diff --git a/ChangeLog.d/platform-setbuf.txt b/ChangeLog.d/platform-setbuf.txt
new file mode 100644
index 0000000000..844f70cf2a
--- /dev/null
+++ b/ChangeLog.d/platform-setbuf.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Provide the missing definition of mbedtls_setbuf() in some configurations
+     with MBEDTLS_PLATFORM_C disabled. Fixes #6118, #6196.