diff --git a/src/common/src/common-b64.c b/src/common/src/common-b64.c
index c41d963b212..0ae3da2082b 100644
--- a/src/common/src/common-b64.c
+++ b/src/common/src/common-b64.c
@@ -263,14 +263,20 @@ static const uint8_t mongoc_b64rmap_invalid = 0xff;
 #if defined(BSON_OS_UNIX)
 #include <pthread.h>
 #define mongoc_common_once_t pthread_once_t
-#define mongoc_common_once pthread_once
+#define mongoc_common_once(o, c)             \
+   do {                                      \
+      Assert (pthread_once ((o), (c)) == 0); \
+   } while (0)
 #define MONGOC_COMMON_ONCE_FUN(n) void n (void)
 #define MONGOC_COMMON_ONCE_RETURN return
 #define MONGOC_COMMON_ONCE_INIT PTHREAD_ONCE_INIT
 #else
 #define mongoc_common_once_t INIT_ONCE
 #define MONGOC_COMMON_ONCE_INIT INIT_ONCE_STATIC_INIT
-#define mongoc_common_once(o, c) InitOnceExecuteOnce (o, c, NULL, NULL)
+#define mongoc_common_once(o, c)                            \
+   do {                                                     \
+      Assert (InitOnceExecuteOnce (o, c, NULL, NULL) != 0); \
+   } while (0)
 #define MONGOC_COMMON_ONCE_FUN(n) BOOL CALLBACK n (PINIT_ONCE _ignored_a, PVOID _ignored_b, PVOID *_ignored_c)
 #define MONGOC_COMMON_ONCE_RETURN return true
 #endif
diff --git a/src/libbson/src/bson/bson-json.c b/src/libbson/src/bson/bson-json.c
index 3952203746d..308df723ca6 100644
--- a/src/libbson/src/bson/bson-json.c
+++ b/src/libbson/src/bson/bson-json.c
@@ -620,9 +620,10 @@ _bson_json_read_integer (bson_json_reader_t *reader, uint64_t val, int64_t sign)
 
    if (rs == BSON_JSON_REGULAR) {
       BASIC_CB_BAIL_IF_NOT_NORMAL ("integer");
+      BSON_ASSERT (mlib_in_range (int, len));
 
       if (val <= INT32_MAX || (sign == -1 && val <= (uint64_t) INT32_MAX + 1)) {
-         bson_append_int32 (STACK_BSON_CHILD, key, (int) len, (int) (val * sign));
+         bson_append_int32 (STACK_BSON_CHILD, key, (int) len, (int32_t) ((int64_t) val * sign));
       } else if (sign == -1) {
 #if defined(_WIN32) && !defined(__MINGW32__)
          // Unary negation of unsigned integer is deliberate.
diff --git a/src/libbson/src/jsonsl/jsonsl.c b/src/libbson/src/jsonsl/jsonsl.c
index 045132aa2dc..5f4a0d04a12 100644
--- a/src/libbson/src/jsonsl/jsonsl.c
+++ b/src/libbson/src/jsonsl/jsonsl.c
@@ -1149,8 +1149,15 @@ void jsonsl_jpr_match_state_init(jsonsl_t jsn,
         return;
     }
     jsn->jprs = (jsonsl_jpr_t *)malloc(sizeof(jsonsl_jpr_t) * njprs);
+    if (!jsn->jprs) {
+        return;
+    }
     jsn->jpr_count = njprs;
     jsn->jpr_root = (size_t*)calloc(1, sizeof(size_t) * njprs * jsn->levels_max);
+    if (!jsn->jpr_root) {
+        free(jsn->jprs);
+        return;
+    }
     memcpy(jsn->jprs, jprs, sizeof(jsonsl_jpr_t) * njprs);
     /* Set the initial jump table values */
 
diff --git a/src/libmongoc/src/mongoc/mongoc-client-session.c b/src/libmongoc/src/mongoc/mongoc-client-session.c
index af595e5544f..0ffbd210d8f 100644
--- a/src/libmongoc/src/mongoc/mongoc-client-session.c
+++ b/src/libmongoc/src/mongoc/mongoc-client-session.c
@@ -886,7 +886,9 @@ _max_time_ms_failure (bson_t *reply)
       return true;
    }
 
-   bson_iter_init (&iter, reply);
+   if (!bson_iter_init (&iter, reply)) {
+      return false;
+   }
    if (bson_iter_find_descendant (&iter, "writeConcernError.codeName", &descendant) &&
        BSON_ITER_HOLDS_UTF8 (&descendant) && 0 == strcmp (bson_iter_utf8 (&descendant, NULL), MAX_TIME_MS_EXPIRED)) {
       return true;
diff --git a/src/libmongoc/src/mongoc/mongoc-collection.c b/src/libmongoc/src/mongoc/mongoc-collection.c
index 64dd81a231a..5bfb38438cd 100644
--- a/src/libmongoc/src/mongoc/mongoc-collection.c
+++ b/src/libmongoc/src/mongoc/mongoc-collection.c
@@ -1255,8 +1255,12 @@ _mongoc_collection_index_keys_equal (const bson_t *expected, const bson_t *actua
    bson_iter_t iter_expected;
    bson_iter_t iter_actual;
 
-   bson_iter_init (&iter_expected, expected);
-   bson_iter_init (&iter_actual, actual);
+   if (!bson_iter_init (&iter_expected, expected)) {
+      return false;
+   }
+   if (!bson_iter_init (&iter_actual, actual)) {
+      return false;
+   }
 
    while (bson_iter_next (&iter_expected)) {
       /* If the key document has fewer items than expected, indexes are unequal
diff --git a/src/libmongoc/src/mongoc/mongoc-counters.c b/src/libmongoc/src/mongoc/mongoc-counters.c
index fef74d29542..3ce070828f4 100644
--- a/src/libmongoc/src/mongoc/mongoc-counters.c
+++ b/src/libmongoc/src/mongoc/mongoc-counters.c
@@ -111,7 +111,9 @@ mongoc_counters_calc_size (void)
            (n_cpu * n_groups * sizeof (mongoc_counter_slots_t)));
 
 #ifdef BSON_OS_UNIX
-   return BSON_MAX (sysconf (_SC_PAGESIZE), size);
+   long pg_sz = sysconf (_SC_PAGESIZE);
+   BSON_ASSERT (pg_sz > 0);
+   return BSON_MAX ((size_t) pg_sz, size);
 #else
    return size;
 #endif
diff --git a/src/libmongoc/src/mongoc/mongoc-server-description.c b/src/libmongoc/src/mongoc/mongoc-server-description.c
index 439e8498e34..248e7ac6ab0 100644
--- a/src/libmongoc/src/mongoc/mongoc-server-description.c
+++ b/src/libmongoc/src/mongoc/mongoc-server-description.c
@@ -790,7 +790,7 @@ mongoc_server_description_new_copy (const mongoc_server_description_t *descripti
          const uint8_t *data = bson_get_data (&copy->last_hello_response) + offset;                                  \
          uint32_t len = description->FIELD.len;                                                                      \
          MONGOC_DEBUG_ASSERT (offset + len <= copy->last_hello_response.len);                                        \
-         bson_init_static (&copy->FIELD, data, len);                                                                 \
+         BSON_ASSERT (bson_init_static (&copy->FIELD, data, len));                                                   \
       } else {                                                                                                       \
          bson_init (&copy->FIELD);                                                                                   \
       }                                                                                                              \
diff --git a/src/libmongoc/src/mongoc/mongoc-socket.c b/src/libmongoc/src/mongoc/mongoc-socket.c
index 5558419caef..6154ef38e4f 100644
--- a/src/libmongoc/src/mongoc/mongoc-socket.c
+++ b/src/libmongoc/src/mongoc/mongoc-socket.c
@@ -1207,6 +1207,7 @@ _mongoc_socket_try_sendv_slow (mongoc_socket_t *sock, /* IN */
          RETURN (ret ? ret : -1);
       }
 
+      BSON_ASSERT (mlib_cmp (wrote, <=, SSIZE_MAX - ret));
       ret += wrote;
 
       if (mlib_cmp (wrote, !=, iov[i].iov_len)) {
diff --git a/src/libmongoc/src/mongoc/mongoc-stream-tls-openssl.c b/src/libmongoc/src/mongoc/mongoc-stream-tls-openssl.c
index 3145f876cea..d9727471027 100644
--- a/src/libmongoc/src/mongoc/mongoc-stream-tls-openssl.c
+++ b/src/libmongoc/src/mongoc/mongoc-stream-tls-openssl.c
@@ -843,6 +843,8 @@ create_stream_with_ctx (
 mongoc_stream_t *
 mongoc_stream_tls_openssl_new (mongoc_stream_t *base_stream, const char *host, mongoc_ssl_opt_t *opt, int client)
 {
+   BSON_ASSERT (opt);
+
    SSL_CTX *ssl_ctx = _mongoc_openssl_ctx_new (opt);
 
    if (!ssl_ctx) {
diff --git a/src/libmongoc/src/mongoc/mongoc-stream-tls.c b/src/libmongoc/src/mongoc/mongoc-stream-tls.c
index 19f62d5afe8..9cb2565ac9a 100644
--- a/src/libmongoc/src/mongoc/mongoc-stream-tls.c
+++ b/src/libmongoc/src/mongoc/mongoc-stream-tls.c
@@ -194,13 +194,13 @@ mongoc_stream_tls_new_with_hostname (mongoc_stream_t *base_stream, const char *h
 
    /* !client is only used for testing,
     * when the streams are pretending to be the server */
-   if (!client || opt->weak_cert_validation) {
+   if (opt && (!client || opt->weak_cert_validation)) {
       opt->allow_invalid_hostname = true;
    }
 
 #ifndef _WIN32
    /* Silly check for Unix Domain Sockets */
-   if (!host || (host[0] == '/' && !access (host, F_OK))) {
+   if (opt && (!host || (host[0] == '/' && !access (host, F_OK)))) {
       opt->allow_invalid_hostname = true;
    }
 #endif
@@ -252,6 +252,7 @@ mongoc_stream_tls_new_with_hostname_and_openssl_context (
    mongoc_stream_t *base_stream, const char *host, mongoc_ssl_opt_t *opt, int client, SSL_CTX *ssl_ctx)
 {
    BSON_ASSERT (base_stream);
+   BSON_ASSERT (opt);
 
    /* !client is only used for testing,
     * when the streams are pretending to be the server */
diff --git a/src/libmongoc/src/mongoc/mongoc-topology-background-monitoring.c b/src/libmongoc/src/mongoc/mongoc-topology-background-monitoring.c
index c42b32b54df..1aaa736f28f 100644
--- a/src/libmongoc/src/mongoc/mongoc-topology-background-monitoring.c
+++ b/src/libmongoc/src/mongoc/mongoc-topology-background-monitoring.c
@@ -311,10 +311,12 @@ _mongoc_topology_background_monitoring_stop (mongoc_topology_t *topology)
    }
 
    /* Signal all RTT monitors to shut down. */
+   bson_mutex_lock (&topology->tpld_modification_mtx);
    for (size_t i = 0u; i < n_rtt_monitors; i++) {
       server_monitor = mongoc_set_get_item (topology->rtt_monitors, i);
       mongoc_server_monitor_request_shutdown (server_monitor);
    }
+   bson_mutex_unlock (&topology->tpld_modification_mtx);
 
    for (size_t i = 0u; i < n_srv_monitors; i++) {
       /* Wait for the thread to shutdown. */
diff --git a/src/libmongoc/src/mongoc/mongoc-uri.c b/src/libmongoc/src/mongoc/mongoc-uri.c
index 6cac707770f..383b04e1ae0 100644
--- a/src/libmongoc/src/mongoc/mongoc-uri.c
+++ b/src/libmongoc/src/mongoc/mongoc-uri.c
@@ -953,7 +953,10 @@ mongoc_uri_options_validate_names (const bson_t *a, const bson_t *b, bson_error_
    /* Scan `a` looking for deprecated names
     * where the canonical name was also used in `a`,
     * or was used in `b`. */
-   bson_iter_init (&key_iter, a);
+   if (!bson_iter_init (&key_iter, a)) {
+      return false;
+   }
+
    while (bson_iter_next (&key_iter)) {
       key = bson_iter_key (&key_iter);
       value = bson_iter_utf8_unsafe (&key_iter, &value_len);
@@ -1017,7 +1020,10 @@ mongoc_uri_apply_options (mongoc_uri_t *uri, const bson_t *options, bool from_dn
    size_t value_len;
    bool bval;
 
-   bson_iter_init (&iter, options);
+   if (!bson_iter_init (&iter, options)) {
+      return false;
+   }
+
    while (bson_iter_next (&iter)) {
       key = bson_iter_key (&iter);
       canon = mongoc_uri_canonicalize_option (key);
diff --git a/src/tools/mongoc-stat.c b/src/tools/mongoc-stat.c
index 4aa0bba763c..a47bb47e8c1 100644
--- a/src/tools/mongoc-stat.c
+++ b/src/tools/mongoc-stat.c
@@ -29,6 +29,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include <mlib/cmp.h>
 
 #pragma pack(1)
 typedef struct {
@@ -108,6 +109,7 @@ mongoc_counters_new_from_pid (unsigned pid)
       return NULL;
    }
 
+   BSON_ASSERT (mlib_in_range (size_t, len));
    size = len;
 
    if (MAP_FAILED == (mem = mmap (NULL, size, PROT_READ, MAP_SHARED, fd, 0))) {