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OmapiCallbackHandlerVpcd: add check to filter short TPDUs

Related: OS#6836
Change-Id: I4e76afd7cf4d63c67b1525202fbe74e0796b2ba3
---
M app/src/main/java/org/osmocom/androidApduProxy/OmapiCallbackHandlerVpcd.java
1 file changed, 10 insertions(+), 0 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/android-apdu-proxy refs/changes/01/41801/1

diff --git a/app/src/main/java/org/osmocom/androidApduProxy/OmapiCallbackHandlerVpcd.java b/app/src/main/java/org/osmocom/androidApduProxy/OmapiCallbackHandlerVpcd.java
index f443ac2..b33d28c 100644
--- a/app/src/main/java/org/osmocom/androidApduProxy/OmapiCallbackHandlerVpcd.java
+++ b/app/src/main/java/org/osmocom/androidApduProxy/OmapiCallbackHandlerVpcd.java
@@ -101,6 +101,16 @@
             @Override
             public byte[] vpcdTransact(byte[] tpdu) {
                 Log.i("PROXY", "Exchanging TPDU...\n");
+
+                //All TPDUs that we receive here should have a minimum length of 5 bytes. Under normal conditins, short
+                //TPDUs should not occurr as they should already be filtered out by the layers that call this method.
+                //To ensure seamless operation, let's check the TPDU length and reject short TPDUs immediately.
+                if (tpdu.length < 5) {
+                    Log.e("PROXY", String.format("Rejecting short TPDU (%s)...\n", Utils.b2h(tpdu)));
+                    //see also ISO/IEC 7816-4, table 5 (wrong length; no further indication)
+                    return (Utils.h2b("6700"));
+                }
+
                 //In case the TPDU contains a SELECT by DF-Name, which is forbidden by OMAPI by design, we must
                 //find an alternative solution: In case the SELECT targets the currently selected application,
                 //we just use the FID 7FFF, which is an alias for the currently selected application. In case the

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