--- a/media/psshparser/PsshParser.cpp
+++ b/media/psshparser/PsshParser.cpp
@@ -102,16 +102,21 @@ private:
 
  // System ID identifying the cenc v2 pssh box format; specified at:
  // https://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/cenc-format.html
 const uint8_t kSystemID[] = {
   0x10, 0x77, 0xef, 0xec, 0xc0, 0xb2, 0x4d, 0x02,
   0xac, 0xe3, 0x3c, 0x1e, 0x52, 0xe2, 0xfb, 0x4b
 };
 
+const uint8_t kPrimetimeID[] = {
+  0xf2, 0x39, 0xe7, 0x69, 0xef, 0xa3, 0x48, 0x50,
+  0x9c, 0x16, 0xa9, 0x03, 0xc6, 0x93, 0x2e, 0xfb
+};
+
 bool
 ParseCENCInitData(const uint8_t* aInitData,
                   uint32_t aInitDataSize,
                   std::vector<std::vector<uint8_t>>& aOutKeyIds)
 {
   aOutKeyIds.clear();
   std::vector<std::vector<uint8_t>> keyIds;
   ByteReader reader(aInitData, aInitDataSize);
@@ -152,16 +157,22 @@ ParseCENCInitData(const uint8_t* aInitDa
     reader.Read(3); // skip flags.
 
     // SystemID
     const uint8_t* sid = reader.Read(sizeof(kSystemID));
     if (!sid) {
       // Insufficient bytes to read SystemID.
       return false;
     }
+    if (!memcmp(kPrimetimeID, sid, sizeof(kSystemID))) {
+      // Allow legacy Primetime key system PSSH boxes, which
+      // don't conform to common encryption format.
+      return true;
+    }
+
     if (memcmp(kSystemID, sid, sizeof(kSystemID))) {
       // Ignore pssh boxes with wrong system ID.
       reader.Seek(std::max<size_t>(reader.Offset(), end));
       continue;
     }
 
     if (!reader.CanRead32()) {
       return false;

--- a/media/psshparser/gtest/TestPsshParser.cpp
+++ b/media/psshparser/gtest/TestPsshParser.cpp
@@ -108,16 +108,25 @@ const uint8_t g2xGoogleWPTCencInitData[]
   0x10, 0x77, 0xEF, 0xEC, 0xC0, 0xB2, 0x4D, 0x02,  // Common SystemID
   0xAC, 0xE3, 0x3C, 0x1E, 0x52, 0xE2, 0xFB, 0x4B,
   0x00, 0x00, 0x00, 0x01,                          // key count
   0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,  // key
   0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
   0x00, 0x00, 0x00, 0x00                           // datasize
 };
 
+const uint8_t gPrimetimePSSH[] = {
+  0x00, 0x00, 0x00, 0x00,                          // size = 0
+  0x70, 0x73, 0x73, 0x68,                          // 'pssh'
+  0x01,                                            // version = 1
+  0x00, 0x00, 0x00,                                // flags
+  0xf2, 0x39, 0xe7, 0x69, 0xef, 0xa3, 0x48, 0x50,  // Primetime system Id
+  0x9c, 0x16, 0xa9, 0x03, 0xc6, 0x93, 0x2e, 0xfb
+};
+
 TEST(PsshParser, ParseCencInitData) {
   std::vector<std::vector<uint8_t>> keyIds;
   bool rv;
 
   rv = ParseCENCInitData(gGoogleWPTCencInitData, MOZ_ARRAY_LENGTH(gGoogleWPTCencInitData), keyIds);
   EXPECT_TRUE(rv);
   EXPECT_EQ(1u, keyIds.size());
   EXPECT_EQ(16u, keyIds[0].size());
@@ -148,9 +157,13 @@ TEST(PsshParser, ParseCencInitData) {
 
   rv = ParseCENCInitData(g2xGoogleWPTCencInitData, MOZ_ARRAY_LENGTH(g2xGoogleWPTCencInitData), keyIds);
   EXPECT_TRUE(rv);
   EXPECT_EQ(2u, keyIds.size());
   EXPECT_EQ(16u, keyIds[0].size());
   EXPECT_EQ(16u, keyIds[1].size());
   EXPECT_EQ(0, memcmp(&keyIds[0].front(), &g2xGoogleWPTCencInitData[32], 16));
   EXPECT_EQ(0, memcmp(&keyIds[1].front(), &g2xGoogleWPTCencInitData[84], 16));
+
+  rv = ParseCENCInitData(gPrimetimePSSH, MOZ_ARRAY_LENGTH(gPrimetimePSSH), keyIds);
+  EXPECT_TRUE(rv);
+  EXPECT_EQ(0u, keyIds.size());
 }