diff --git a/capi20.old/capiutils.h b/capi20.old/capiutils.h
index 27b77fe6..2f2fcdbd 100644
--- a/capi20.old/capiutils.h
+++ b/capi20.old/capiutils.h
@@ -322,6 +322,10 @@ char *capi_info2str(_cword reason);
 #define capi20_cmd2str	capi_cmd2str
 char *capi_cmd2str(_cbyte cmd, _cbyte subcmd);
 
+/*
+ * WARNING: The following two functions use a single static buffer and
+ * are not thread-safe.
+ */
 #define capi20_cmsg2str	capi_cmsg2str
 char *capi_cmsg2str(_cmsg * cmsg);
 
diff --git a/capi20.old/convert.c b/capi20.old/convert.c
index fa513c87..2cf466f7 100644
--- a/capi20.old/convert.c
+++ b/capi20.old/convert.c
@@ -897,10 +897,14 @@ static char *p = 0;
 static void bufprint(char *fmt,...)
 {
 	va_list f;
+	size_t space = buf + sizeof(buf) - p, len;
 	va_start(f, fmt);
-	vsprintf(p, fmt, f);
+	len = vsnprintf(p, space, fmt, f);
 	va_end(f);
-	p += strlen(p);
+	if (len < space - 1)
+		p += len;
+	else
+		p += space - 1;
 }
 
 static void printstructlen(_cbyte * m, unsigned len)
diff --git a/capi20/capiutils.h b/capi20/capiutils.h
index 51233966..a88e8d46 100644
--- a/capi20/capiutils.h
+++ b/capi20/capiutils.h
@@ -370,6 +370,10 @@ char *capi_info2str(_cword reason);
 #define capi20_cmd2str	capi_cmd2str
 char *capi_cmd2str(_cbyte cmd, _cbyte subcmd);
 
+/*
+ * WARNING: The following two functions use a single static buffer and
+ * are not thread-safe.
+ */
 #define capi20_cmsg2str	capi_cmsg2str
 char *capi_cmsg2str(_cmsg * cmsg);
 
diff --git a/capi20/convert.c b/capi20/convert.c
index f9bc7c7c..393409ee 100644
--- a/capi20/convert.c
+++ b/capi20/convert.c
@@ -842,10 +842,14 @@ static char *p = 0;
 static void bufprint(char *fmt,...)
 {
 	va_list f;
+	size_t space = buf + sizeof(buf) - p, len;
 	va_start(f, fmt);
-	vsprintf(p, fmt, f);
+	len = vsnprintf(p, space, fmt, f);
 	va_end(f);
-	p += strlen(p);
+	if (len < space - 1)
+		p += len;
+	else
+		p += space - 1;
 }
 
 static void printstructlen(_cbyte * m, unsigned len)