gnark is a zero-knowledge proof system framework. In versions prior to 0.14.0, the Verify function in eddsa.go and ecdsa.go used the S value from a signature without asserting that 0 ≤ S < order, leading to a signature malleability vulnerability. Because gnark’s native EdDSA and ECDSA circuits lack essential constraints, multiple distinct witnesses can satisfy the same public inputs. In protocols where nullifiers or anti-replay checks are derived from R and S, this enables signature malleability and may allow double spending. This issue has been addressed in version 0.14.0.

Affected: Go: github.com/consensys/gnark

In version before, `sig.s` used without asserting `0 ≤ S < order` in `Verify function` in [eddsa.go](https://github.com/Consensys/gnark/blob/d9a42397979b05f95f21a601fd219b06a8d60b7b/std/signature/eddsa/eddsa.go) and [ecdsa.go](https://github.com/Consensys/gnark/blob/d9a42397979b05f95f21a601fd219b06a8d60b7b/std/signature/ecdsa/ecdsa.go), which will lead to *signature malleability* vulnerability.

### Impact

Since gnark’s native EdDSA and ECDSA circuits lack essential constraints, multiple distinct witnesses can satisfy the same public inputs. In protocols where nullifiers or anti-replay checks are derived from `(R, S)`, this enables signature malleability and may lead to double spending.

### Exploitation

```go package main

import ( "crypto/rand" "fmt" "math/big"

"github.com/consensys/gnark-crypto/ecc" mimcHash "github.com/consensys/gnark-crypto/ecc/bn254/fr/mimc" eddsaCrypto "github.com/consensys/gnark-crypto/ecc/bn254/twistededwards/eddsa"

"github.com/consensys/gnark/backend/groth16" "github.com/consensys/gnark/frontend" "github.com/consensys/gnark/frontend/cs/r1cs" "github.com/consensys/gnark/std/algebra/native/twistededwards" stdMimc "github.com/consensys/gnark/std/hash/mimc" stdEddsa "github.com/consensys/gnark/std/signature/eddsa"

te "github.com/consensys/gnark-crypto/ecc/twistededwards" )

// Circuit type eddsaCircuit struct { Msg frontend.Variable `gnark:",public"` Pk stdEddsa.PublicKey `gnark:",public"` Sig stdEddsa.Signature }

func (c *…