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Sanitize redirect URLs to prevent XSS and header injection #101

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46 changes: 43 additions & 3 deletions darkhttpd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1728,27 +1728,66 @@ static void default_reply(struct connection *conn,
conn->reply_start = 0;
}

static char *html_escape(const char *src) {
size_t i, len = strlen(src), alloc = len;
char *out;
for (i = 0; i < len; i++) {
switch (src[i]) {
case '<': case '>': alloc += 3; break;
case '&': alloc += 4; break;
case '"': case '\'': alloc += 5; break;
}
}
out = xmalloc(alloc + 1);
size_t pos = 0;
for (i = 0; i < len; i++) {
switch (src[i]) {
case '<': memcpy(out+pos, "&lt;", 4); pos += 4; break;
case '>': memcpy(out+pos, "&gt;", 4); pos += 4; break;
case '&': memcpy(out+pos, "&amp;", 5); pos += 5; break;
case '"': memcpy(out+pos, "&quot;", 6); pos += 6; break;
case '\'': memcpy(out+pos, "&apos;", 6); pos += 6; break;
default: out[pos++] = src[i]; break;
}
}
out[pos] = '\0';
return out;
}

static void strip_crlf(char *s) {
size_t r = 0, w = 0;
while (s[r] != '\0') {
if (s[r] != '\r' && s[r] != '\n')
s[w++] = s[r];
r++;
}
s[w] = '\0';
}

static void redirect(struct connection *conn, const char *format, ...)
__printflike(2, 3);
static void redirect(struct connection *conn, const char *format, ...) {
char *where, date[DATE_LEN];
char *where, *where_escaped, date[DATE_LEN];
va_list va;

va_start(va, format);
xvasprintf(&where, format, va);
va_end(va);

strip_crlf(where);
where_escaped = html_escape(where);

/* Only really need to calculate the date once. */
rfc1123_date(date, now);

conn->reply_length = xasprintf(&(conn->reply),
"<!DOCTYPE html><html><head><title>301 Moved Permanently</title></head><body>\n"
"<h1>Moved Permanently</h1>\n"
"Moved to: <a href=\"%s\">%s</a>\n" /* where x 2 */
"Moved to: <a href=\"%s\">%s</a>\n" /* where_escaped x 2 */

@emikulic emikulic Aug 22, 2026

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Should the href="..." be url-escaped?

"<hr>\n"
"%s" /* generated on */
"</body></html>\n",
where, where, generated_on(date));
where_escaped, where_escaped, generated_on(date));

conn->header_length = xasprintf(&(conn->header),
"HTTP/1.1 301 Moved Permanently\r\n"
Expand All @@ -1764,6 +1803,7 @@ static void redirect(struct connection *conn, const char *format, ...) {
date, server_hdr, where, keep_alive(conn),
custom_hdrs, llu(conn->reply_length));

free(where_escaped);
free(where);
conn->reply_type = REPLY_GENERATED;
conn->http_code = 301;
Expand Down