diff --git a/libfetch/common.c b/libfetch/common.c
index bcba889..01c5f2b 100644
--- a/libfetch/common.c
+++ b/libfetch/common.c
@@ -171,6 +171,30 @@ fetch_info(const char *fmt, ...)
 
 /*** Network-related utility functions ***************************************/
 
+uintmax_t
+fetch_parseuint(const char *str, const char **endptr, int radix, uintmax_t max)
+{
+	uintmax_t val = 0, maxx = max / radix, d;
+	const char *p;
+
+	for (p = str; isxdigit((unsigned char)*p); p++) {
+		unsigned char ch = (unsigned char)*p;
+		if (isdigit(ch))
+			d = ch - '0';
+		else	d = tolower(ch - 'a');
+		if (d > radix || val > maxx) goto err;
+		val *= radix;
+		if (val > max-d) goto err;
+		val += d;
+	}
+	if (p == str || val > max) goto err;
+	*endptr = p;
+	return val;
+err:
+	*endptr = "\xff";
+	return 0;
+}
+
 /*
  * Return the default port for a scheme
  */
diff --git a/libfetch/common.h b/libfetch/common.h
index dd5c14c..2c16bf7 100644
--- a/libfetch/common.h
+++ b/libfetch/common.h
@@ -38,6 +38,8 @@
 #define FTP_DEFAULT_PROXY_PORT	21
 #define HTTP_DEFAULT_PROXY_PORT	3128
 
+#include <sys/types.h>
+#include <limits.h>
 #include "openssl-compat.h"
 
 #if defined(__GNUC__) && __GNUC__ >= 3
@@ -53,6 +55,14 @@
 #define HAVE_SA_LEN
 #endif
 
+#ifndef IPPORT_MAX
+# define IPPORT_MAX 65535
+#endif
+
+#ifndef OFF_MAX
+# define OFF_MAX (((((off_t)1 << (sizeof(off_t) * CHAR_BIT - 2)) - 1) << 1) + 1)
+#endif
+
 /* Connection */
 typedef struct fetchconn conn_t;
 
@@ -86,6 +96,7 @@ struct fetcherr {
 void		 fetch_seterr(struct fetcherr *, int);
 void		 fetch_syserr(void);
 void		 fetch_info(const char *, ...)  LIBFETCH_PRINTFLIKE(1, 2);
+uintmax_t	 fetch_parseuint(const char *p, const char **endptr, int radix, uintmax_t max);
 int		 fetch_default_port(const char *);
 int		 fetch_default_proxy_port(const char *);
 int		 fetch_bind(int, int, const char *);
@@ -125,7 +136,6 @@ fetchIO		*http_request(struct url *, const char *,
 fetchIO		*ftp_request(struct url *, const char *, const char *,
 		     struct url_stat *, struct url *, const char *);
 
-
 /*
  * Check whether a particular flag is set
  */
diff --git a/libfetch/fetch.c b/libfetch/fetch.c
index a0d4dbd..45c92aa 100644
--- a/libfetch/fetch.c
+++ b/libfetch/fetch.c
@@ -473,15 +473,12 @@ find_user:
 
 	/* port */
 	if (*p == ':') {
-		for (q = ++p; *q && (*q != '/'); q++)
-			if (isdigit((unsigned char)*q))
-				u->port = u->port * 10 + (*q - '0');
-			else {
-				/* invalid port */
-				url_seterr(URL_BAD_PORT);
-				goto ouch;
-			}
-		p = q;
+		u->port = fetch_parseuint(p + 1, &p, 10, IPPORT_MAX);
+		if (*p && *p != '/') {
+			/* invalid port */
+			url_seterr(URL_BAD_PORT);
+			goto ouch;
+		}
 	}
 
 	/* document */
diff --git a/libfetch/ftp.c b/libfetch/ftp.c
index 8f9f04f..77790aa 100644
--- a/libfetch/ftp.c
+++ b/libfetch/ftp.c
@@ -471,8 +471,7 @@ ftp_stat(conn_t *conn, const char *file, struct url_stat *us)
 	}
 	for (ln = conn->buf + 4; *ln && isspace((unsigned char)*ln); ln++)
 		/* nothing */ ;
-	for (us->size = 0; *ln && isdigit((unsigned char)*ln); ln++)
-		us->size = us->size * 10 + *ln - '0';
+	us->size = fetch_parseuint(ln, (const char **) &ln, 10, OFF_MAX);
 	if (*ln && !isspace((unsigned char)*ln)) {
 		ftp_seterr(FTP_PROTOCOL_ERROR);
 		us->size = -1;
@@ -700,7 +699,7 @@ retry_mode:
 
 	if (pasv) {
 		unsigned char addr[64];
-		char *ln, *p;
+		const char *ln, *p;
 		unsigned int i;
 		int port;
 
@@ -737,10 +736,15 @@ retry_mode:
 			for (p = ln + 3; *p && !isdigit((unsigned char)*p); p++)
 				/* nothing */ ;
 			if (!*p) goto protocol_error;
-			l = (e == FTP_PASSIVE_MODE ? 6 : 21);
-			for (i = 0; *p && i < l; i++, p++)
-				addr[i] = strtol(p, &p, 10);
-			if (i < l) goto protocol_error;
+			l = (e == FTP_PASSIVE_MODE ? 6 : 21) - 1;
+			for (i = 0; *p && i < l; i++, p++) {
+				while (isspace((unsigned char)*p)) p++;
+				addr[i] = fetch_parseuint(p, &p, 10, UCHAR_MAX);
+				if (*p != ',') goto protocol_error;
+			}
+			while (isspace((unsigned char)*p)) p++;
+			addr[i] = fetch_parseuint(p, &p, 10, UCHAR_MAX);
+			if (*p && *p != ')') goto protocol_error;
 			break;
 		case FTP_EPASSIVE_MODE:
 			for (p = ln + 3; *p && *p != '('; p++)
diff --git a/libfetch/http.c b/libfetch/http.c
index 59d6292..abc3ae6 100644
--- a/libfetch/http.c
+++ b/libfetch/http.c
@@ -134,29 +134,19 @@ struct httpio
 static int
 http_new_chunk(struct httpio *io)
 {
-	char *p;
+	const char *p;
 
 	if (fetch_getln(io->conn) == -1)
-		return (-1);
+		return -1;
 
-	if (io->conn->buflen < 2 || !isxdigit((unsigned char)*io->conn->buf))
-		return (-1);
+	if (io->conn->buflen < 2)
+		return -1;
 
-	for (p = io->conn->buf; *p && !isspace((unsigned char)*p); ++p) {
-		if (*p == ';')
-			break;
-		if (!isxdigit((unsigned char)*p))
-			return (-1);
-		if (isdigit((unsigned char)*p)) {
-			io->chunksize = io->chunksize * 16 +
-			    *p - '0';
-		} else {
-			io->chunksize = io->chunksize * 16 +
-			    10 + tolower((unsigned char)*p) - 'a';
-		}
-	}
+	io->chunksize = fetch_parseuint(io->conn->buf, &p, 16, SIZE_MAX);
+	if (*p && *p != ';' && !isspace(*p))
+		return -1;
 
-	return (io->chunksize);
+	return io->chunksize;
 }
 
 /*
@@ -501,22 +491,6 @@ http_parse_mtime(const char *p, time_t *mtime)
 	return (0);
 }
 
-/*
- * Parse a content-length header
- */
-static int
-http_parse_length(const char *p, off_t *length)
-{
-	off_t len;
-
-	for (len = 0; *p && isdigit((unsigned char)*p); ++p)
-		len = len * 10 + (*p - '0');
-	if (*p)
-		return (-1);
-	*length = len;
-	return (0);
-}
-
 /*
  * Parse a content-range header
  */
@@ -532,17 +506,14 @@ http_parse_range(const char *p, off_t *offset, off_t *length, off_t *size)
 		first = last = -1;
 		++p;
 	} else {
-		for (first = 0; *p && isdigit((unsigned char)*p); ++p)
-			first = first * 10 + *p - '0';
+		first = fetch_parseuint(p, &p, 10, OFF_MAX);
 		if (*p != '-')
 			return (-1);
-		for (last = 0, ++p; *p && isdigit((unsigned char)*p); ++p)
-			last = last * 10 + *p - '0';
+		last = fetch_parseuint(p+1, &p, 10, OFF_MAX);
 	}
 	if (first > last || *p != '/')
 		return (-1);
-	for (len = 0, ++p; *p && isdigit((unsigned char)*p); ++p)
-		len = len * 10 + *p - '0';
+	len = fetch_parseuint(p+1, &p, 10, OFF_MAX);
 	if (*p || len < last - first + 1)
 		return (-1);
 	if (first == -1)
@@ -850,7 +821,7 @@ http_request(struct url *URL, const char *op, struct url_stat *us,
 	int e, i, n;
 	off_t offset, clength, length, size;
 	time_t mtime;
-	const char *p;
+	const char *p, *q;
 	fetchIO *f;
 	hdr_t h;
 	char hbuf[URL_HOSTLEN + 7], *host;
@@ -1050,13 +1021,16 @@ http_request(struct url *URL, const char *op, struct url_stat *us,
 				keep_alive = (strcasecmp(p, "keep-alive") == 0);
 				break;
 			case hdr_content_length:
-				http_parse_length(p, &clength);
+				clength = fetch_parseuint(p, &q, 10, OFF_MAX);
+				if (*q) goto protocol_error;
 				break;
 			case hdr_content_range:
-				http_parse_range(p, &offset, &length, &size);
+				if (http_parse_range(p, &offset, &length, &size) < 0)
+					goto protocol_error;
 				break;
 			case hdr_last_modified:
-				http_parse_mtime(p, &mtime);
+				if (http_parse_mtime(p, &mtime) < 0)
+					goto protocol_error;
 				break;
 			case hdr_location:
 				if (!HTTP_REDIRECT(conn->err))