Not a security bug because this is in the test harness, not NSS itself. While running the ssl_gtests under Address Sanitizer (Clang 3.6, Linux, x86_64, with edge coverage enabled (-fsanitize-coverage=3 and ASAN_OPTIONS=coverage=1) and with optimization, if any of that matters), I got a heap-use-after-free on one run. It doesn't happen reliably, and I saw it only once, but I haven't actively tried to reproduce it. I don't know if the specific test is important. Output was: [ RUN ] VariantsAll/TlsConnectGeneric.ConnectResumeSupportBoth/3 Version: DTLS 1.2 server: Changing state from INIT to CONNECTING client: Changing state from INIT to CONNECTING client: Would have blocked server: Would have blocked Poll() client: Readable client: Would have blocked Poll() server: Readable server: Handshake success server: Changing state from CONNECTING to CONNECTED client: Readable client: Handshake success client: Changing state from CONNECTING to CONNECTED ================================================================= ==87544==ERROR: AddressSanitizer: heap-use-after-free on address 0x603000051da0 at pc 0x0000006e0c4e bp 0x7ffceb7d5d50 sp 0x7ffceb7d5d48 WRITE of size 8 at 0x603000051da0 thread T0 The allocation was here: nss_test::Poller::SetTimer(unsigned int, nss_test::PollTarget*, void (*)(nss_test::PollTarget*, nss_test::Event), nss_test::Poller::Timer**) /home/jld/src/NSS/nss/external_tests/ssl_gtest/test_io.cc:429 Timeout /home/jld/src/NSS/nss/external_tests/ssl_gtest/./gtest_utils.h:20 nss_test::TlsConnectTestBase::Handshake() /home/jld/src/NSS/nss/external_tests/ssl_gtest/tls_connect.cc:137 nss_test::TlsConnectTestBase::Connect() /home/jld/src/NSS/nss/external_tests/ssl_gtest/tls_connect.cc:151 testing::Test::Run() /home/jld/src/NSS/nss/external_tests/google_test/gtest/src/gtest.cc:2434 [...] The deletion was here (where timers that have already fired are deleted): operator delete(void*) ??:? nss_test::Poller::Poll() /home/jld/src/NSS/nss/external_tests/ssl_gtest/test_io.cc:487 (discriminator 1) The misuse was here: ~Timeout /home/jld/src/NSS/nss/external_tests/ssl_gtest/./test_io.h:97 nss_test::TlsConnectTestBase::Handshake() /home/jld/src/NSS/nss/external_tests/ssl_gtest/tls_connect.cc:137 nss_test::TlsConnectTestBase::Connect() /home/jld/src/NSS/nss/external_tests/ssl_gtest/tls_connect.cc:151 nss_test::TlsConnectGeneric_ConnectResumeSupportBoth_Test::TestBody() /home/jld/src/NSS/nss/external_tests/ssl_gtest/ssl_loopback_unittest.cc:145 i.e., an nss_test::Timeout being destroyed had a pointer to the already-deleted Timer and tried to call Cancel() on it. Fixing this might be a simple matter of making Timeout::ExpiredCallback clear handle_ (and checking it before use), but what ought to happen longer-term is converting this code to use safe pointer classes (unique_ptr, shared_ptr, etc.).