diff --git a/ChangeLog.d/mpi_sub_abs.txt b/ChangeLog.d/mpi_sub_abs.txt
new file mode 100644
index 0000000..9f34ee7
--- /dev/null
+++ b/ChangeLog.d/mpi_sub_abs.txt
@@ -0,0 +1,7 @@
+Security
+   * Fix a buffer overflow in mbedtls_mpi_sub_abs() when calculating
+     |A| - |B| where |B| is larger than |A| and has more limbs (so the
+     function should return MBEDTLS_ERR_MPI_NEGATIVE_VALUE). Only
+     applications calling mbedtls_mpi_sub_abs() directly are affected:
+     all calls inside the library were safe since this function is
+     only called with |A| >= |B|. Reported by Guido Vranken in #4042.
diff --git a/library/bignum.c b/library/bignum.c
index 9cc5d66..e74a1ad 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1401,6 +1401,12 @@
     for( n = B->n; n > 0; n-- )
         if( B->p[n - 1] != 0 )
             break;
+    if( n > A->n )
+    {
+        /* B >= (2^ciL)^n > A */
+        ret = MBEDTLS_ERR_MPI_NEGATIVE_VALUE;
+        goto cleanup;
+    }
 
     carry = mpi_sub_hlp( n, X->p, B->p );
     if( carry != 0 )