--- a/netwerk/dns/TRR.cpp
+++ b/netwerk/dns/TRR.cpp
@@ -617,16 +617,17 @@ TRR::DohDecode()
       break;
 
     case TRRTYPE_NS:
       break;
     case TRRTYPE_CNAME:
       if (mCname.IsEmpty()) {
         uint8_t clength = 0;
         unsigned int cindex = index;
+        unsigned int loop = 128; // a valid DNS name can never loop this much
         do {
           if (cindex >= mBodySize) {
             LOG(("TRR: bad cname packet\n"));
             return NS_ERROR_ILLEGAL_VALUE;
           }
           clength = static_cast<uint8_t>(mResponse[cindex]);
           if ((clength & 0xc0) == 0xc0) {
             // name pointer, get the new offset (14 bits)
@@ -649,17 +650,22 @@ TRR::DohDecode()
               mCname.Append(".");
             }
             if ((cindex + clength) > mBodySize) {
               return NS_ERROR_ILLEGAL_VALUE;
             }
             mCname.Append((const char *)(&mResponse[cindex]), clength);
             cindex += clength; // skip label
           }
-        } while (clength);
+        } while (clength && --loop);
+
+        if (!loop) {
+          LOG(("TRR::DohDecode pointer loop error\n"));
+          return NS_ERROR_ILLEGAL_VALUE;
+        }
 
         LOG(("TRR::DohDecode CNAME host %s => %s\n",
              host.get(), mCname.get()));
       }
       else {
         LOG(("TRR::DohDecode CNAME - ignoring another entry\n"));
       }
       break;