diff --git a/ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt b/ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt
new file mode 100644
index 0000000000..0a90721eaf
--- /dev/null
+++ b/ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix potential undefined behavior in mbedtls_mpi_sub_abs().  Reported by
+     Pascal Cuoq using TrustInSoft Analyzer in #6701; observed independently by
+     Aaron Ucko under Valgrind.
diff --git a/library/bignum.c b/library/bignum.c
index 9bc1c2d43f..41b3a26914 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1009,7 +1009,7 @@ int mbedtls_mpi_sub_abs(mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
     /* Set the high limbs of X to match A. Don't touch the lower limbs
      * because X might be aliased to B, and we must not overwrite the
      * significant digits of B. */
-    if (A->n > n) {
+    if (A->n > n && A != X) {
         memcpy(X->p + n, A->p + n, (A->n - n) * ciL);
     }
     if (X->n > A->n) {