diff --git a/ChangeLog.d/sha384-blocksize.txt b/ChangeLog.d/sha384-blocksize.txt
new file mode 100644
index 000000000..a064264cb
--- /dev/null
+++ b/ChangeLog.d/sha384-blocksize.txt
@@ -0,0 +1,6 @@
+Security
+   * Fix definition of MBEDTLS_MD_MAX_BLOCK_SIZE, which was too
+     small when MBEDTLS_SHA384_C was defined and MBEDTLS_SHA512_C was
+     undefined. Mbed TLS itself was unaffected by this, but user code
+     which used MBEDTLS_MD_MAX_BLOCK_SIZE could be affected. This bug
+     was first released in Mbed TLS 3.4.0.