an unintended way. It is interesting to note that the same bug would likely be prevented in Rust, as the compiler would prevent having a mutable and non-mutable references to a same memory location. If that is the case, Rust’s memory safety principles would have prevented this hard fork bug from happening.

Mapping to EVM: CVE-2021-39137 identifies the previously described pattern inside go-ethereum‘s EVM. Let’s start from looking at the key line of the contract that demonstrated the vulnerability, decompiled from the statetest provided at the end of post-mortem.

contract Contract {
    function main() {
        memory[0x00:0x01] = 0x01;
        memory[0x01:0x02] = 0x02;
        memory[0x02:0x03] = 0x03;
        memory[0x03:0x04] = 0x04;
        memory[0x04:0x05] = 0x05;
        memory[0x05:0x06] = 0x06;
        var temp0;
        temp0, memory[0x02:0x08] = address(0x04).call.gas(0x7ef0367e633852132a0ebbf70eb714015dd44bc82e1e55a96ef1389c999c1bca)(memory[0x00:0x06]);     // (1) 
        // [...]

After filling up the EVM contract memory with an increasing sequence of bytes, a contract at address 0x04 is called at the line denoted by (1).

The contract at address 0x04 is a special, "pre-compiled" contract and it is implemented natively in go-ethereum. Pre-compiled contracts implement commonly used functionalities such as hash function computation and elliptic curve operations. In this case, the dataCopy pre-compiled contract is very simple, it implements the "identity" function, which just returns its one argument:

func (c *dataCopy) Run(in []byte) ([]byte, error) {
        return in, nil

Let’s next look what happens inside opCall, right after dataCopy is called:

func opCall(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext) ([]byte, error) {

  // ..SNIP..

        ret, returnGas, err := interpreter.evm.Call(scope.Contract, toAddr, args, gas, bigVal)  // (2)

	if err != nil {
        } else {
        if err == nil || err == ErrExecutionReverted {
                //  ret = common.CopyBytes(ret)       //  (3) (FIX)
                scope.Memory.Set(retOffset.Uint64(), retSize.Uint64(), ret)    // (4) overwrite
        scope.Contract.Gas += returnGas

	interpreter.returnData = ret    // (5) returnData set
        return ret, nil

Relevant to CVE-2021-39137 are the following aspects:

  • In the line denoted with (2), the ret value is a slice that points to the first 6 bytes of the contract memory. That portion of the memory was chosen by the caller in the offending contract (memory[0x00:0x06], see line (1)). The dataCopy contract does not create a new copy of memory before returning.
  • The call to scope.Memory.Set copies ret to another location, see line (4). The copy target location is chosen by the caller and in the case of the decompiled contract above, it is memory[0x02:0x08], see line (1).
  • Close to the end of the opCall implementation at line (5), interpreter.returnData is set to be equal to ret, that is, memory[0x00:0x06]. In EVM, the interpreter keeps track of "returnData": this is an internal holder opCode that calls other contracts returned.

The scope.Memory.Set line corrupts the memory that returnData will point to and returnData ends up being incorrect (0|1|0|1|2|3 as opposed to 0|1|2|3|4|5). The fix is to add line (3), which preserves the original subslice that will be returned before it is overwritten.

Note: A related issue in Golang concerns memory reallocation which happens in the context of the append function. This type of edge-case behavior can be more likely to cause issues as it may evade detection in testing.

Parent scope variables in deferred execution

This bug is similar to a general bug pattern where global variables change underneath code in an unforeseen way. Recall that functions are "first-class values" (pg. 135, sect 5.6 in the Golang book):

func main() {

	var f func(k int) int

	f = func(n int) int {
		return n*n

        fmt.Println(f(2))      // 4 


This means that functions can be assigned to variables, passed to and returned from functions. Functions cannot be compared, and their zero value is nil. It is interesting to note that functions have state. See the following example:

func createIncrease() func() int {

	x := 0
	increase := func() int {
                x = x + 1
                return x

	return increase
}  // x goes out of scope

func main() {

	f := createIncrease()

        fmt.Println(f())  // 1
        fmt.Println(f())  // 2

Even though x went out of scope (as a local variable inside createIncrease function), x still exists as a part of the f function’s state. We don’t see x anymore, but it’s there. There is also the question what happens if multiple functions pick up on the same parent scope variable:

func createIncreaseAndSquare() (func() int, func() int) {
        x := 2
        increase := func() int {
                x = x + 1
                return x

        square := func() int {
                x = x*x
                return x

	return increase, square

func main() {

	// x is created inside createIncrease and goes out of scope
	f, g := createIncreaseAndSquare()

	// x lives on
	fmt.Println(g())  // 4
        fmt.Println(g())  // 16

	// f shares the same x as g
        fmt.Println(f())  // 17

As can be seen, the parent-scope variable is shared between the two functions. This is similar to the usage of global variables by which a global variable may change underneath the deferred function from multiple locations in the code.

func main() {

        keys := []string{"first", "second", "third"}
        m := make(map[string]bool)
	var cleanupFs []func()

        for _, k := range keys {

		m[k] = true

		// k := k       <-- FIX

		cleanupFs = append(cleanupFs, func() {
                        fmt.Println("deleting ", k)
		        delete(m, k)

	// ..do something with m..

	// clean up
        for _, cleanup := range cleanupFs {

The map does not properly get cleaned up. The "clean up" for loop only deletes the "third" key from the map and does so 3 times:

	// deleting  third
	// deleting  third
	// deleting  third

As the loop unwinds, the cleanup functions are picking up the parent scope’s k variable. By the end of the for loop, the delayed functions and the parent function share the same k, which is equal to "third".


Both bug types described in this blog post can roughly be described as variable content unexpectedly changing underneath code. Whether a particular logical issue of this type is labeled as a security issue or not is less important. Audits of correctness-critical Golang code such as Cosmos blockchain code should include checking for these types of issues.

Written by Aleks Kircanski of NCC Group Cryptography Services