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Re: [PATCH 1 of 2] Rewritten host header validation to follow generic parsing rules

J Carter
June 15, 2024 08:44PM
Hello Sergey,

I had some trouble importing your patches in one go due to missing
whitespace after patches 2 and 3. In case anyone else has the same
issue just add 1 new line after each of those patches.

On Fri, 7 Jun 2024 20:48:23 +0400
Sergey Kandaurov <pluknet@nginx.com> wrote:

> On Tue, May 28, 2024 at 12:53:46PM +0100, J Carter wrote:
> > Hello Sergey,
> >
> > On Mon, 27 May 2024 14:21:43 +0400
> > Sergey Kandaurov <pluknet@nginx.com> wrote:
> >
> > > # HG changeset patch
> > > # User Sergey Kandaurov <pluknet@nginx.com>
> > > # Date 1716805272 -14400
> > > # Mon May 27 14:21:12 2024 +0400
> > > # Node ID e82a7318ed48fdbc1273771bc96357e9dc232975
> > > # Parent f58b6f6362387eeace46043a6fc0bceb56a6786a
> > > Rewritten host header validation to follow generic parsing rules.
> > >
> > > It now uses a generic model of state-based machine, with more strict
> > > parsing rules borrowed from ngx_http_validate_host(),
> >
> > I think you mean "borrowed from ngx_http_parse_request_line()".
> >
>
> Sure, tnx.
>
> The problem is that both functions make a subset of parsing
> and validation, and these sets just partially intersect.
> In particular, ngx_http_parse_request_line() currently detects
> invalid characters in a port subcomponent of authority, and
> ngx_http_validate_host() handles a trailing dot.
> So I think it makes sense to make them unifined, this will also
> remove the need to validate host in absolute URIs.
> Below is an updated version with both parsers further polished
> (stream part is excluded for now).
>
> Also, it may have sense to rename ngx_http_validate_host()
> to something like ngx_http_parse_host(), similar to
> ngx_http_parse_uri(), out of this series.

Makes sense.

>
> # HG changeset patch
> # User Sergey Kandaurov <pluknet@nginx.com>
> # Date 1717777582 -14400
> # Fri Jun 07 20:26:22 2024 +0400
> # Node ID 0cba4301e4980871de7aceb46acddf8f2b5a7318
> # Parent 02e9411009b987f408214ab4a8b6b6093f843bcd
> Improved parsing of host in absolute URIs.
>
> When the request line is in the absolute-URI form, a host identified
> by a registered name (reg-name) is now restricted to start with an
> alphanumeric character (see RFC 1123, RFC 3986). Previously, empty
> domain labels or host starting with a hyphen were accepted.
>
> Additionally, host with a trailing dot is taken into account.
>
> diff --git a/src/http/ngx_http_parse.c b/src/http/ngx_http_parse.c
> --- a/src/http/ngx_http_parse.c
> +++ b/src/http/ngx_http_parse.c
> @@ -113,8 +113,10 @@ ngx_http_parse_request_line(ngx_http_req
> sw_schema_slash_slash,
> sw_host_start,
> sw_host,
> + sw_host_dot,
> sw_host_end,
> sw_host_ip_literal,
> + sw_host_ip_literal_dot,
> sw_port,
> sw_after_slash_in_uri,
> sw_check_uri,
> @@ -354,27 +356,50 @@ ngx_http_parse_request_line(ngx_http_req
> break;
> }
>
> - state = sw_host;
> + if (ch == '.' || ch == '-') {
> + return NGX_HTTP_PARSE_INVALID_REQUEST;

One inconsistency I noticed while testing these patches is the difference
in the usefulness of error logs between invalid host header vs invalid
host value in absolute uri.

Absolute uri parsing simply includes a vague "error while parsing
client request line" for everything, including failed host checks,
whereas host header failure error is naturally more specific to what
the error is. Perhaps improving that is beyond scope of this series
however.

> + }
>
> /* fall through */
>
> case sw_host:
> + case sw_host_dot:
> +
> + if (ch == '.') {
> + if (state == sw_host_dot) {
> + return NGX_HTTP_PARSE_INVALID_REQUEST;
> + }
> +
> + state = sw_host_dot;
> + break;
> + }
>
> c = (u_char) (ch | 0x20);
> if (c >= 'a' && c <= 'z') {
> + state = sw_host;
> + break;
> + }
> +
> + if ((ch >= '0' && ch <= '9') || ch == '-') {
> + state = sw_host;
> break;
> }
>
> - if ((ch >= '0' && ch <= '9') || ch == '.' || ch == '-') {
> - break;
> + if (state == sw_host_start) {
> + return NGX_HTTP_PARSE_INVALID_REQUEST;
> + }
> +
> + if (state == sw_host_dot) {
> + r->host_end = p - 1;
> +
> + } else {
> + r->host_end = p;
> }
>
> /* fall through */
>
> case sw_host_end:
>
> - r->host_end = p;
> -
> switch (ch) {
> case ':':
> state = sw_port;
> @@ -404,6 +429,18 @@ ngx_http_parse_request_line(ngx_http_req
> break;
>
> case sw_host_ip_literal:
> + case sw_host_ip_literal_dot:
> +
> + if (ch == '.') {
> + if (state == sw_host_ip_literal_dot) {
> + return NGX_HTTP_PARSE_INVALID_REQUEST;
> + }
> +
> + state = sw_host_ip_literal_dot;
> + break;
> + }
> +
> + state = sw_host_ip_literal;
>
> if (ch >= '0' && ch <= '9') {
> break;
> @@ -418,10 +455,10 @@ ngx_http_parse_request_line(ngx_http_req
> case ':':
> break;
> case ']':
> + r->host_end = p + 1;
> state = sw_host_end;
> break;
> case '-':
> - case '.':
> case '_':
> case '~':
> /* unreserved */
> # HG changeset patch
> # User Sergey Kandaurov <pluknet@nginx.com>
> # Date 1717777646 -14400
> # Fri Jun 07 20:27:26 2024 +0400
> # Node ID 3f7ac1d90a6d4eceabaa5ce45dabf53efd99ed67
> # Parent 0cba4301e4980871de7aceb46acddf8f2b5a7318
> Rewritten host validation to match host parsing in absolute URIs.
>
> It is reimplemented based on ngx_http_parse_request_line() state machine.
> This introduces several changes, in particular:
> - host name with underscores is rejected;
> - a port subcomponent is restricted to digits;
> - for IP literals, a missing closing bracket and trailing dot are detected.
>

I see the changes in this patch cause a fail on 'double port hack'
test from http_host.t. I'm not sure on the context behind that test,
as it's quite an odd thing to test for (perhaps a workaround for something?)

The author of that test was Valentin Bartenev, perhaps he will kindly
let us know the context if he remembers, and we do not.

> diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
> --- a/src/http/ngx_http_request.c
> +++ b/src/http/ngx_http_request.c
> @@ -2145,72 +2145,157 @@ ngx_int_t
> ngx_http_validate_host(ngx_str_t *host, ngx_pool_t *pool, ngx_uint_t alloc)
> {
> u_char *h, ch;
> - size_t i, dot_pos, host_len;
> + size_t i, host_len;
>
> enum {
> - sw_usual = 0,
> - sw_literal,
> - sw_rest
> + sw_host_start = 0,
> + sw_host,
> + sw_host_dot,
> + sw_host_end,
> + sw_host_ip_literal,
> + sw_host_ip_literal_dot,
> + sw_port,
> } state;
>
> - dot_pos = host->len;
> host_len = host->len;
>
> h = host->data;
>
> - state = sw_usual;
> + state = sw_host_start;
>
> for (i = 0; i < host->len; i++) {
> ch = h[i];
>
> - switch (ch) {
> -
> - case '.':
> - if (dot_pos == i - 1) {
> + switch (state) {
> +
> + case sw_host_start:
> +
> + if (ch == '[') {
> + state = sw_host_ip_literal;
> + break;
> + }
> +
> + if (ch == '.' || ch == '-') {
> return NGX_DECLINED;
> }
> - dot_pos = i;
> - break;
> -
> - case ':':
> - if (state == sw_usual) {
> - host_len = i;
> - state = sw_rest;
> - }
> - break;
> -
> - case '[':
> - if (i == 0) {
> - state = sw_literal;
> - }
> - break;
> -
> - case ']':
> - if (state == sw_literal) {
> - host_len = i + 1;
> - state = sw_rest;
> - }
> - break;
> -
> - default:
> -
> - if (ngx_path_separator(ch)) {
> - return NGX_DECLINED;
> - }
> -
> - if (ch <= 0x20 || ch == 0x7f) {
> - return NGX_DECLINED;
> +
> + /* fall through */
> +
> + case sw_host:
> + case sw_host_dot:
> +
> + if (ch == '.') {
> + if (state == sw_host_dot) {
> + return NGX_DECLINED;
> + }
> +
> + state = sw_host_dot;
> + break;
> }
>
> if (ch >= 'A' && ch <= 'Z') {
> alloc = 1;
> + state = sw_host;
> + break;
> }
>
> + if (ch >= 'a' && ch <= 'z') {
> + state = sw_host;
> + break;
> + }
> +
> + if ((ch >= '0' && ch <= '9') || ch == '-') {
> + state = sw_host;
> + break;
> + }
> +
> + if (state == sw_host_dot) {
> + host_len = i - 1;
> +
> + } else {
> + host_len = i;
> + }
> +
> + /* fall through */
> +
> + case sw_host_end:
> +
> + if (ch == ':') {
> + state = sw_port;
> + break;
> + }
> + return NGX_DECLINED;
> +
> + case sw_host_ip_literal:
> + case sw_host_ip_literal_dot:
> +
> + if (ch == '.') {
> + if (state == sw_host_ip_literal_dot) {
> + return NGX_DECLINED;
> + }
> +
> + state = sw_host_ip_literal_dot;
> + break;
> + }
> +
> + state = sw_host_ip_literal;
> +
> + if (ch >= 'A' && ch <= 'Z') {
> + alloc = 1;
> + break;
> + }
> +
> + if (ch >= 'a' && ch <= 'z') {
> + break;
> + }
> +
> + if (ch >= '0' && ch <= '9') {
> + break;
> + }
> +
> + switch (ch) {
> + case ':':
> + break;
> + case ']':
> + host_len = i + 1;
> + state = sw_host_end;
> + break;
> + case '-':
> + case '_':
> + case '~':
> + /* unreserved */
> + break;
> + case '!':
> + case '$':
> + case '&':
> + case '\'':
> + case '(':
> + case ')':
> + case '*':
> + case '+':
> + case ',':
> + case ';':
> + case '=':
> + /* sub-delims */
> + break;
> + default:
> + return NGX_DECLINED;
> + }
> break;
> +
> + case sw_port:
> + if (ch >= '0' && ch <= '9') {
> + break;
> + }

Although likely beyond scope of this series, it does seem a bit
inconsistent to reject request for non-numeric characters in port, while
not also doing so for obviously incorrect port values.

For example: Host: 127.0.0.1:77777 is accepted.

> + return NGX_DECLINED;
> }
> }
>
> - if (dot_pos == host_len - 1) {
> + if (state == sw_host_ip_literal) {
> + return NGX_DECLINED;
> + }
> +
> + if (h[host_len - 1] == '.') {
> host_len--;
> }
>
> # HG changeset patch
> # User Sergey Kandaurov <pluknet@nginx.com>
> # Date 1717777737 -14400
> # Fri Jun 07 20:28:57 2024 +0400
> # Node ID 722bcffe3d3c9ff4314a2813227c47dc5eff660e
> # Parent 3f7ac1d90a6d4eceabaa5ce45dabf53efd99ed67
> Skip host validation in absolute URIs.
>
> Now that parsing of host in the absolute-URI form and of the host header
> were made the same in previous changes, it makes no sense to validate
> host once again. Only host case normalization to lowercase is applied
> (RFC 3986, 6.2.2.1) after parsing absolute URI as this is out of scope.
>
> No functional changes intended.
>
> diff --git a/src/http/ngx_http_parse.c b/src/http/ngx_http_parse.c
> --- a/src/http/ngx_http_parse.c
> +++ b/src/http/ngx_http_parse.c
> @@ -374,8 +374,13 @@ ngx_http_parse_request_line(ngx_http_req
> break;
> }
>
> - c = (u_char) (ch | 0x20);
> - if (c >= 'a' && c <= 'z') {
> + if (ch >= 'A' && ch <= 'Z') {
> + r->host_normalize = 1;
> + state = sw_host;
> + break;
> + }
> +
> + if (ch >= 'a' && ch <= 'z') {
> state = sw_host;
> break;
> }
> @@ -446,8 +451,12 @@ ngx_http_parse_request_line(ngx_http_req
> break;
> }
>
> - c = (u_char) (ch | 0x20);
> - if (c >= 'a' && c <= 'z') {
> + if (ch >= 'A' && ch <= 'Z') {
> + r->host_normalize = 1;
> + break;
> + }
> +
> + if (ch >= 'a' && ch <= 'z') {
> break;
> }
>
> diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
> --- a/src/http/ngx_http_request.c
> +++ b/src/http/ngx_http_request.c
> @@ -1147,18 +1147,15 @@ ngx_http_process_request_line(ngx_event_
> host.len = r->host_end - r->host_start;
> host.data = r->host_start;
>
> - rc = ngx_http_validate_host(&host, r->pool, 0);
> -
> - if (rc == NGX_DECLINED) {
> - ngx_log_error(NGX_LOG_INFO, c->log, 0,
> - "client sent invalid host in request line");
> - ngx_http_finalize_request(r, NGX_HTTP_BAD_REQUEST);
> - break;
> - }
> -
> - if (rc == NGX_ERROR) {
> - ngx_http_close_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
> - break;
> + if (r->host_normalize) {
> + host.data = ngx_pnalloc(r->pool, host.len);
> + if (host.data == NULL) {
> + ngx_http_close_request(r,
> + NGX_HTTP_INTERNAL_SERVER_ERROR);
> + break;
> + }
> +
> + ngx_strlow(host.data, r->host_start, host.len);
> }
>
> if (ngx_http_set_virtual_server(r, &host) == NGX_ERROR) {
> diff --git a/src/http/ngx_http_request.h b/src/http/ngx_http_request.h
> --- a/src/http/ngx_http_request.h
> +++ b/src/http/ngx_http_request.h
> @@ -466,6 +466,9 @@ struct ngx_http_request_s {
>
> unsigned http_state:4;
>
> + /* host with upper case */
> + unsigned host_normalize:1;
> +
> /* URI with "/." and on Win32 with "//" */
> unsigned complex_uri:1;
>

Functionally everything appears to be correct in these patches.

I do have a question about real world impact of these changes
particularly around underscore in Host (following on from concerns
raised by others out-of-band). Do we have any statistics on the use of
underscores in Host, today, on the open internet?

If not, is it worth attempting to source that information before
making these changes to better assess the impact / if 'no underscore in
host header' enforcement should be optional, behind a flag?
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Subject Author Views Posted

[PATCH 1 of 2] Rewritten host header validation to follow generic parsing rules

Sergey Kandaurov 265 May 27, 2024 06:22AM

[PATCH 2 of 2] Stream: do not reallocate a parsed SNI host

Sergey Kandaurov 51 May 27, 2024 06:22AM

Re: [PATCH 2 of 2] Stream: do not reallocate a parsed SNI host

Roman Arutyunyan 58 May 30, 2024 12:50PM

Re: [PATCH 2 of 2] Stream: do not reallocate a parsed SNI host

Sergey Kandaurov 44 June 06, 2024 10:34AM

Re: [PATCH 1 of 2] Rewritten host header validation to follow generic parsing rules

J Carter 67 May 28, 2024 07:56AM

Re: [PATCH 1 of 2] Rewritten host header validation to follow generic parsing rules

Sergey Kandaurov 41 June 07, 2024 12:50PM

Re: [PATCH 1 of 2] Rewritten host header validation to follow generic parsing rules

J Carter 39 June 15, 2024 08:44PM



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