diff --git a/src/main/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLV.java b/src/main/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLV.java
index a952dbcc..56c2e7c8 100644
--- a/src/main/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLV.java
+++ b/src/main/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLV.java
@@ -15,16 +15,39 @@ public class TinyBERTLV {
     private byte[] buffer;
     private int pos;
 
+    /**
+     * Reads the length at the given offset, on one to four bytes. The returned length is guaranteed to be present in
+     * the given buffer.
+     *
+     * @param buf the buffer to read from
+     * @param off the offset of the length
+     * @return the length, and the offset of the body following it
+     * @throws IllegalArgumentException if the length is malformed, or declares more bytes than the buffer holds
+     */
     public static int[] readNum(byte[] buf, int off) {
+        if(off >= buf.length) {
+            throw new IllegalArgumentException("Truncated TLV: no length byte at offset " + off);
+        }
+
         int len = buf[off++] & 0xff;
         int lenlen = 0;
 
         if((len & 0x80) == 0x80) {
             lenlen = len & 0x7f;
+            if(lenlen < 1 || lenlen > 4 || (off + lenlen) > buf.length) {
+                throw new IllegalArgumentException("Truncated TLV: length header of " + lenlen + " bytes is unsupported or exceeds the " + (buf.length - off) + " bytes remaining");
+            }
+
             len = readVal(buf, off, lenlen);
         }
 
-        return new int[]{len, off + lenlen};
+        off += lenlen;
+
+        if(len < 0 || len > (buf.length - off)) {
+            throw new IllegalArgumentException("Truncated TLV: declared length of " + Integer.toUnsignedString(len) + " exceeds the " + (buf.length - off) + " bytes remaining");
+        }
+
+        return new int[]{len, off};
     }
 
     public static int readVal(byte[] val, int off, int len) {
@@ -76,7 +99,7 @@ public class TinyBERTLV {
      *
      * @param tag the tag to enter
      * @return the length of the TLV
-     * @throws IllegalArgumentException if the next tag does not match the given one
+     * @throws IllegalArgumentException if the next tag does not match the given one, or the TLV extends past the end of the buffer
      */
     public int enterConstructed(int tag) throws IllegalArgumentException {
         checkTag(tag, readTag());
@@ -88,7 +111,7 @@ public class TinyBERTLV {
      *
      * @param tag the tag to read
      * @return the body of the TLV
-     * @throws IllegalArgumentException if the next tag does not match the given one
+     * @throws IllegalArgumentException if the next tag does not match the given one, or the TLV extends past the end of the buffer
      */
     public byte[] readPrimitive(int tag) throws IllegalArgumentException {
         checkTag(tag, readTag());
@@ -148,9 +171,10 @@ public class TinyBERTLV {
     }
 
     /**
-     * Reads the next tag. The current implementation only reads length on one and two bytes. Can be extended if needed.
+     * Reads the next length. The current implementation reads lengths on one to four bytes. Can be extended if needed.
      *
-     * @return the tag
+     * @return the length
+     * @throws IllegalArgumentException if the length is malformed, or declares more bytes than the buffer holds
      */
     public int readLength() {
         int[] len = TinyBERTLV.readNum(buffer, pos);
diff --git a/src/test/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLVTest.java b/src/test/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLVTest.java
new file mode 100644
index 00000000..bb512d44
--- /dev/null
+++ b/src/test/java/com/sparrowwallet/sparrow/io/keycard/TinyBERTLVTest.java
@@ -0,0 +1,131 @@
+package com.sparrowwallet.sparrow.io.keycard;
+
+import com.sparrowwallet.drongo.Utils;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.io.ByteArrayOutputStream;
+
+public class TinyBERTLVTest {
+    @Test
+    public void testFourByteLengthRejected() {
+        //tag 0x80 declaring a ~2GiB body from six bytes of card response
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("80847fffffff"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testNegativeLengthRejected() {
+        //a four byte length with the sign bit set
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("8084ffffffff"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testShortBodyRejectedNotZeroPadded() {
+        //five bytes declared, two supplied - previously returned three zero bytes of padding
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("80050102"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testLongFormShortBodyRejectedNotZeroPadded() {
+        //a long form length of one million, four bytes supplied - previously returned a megabyte of zero padding
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("8084000f424001020304"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testMissingLengthByteRejected() {
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("80"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testTruncatedLengthHeaderRejected() {
+        //0x84 promises four length bytes, only one follows
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("808400"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testIndefiniteLengthRejected() {
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("8080"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testOversizedConstructedLengthRejected() {
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("a4847fffffff"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> tlv.enterConstructed(ApplicationInfo.TLV_APPLICATION_INFO_TEMPLATE));
+    }
+
+    @Test
+    public void testShortFormLengthStillParses() {
+        TinyBERTLV tlv = new TinyBERTLV(Utils.hexToBytes("80020102"));
+        Assertions.assertArrayEquals(Utils.hexToBytes("0102"), tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testLongFormLengthStillParses() {
+        //a 200 byte body, which requires the 0x81 length form
+        byte[] body = new byte[200];
+        for(int i = 0; i < body.length; i++) {
+            body[i] = (byte)i;
+        }
+
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        baos.write(ApplicationInfo.TLV_PUB_KEY);
+        TinyBERTLV.writeNum(baos, body.length);
+        baos.writeBytes(body);
+
+        TinyBERTLV tlv = new TinyBERTLV(baos.toByteArray());
+        Assertions.assertArrayEquals(body, tlv.readPrimitive(ApplicationInfo.TLV_PUB_KEY));
+    }
+
+    @Test
+    public void testUninitializedCardSelectResponseStillParses() {
+        byte[] pubKey = new byte[65];
+        pubKey[0] = 0x04;
+
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        writePrimitive(baos, ApplicationInfo.TLV_PUB_KEY, pubKey);
+
+        ApplicationInfo applicationInfo = new ApplicationInfo(baos.toByteArray());
+        Assertions.assertFalse(applicationInfo.isInitializedCard());
+        Assertions.assertArrayEquals(pubKey, applicationInfo.getSecureChannelPubKey());
+        Assertions.assertTrue(applicationInfo.hasSecureChannelCapability());
+    }
+
+    @Test
+    public void testInitializedCardSelectResponseStillParses() {
+        byte[] instanceUID = new byte[16];
+        byte[] pubKey = new byte[65];
+        pubKey[0] = 0x04;
+        byte[] keyUID = new byte[32];
+
+        ByteArrayOutputStream template = new ByteArrayOutputStream();
+        writePrimitive(template, ApplicationInfo.TLV_UID, instanceUID);
+        writePrimitive(template, ApplicationInfo.TLV_PUB_KEY, pubKey);
+        writePrimitive(template, TinyBERTLV.TLV_INT, new byte[] {0x03, 0x01});
+        writePrimitive(template, TinyBERTLV.TLV_INT, new byte[] {0x05});
+        writePrimitive(template, ApplicationInfo.TLV_KEY_UID, keyUID);
+        writePrimitive(template, ApplicationInfo.TLV_CAPABILITIES, new byte[] {ApplicationInfo.CAPABILITIES_ALL});
+
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        writePrimitive(baos, ApplicationInfo.TLV_APPLICATION_INFO_TEMPLATE, template.toByteArray());
+
+        ApplicationInfo applicationInfo = new ApplicationInfo(baos.toByteArray());
+        Assertions.assertTrue(applicationInfo.isInitializedCard());
+        Assertions.assertEquals("3.1", applicationInfo.getAppVersionString());
+        Assertions.assertEquals(5, applicationInfo.getFreePairingSlots());
+        Assertions.assertArrayEquals(keyUID, applicationInfo.getKeyUID());
+        Assertions.assertEquals(ApplicationInfo.CAPABILITIES_ALL, applicationInfo.getCapabilities());
+    }
+
+    private void writePrimitive(ByteArrayOutputStream baos, byte tag, byte[] body) {
+        baos.write(tag);
+        TinyBERTLV.writeNum(baos, body.length);
+        baos.writeBytes(body);
+    }
+}