[PATCH v2 4/6] http: make redirects more obvious
We instruct curl to always follow HTTP redirects. This is convenient, but it creates opportunities for malicious servers to create confusing situations. For instance, imagine Alice is a git user with access to a private repository on Bob's server. Mallory runs her own server and wants to access objects from Bob's repository.
Mallory may try a few tricks that involve asking Alice to clone from her, build on top, and then push the result:
1. Mallory may simply redirect all fetch requests to Bob's
server. Git will transparently follow those redirects
and fetch Bob's history, which Alice may believe she
got from Mallory. The subsequent push seems like it is
just feeding Mallory back her own objects, but is
actually leaking Bob's objects. There is nothing in
git's output to indicate that Bob's repository was
involved at all. The downside (for Mallory) of this attack is that Alice
will have received Bob's entire repository, and is
likely to notice that when building on top of it. 2. If Mallory happens to know the sha1 of some object X in
Bob's repository, she can instead build her own history
that references that object. She then runs a dumb http
server, and Alice's client will fetch each object
individually. When it asks for X, Mallory redirects her
to Bob's server. The end result is that Alice obtains
objects from Bob, but they may be buried deep in
history. Alice is less likely to notice.Both of these attacks are fairly hard to pull off. There's a social component in getting Mallory to convince Alice to work with her. Alice may be prompted for credentials in accessing Bob's repository (but not always, if she is using a credential helper that caches). Attack (1) requires a certain amount of obliviousness on Alice's part while making a new commit. Attack (2) requires that Mallory knows a sha1 in Bob's repository, that Bob's server supports dumb http, and that the object in question is loose on Bob's server.
But we can probably make things a bit more obvious without any loss of functionality. This patch does two things to that end.
First, when we encounter a whole-repo redirect during the initial ref discovery, we now inform the user on stderr, making attack (1) much more obvious.
Second, the decision to follow redirects is now configurable. The truly paranoid can set the new http.followRedirects to false to avoid any redirection entirely. But for a more practical default, we will disallow redirects only after the initial ref discovery. This is enough to thwart attacks similar to (2), while still allowing the common use of redirects at the repository level. Since c93c92f30 (http: update base URLs when we see redirects, 2013-09-28) we re-root all further requests from the redirect destination, which should generally mean that no further redirection is necessary.
As an escape hatch, in case there really is a server that needs to redirect individual requests, the user can set http.followRedirects to "true" (and this can be done on a per-server basis via http.*.followRedirects config).
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jeff King <peff@peff.net>
---
Documentation/config.txt | 10 ++++++++++
http.c | 31 +++++++++++++++++++++++++++++--
http.h | 10 +++++++++-
remote-curl.c | 4 ++++
t/lib-httpd/apache.conf | 6 ++++++
t/t5550-http-fetch-dumb.sh | 23 +++++++++++++++++++++++
6 files changed, 81 insertions(+), 3 deletions(-)
Show changes to 6 files +81 −3
Documentation/config.txt, http.c, http.h, remote-curl.c, t/lib-httpd/apache.conf, t/t5550-http-fetch-dumb.sh
diff --git a/Documentation/config.txt b/Documentation/config.txt
index f4721a048..815333643 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1833,6 +1833,16 @@ http.userAgent::
of common USER_AGENT strings (but not including those like git/1.7.1).
Can be overridden by the `GIT_HTTP_USER_AGENT` environment variable.
+http.followRedirects::
+ Whether git should follow HTTP redirects. If set to `true`, git
+ will transparently follow any redirect issued by a server it
+ encounters. If set to `false`, git will treat all redirects as
+ errors. If set to `initial`, git will follow redirects only for
+ the initial request to a remote, but not for subsequent
+ follow-up HTTP requests. Since git uses the redirected URL as
+ the base for the follow-up requests, this is generally
+ sufficient. The default is `initial`.
+
http.<url>.*::
Any of the http.* options above can be applied selectively to some URLs.
For a config key to match a URL, each element of the config key is
diff --git a/http.c b/http.c
index 718d2109b..b99ade5fa 100644
--- a/http.c
+++ b/http.c
@@ -98,6 +98,8 @@ static int http_proactive_auth;
static const char *user_agent;
static int curl_empty_auth;
+enum http_follow_config http_follow_config = HTTP_FOLLOW_INITIAL;
+
#if LIBCURL_VERSION_NUM >= 0x071700
/* Use CURLOPT_KEYPASSWD as is */
#elif LIBCURL_VERSION_NUM >= 0x070903
@@ -337,6 +339,16 @@ static int http_options(const char *var, const char *value, void *cb)
return 0;
}
+ if (!strcmp("http.followredirects", var)) {
+ if (value && !strcmp(value, "initial"))
+ http_follow_config = HTTP_FOLLOW_INITIAL;
+ else if (git_config_bool(var, value))
+ http_follow_config = HTTP_FOLLOW_ALWAYS;
+ else
+ http_follow_config = HTTP_FOLLOW_NONE;
+ return 0;
+ }
+
/* Fall back on the default ones */
return git_default_config(var, value, cb);
}
@@ -553,7 +565,6 @@ static CURL *get_curl_handle(void)
curl_low_speed_time);
}
- curl_easy_setopt(result, CURLOPT_FOLLOWLOCATION, 1);
curl_easy_setopt(result, CURLOPT_MAXREDIRS, 20);
#if LIBCURL_VERSION_NUM >= 0x071301
curl_easy_setopt(result, CURLOPT_POSTREDIR, CURL_REDIR_POST_ALL);
@@ -882,6 +893,16 @@ struct active_request_slot *get_active_slot(void)
curl_easy_setopt(slot->curl, CURLOPT_FAILONERROR, 1);
curl_easy_setopt(slot->curl, CURLOPT_RANGE, NULL);
+ /*
+ * Default following to off unless "ALWAYS" is configured; this gives
+ * callers a sane starting point, and they can tweak for individual
+ * HTTP_FOLLOW_* cases themselves.
+ */
+ if (http_follow_config == HTTP_FOLLOW_ALWAYS)
+ curl_easy_setopt(slot->curl, CURLOPT_FOLLOWLOCATION, 1);
+ else
+ curl_easy_setopt(slot->curl, CURLOPT_FOLLOWLOCATION, 0);
+
#if LIBCURL_VERSION_NUM >= 0x070a08
curl_easy_setopt(slot->curl, CURLOPT_IPRESOLVE, git_curl_ipresolve);
#endif
@@ -1122,9 +1143,12 @@ static int handle_curl_result(struct slot_results *results)
* If we see a failing http code with CURLE_OK, we have turned off
* FAILONERROR (to keep the server's custom error response), and should
* translate the code into failure here.
+ *
+ * Likewise, if we see a redirect (30x code), that means we turned off
+ * redirect-following, and we should treat the result as an error.
*/
if (results->curl_result == CURLE_OK &&
- results->http_code >= 400) {
+ results->http_code >= 300) {
results->curl_result = CURLE_HTTP_RETURNED_ERROR;
/*
* Normally curl will already have put the "reason phrase"
@@ -1443,6 +1467,9 @@ static int http_request(const char *url,
strbuf_addstr(&buf, " no-cache");
if (options && options->keep_error)
curl_easy_setopt(slot->curl, CURLOPT_FAILONERROR, 0);
+ if (options && options->initial_request &&
+ http_follow_config == HTTP_FOLLOW_INITIAL)
+ curl_easy_setopt(slot->curl, CURLOPT_FOLLOWLOCATION, 1);
headers = curl_slist_append(headers, buf.buf);
diff --git a/http.h b/http.h
index 36f558bfb..31b4cc94b 100644
--- a/http.h
+++ b/http.h
@@ -116,6 +116,13 @@ extern struct credential http_auth;
extern char curl_errorstr[CURL_ERROR_SIZE];
+enum http_follow_config {
+ HTTP_FOLLOW_NONE,
+ HTTP_FOLLOW_ALWAYS,
+ HTTP_FOLLOW_INITIAL
+};
+extern enum http_follow_config http_follow_config;
+
static inline int missing__target(int code, int result)
{
return /* file:// URL -- do we ever use one??? */
@@ -139,7 +146,8 @@ extern char *get_remote_object_url(const char *url, const char *hex,
/* Options for http_get_*() */
struct http_get_options {
unsigned no_cache:1,
- keep_error:1;
+ keep_error:1,
+ initial_request:1;
/* If non-NULL, returns the content-type of the response. */
struct strbuf *content_type;
diff --git a/remote-curl.c b/remote-curl.c
index e3803daa3..05ae8dead 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -276,6 +276,7 @@ static struct discovery *discover_refs(const char *service, int for_push)
http_options.charset = &charset;
http_options.effective_url = &effective_url;
http_options.base_url = &url;
+ http_options.initial_request = 1;
http_options.no_cache = 1;
http_options.keep_error = 1;
@@ -294,6 +295,9 @@ static struct discovery *discover_refs(const char *service, int for_push)
die("unable to access '%s': %s", url.buf, curl_errorstr);
}
+ if (options.verbosity && !starts_with(refs_url.buf, url.buf))
+ warning(_("redirecting to %s"), url.buf);
+
last= xcalloc(1, sizeof(*last_discovery));
last->service = service;
last->buf_alloc = strbuf_detach(&buffer, &last->len);
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index 9a355fb1c..5b408d649 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -123,6 +123,7 @@ ScriptAlias /error/ error.sh/
</Files>
RewriteEngine on
+RewriteRule ^/dumb-redir/(.*)$ /dumb/$1 [R=301]
RewriteRule ^/smart-redir-perm/(.*)$ /smart/$1 [R=301]
RewriteRule ^/smart-redir-temp/(.*)$ /smart/$1 [R=302]
RewriteRule ^/smart-redir-auth/(.*)$ /auth/smart/$1 [R=301]
@@ -140,6 +141,11 @@ RewriteRule ^/loop-redir/(.*)$ /loop-redir/x-$1 [R=302]
RewriteRule ^/insane-redir/(.*)$ /intern-redir/$1/foo [R=301]
RewriteRule ^/intern-redir/(.*)/foo$ /smart/$1 [PT]
+# Serve info/refs internally without redirecting, but
+# issue a redirect for any object requests.
+RewriteRule ^/redir-objects/(.*/info/refs)$ /dumb/$1 [PT]
+RewriteRule ^/redir-objects/(.*/objects/.*)$ /dumb/$1 [R=301]
+
# Apache 2.2 does not understand <RequireAll>, so we use RewriteCond.
# And as RewriteCond does not allow testing for non-matches, we match
# the desired case first (one has abra, two has cadabra), and let it
diff --git a/t/t5550-http-fetch-dumb.sh b/t/t5550-http-fetch-dumb.sh
index 3484b6f0f..ad94ed7b1 100755
--- a/t/t5550-http-fetch-dumb.sh
+++ b/t/t5550-http-fetch-dumb.sh
@@ -299,5 +299,28 @@ test_expect_success 'git client does not send an empty Accept-Language' '
! grep "^Accept-Language:" stderr
'
+test_expect_success 'redirects can be forbidden/allowed' '
+ test_must_fail git -c http.followRedirects=false \
+ clone $HTTPD_URL/dumb-redir/repo.git dumb-redir &&
+ git -c http.followRedirects=true \
+ clone $HTTPD_URL/dumb-redir/repo.git dumb-redir 2>stderr
+'
+
+test_expect_success 'redirects are reported to stderr' '
+ # just look for a snippet of the redirected-to URL
+ test_i18ngrep /dumb/ stderr
+'
+
+test_expect_success 'non-initial redirects can be forbidden' '
+ test_must_fail git -c http.followRedirects=initial \
+ clone $HTTPD_URL/redir-objects/repo.git redir-objects &&
+ git -c http.followRedirects=true \
+ clone $HTTPD_URL/redir-objects/repo.git redir-objects
+'
+
+test_expect_success 'http.followRedirects defaults to "initial"' '
+ test_must_fail git clone $HTTPD_URL/redir-objects/repo.git default
+'
+
stop_httpd
test_done
--
2.11.0.191.gdb26c57
Jeff King