[PATCH] http: match headers case-insensitively when redacting

When HTTP/2 is in use, we fail to correctly redact "Authorization" (and other) headers in our GIT_TRACE_CURL output.

We get the headers in our CURLOPT_DEBUGFUNCTION callback, curl_trace(). It passes them along to curl_dump_header(), which in turn checks redact_sensitive_header(). We see the headers as a text buffer like:

  Host: ...
  Authorization: Basic ...

After breaking it into lines, we match each header using skip_prefix(). This is case-insensitive, even though HTTP headers are case-insensitive. This has worked reliably in the past because these headers are generated by curl itself, which is predictable in what it sends.

But when HTTP/2 is in use, instead we get a lower-case "authorization:" header, and we fail to match it. The fix is simple: we should match with skip_iprefix().

Testing is more complicated, though. We do have a test for the redacting feature, but we don't hit the problem case because our test Apache setup does not understand HTTP/2. You can reproduce the issue by applying this on top of the test change in this patch:

	diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
	index afa91e38b0..19267c7107 100644
	--- a/t/lib-httpd/apache.conf
	+++ b/t/lib-httpd/apache.conf
	@@ -29,6 +29,9 @@ ErrorLog error.log
	 	LoadModule setenvif_module modules/mod_setenvif.so
	 </IfModule>

	+LoadModule http2_module modules/mod_http2.so
	+Protocols h2c
	+
	 <IfVersion < 2.4>
	 LockFile accept.lock
	 </IfVersion>
	@@ -64,8 +67,8 @@ LockFile accept.lock
	 <IfModule !mod_access_compat.c>
	 	LoadModule access_compat_module modules/mod_access_compat.so
	 </IfModule>
	-<IfModule !mod_mpm_prefork.c>
	-	LoadModule mpm_prefork_module modules/mod_mpm_prefork.so
	+<IfModule !mod_mpm_event.c>
	+	LoadModule mpm_event_module modules/mod_mpm_event.so
	 </IfModule>
	 <IfModule !mod_unixd.c>
	 	LoadModule unixd_module modules/mod_unixd.so
	diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
	index 1c2a444ae7..ff74f0ae8a 100755
	--- a/t/t5551-http-fetch-smart.sh
	+++ b/t/t5551-http-fetch-smart.sh
	@@ -24,6 +24,10 @@ test_expect_success 'create http-accessible bare repository' '
	 	git push public main:main
	 '

	+test_expect_success 'prefer http/2' '
	+	git config --global http.version HTTP/2
	+'
	+
	 setup_askpass_helper

	 test_expect_success 'clone http repository' '

but this has a few issues:

  - it's not necessarily portable. The http2 apache module might not be
    available on all systems. Further, the http2 module isn't compatible
    with the prefork mpm, so we have to switch to something else. But we
    don't necessarily know what's available. It would be nice if we
    could have conditional config, but IfModule only tells us if a
    module is already loaded, not whether it is available at all.

    This might be a non-issue. The http tests are already optional, and
    modern-enough systems may just have both of these. But...

  - if we do this, then we'd no longer be testing HTTP/1.1 at all. I'm
    not sure how much that matters since it's all handled by curl under
    the hood, but I'd worry that some detail leaks through. We'd
    probably want two scripts running similar tests, one with HTTP/2 and
    one with HTTP/1.1.

  - speaking of which, a later test fails with the patch above! The
    problem is that it is making sure we used a chunked
    transfer-encoding by looking for that header in the trace. But
    HTTP/2 doesn't support that, as it has its own streaming mechanisms
    (the overall operation works fine; we just don't see the header in
    the trace).

On top of that, we also need the test change that this patch _does_ do: grepping the trace file case-insensitively. Otherwise the test continues to pass even over HTTP/2, because it sees _both_ forms of the header (redacted and unredacted), as we upgrade from HTTP/1.1 to HTTP/2. So our double grep:

	# Ensure that there is no "Basic" followed by a base64 string, but that
	# the auth details are redacted
	! grep "Authorization: Basic [0-9a-zA-Z+/]" trace &&
	grep "Authorization: Basic <redacted>" trace

gets confused. It sees the "<redacted>" one from the pre-upgrade HTTP/1.1 request, but fails to see the unredacted HTTP/2 one, because it does not match the lower-case "authorization". Even without the rest of the test changes, we can still make this test more robust by matching case-insensitively. That will future-proof the test for a day when HTTP/2 is finally enabled by default, and doesn't hurt in the meantime.

And finally, there's one other way to demonstrate the issue (and how I actually found it originally). Looking at GIT_TRACE_CURL output against github.com, you'll see the unredacted output, even if you didn't set http.version. That's because setting it is only necessary for curl to send the extra headers in its HTTP/1.1 request that say "Hey, I speak HTTP/2; upgrade if you do, too". But for a production site speaking https, the server advertises via ALPN, a TLS extension, that it supports HTTP/2, and the client can immediately start using it.

Signed-off-by: Jeff King <peff@peff.net>
---
 http.c                      |  6 +++---
 t/t5551-http-fetch-smart.sh | 24 ++++++++++++------------
 2 files changed, 15 insertions(+), 15 deletions(-)