diff --git a/README.md b/README.md
index ac31f58..455f644 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,8 @@ the next pseudo IMSI and sends it as SMS to the SIM. The SIM applet overwrites
 its current IMSI with the new one, and uses it in the next location update.
 Afterwards, the HLR will generate the next IMSI and so on.
 
+**FIXME:** details below need updating, see [OS#4400](https://osmocom.org/issues/4400).
+
 ```
 HLR <-> SIM  LOCATION UPDATE, imsi_pseudo=200
 HLR  -> SIM  NEW PSEUDO IMSI REQ, session_id=1, imsi_pseudo=123
diff --git a/docs/specification.md b/docs/specification.md
index 7d4c9bc..3f8b53c 100644
--- a/docs/specification.md
+++ b/docs/specification.md
@@ -1,5 +1,7 @@
 # [WIP] Make IMSI Pseudonymization an optional extension of 3GPP TS
 
+**FIXME:** needs to be updated, see [OS#4400](https://osmocom.org/issues/4400).
+
 Optional additions we need to make, and where to make them:
 
 * Initial provisioning of the SIM: can optionally have a pseudo IMSI