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authentication: use hidden form instead of referer

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This also gives us some CSRF protection. Note that we make use of the
hmac to protect the redirect value.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
This commit is contained in:
Jason A. Donenfeld 2014-01-16 11:39:17 +01:00
parent d6e9200cc3
commit b826537cb4
3 changed files with 133 additions and 96 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
cgit.c
View File

@ -614,22 +614,19 @@ static inline void open_auth_filter(struct cgit_context *ctx, const char *functi
ctx->qry.url ? ctx->qry.url : "");
}
/* We intentionally keep this rather small, instead of looping and
* feeding it to the filter a couple bytes at a time. This way, the
* filter itself does not need to handle any denial of service or
* buffer bloat issues. If this winds up being too small, people
* will complain on the mailing list, and we'll increase it as needed. */
#define MAX_AUTHENTICATION_POST_BYTES 4096
/* The filter is expected to spit out "Status: " and all headers. */
static inline void authenticate_post(struct cgit_context *ctx)
{
if (ctx->env.http_referer && strlen(ctx->env.http_referer) > 0) {
html("Status: 302 Redirect\n");
html("Cache-Control: no-cache, no-store\n");
htmlf("Location: %s\n", ctx->env.http_referer);
} else {
html("Status: 501 Missing Referer\n");
html("Cache-Control: no-cache, no-store\n\n");
exit(0);
}
open_auth_filter(ctx, "authenticate-post");
char buffer[MAX_AUTHENTICATION_POST_BYTES];
int len;
open_auth_filter(ctx, "authenticate-post");
len = ctx->env.content_length;
if (len > MAX_AUTHENTICATION_POST_BYTES)
len = MAX_AUTHENTICATION_POST_BYTES;
@ -637,10 +634,7 @@ static inline void authenticate_post(struct cgit_context *ctx)
die_errno("Could not read POST from stdin");
if (write(STDOUT_FILENO, buffer, len) < 0)
die_errno("Could not write POST to stdout");
/* The filter may now spit out a Set-Cookie: ... */
cgit_close_filter(ctx->cfg.auth_filter);
html("\n");
exit(0);
}

3
cgitrc.5.txt
View File

@ -662,7 +662,8 @@ auth filter::
the http cookie and return a 0 if it is invalid or 1 if it is invalid,
in the exit code / close function. If the filter action is
"authenticate-post", this filter receives POST'd parameters on
standard input, and should write to output one or more "Set-Cookie"
standard input, and should write a complete CGI request, preferably
with a 302 redirect, and write to output one or more "Set-Cookie"
HTTP headers, each followed by a newline.
Please see `filters/simple-authentication.lua` for a clear example

204
filters/simple-authentication.lua
View File

@ -33,15 +33,28 @@ local secret = "BE SURE TO CUSTOMIZE THIS STRING TO SOMETHING BIG AND RANDOM"
--
--
-- Sets HTTP cookie headers based on post
-- Sets HTTP cookie headers based on post and sets up redirection.
function authenticate_post()
local password = users[post["username"]]
-- TODO: Implement time invariant string comparison function to mitigate against timing attack.
if password == nil or password ~= post["password"] then
construct_cookie("", "cgitauth")
else
construct_cookie(post["username"], "cgitauth")
local redirect = validate_value(post["redirect"])
if redirect == nil then
not_found()
return 0
end
redirect_to(redirect)
-- TODO: Implement time invariant string comparison function to mitigate timing attack.
if password == nil or password ~= post["password"] then
set_cookie("cgitauth", "")
else
-- One week expiration time
local username = secure_value(post["username"], os.time() + 604800)
set_cookie("cgitauth", username)
end
html("\n")
return 0
end
@ -54,8 +67,8 @@ function authenticate_cookie()
return 1
end
local username = validate_cookie(get_cookie(http["cookie"], "cgitauth"))
if username == nil or not accepted_users[username] then
local username = validate_value(get_cookie(http["cookie"], "cgitauth"))
if username == nil or not accepted_users[username:lower()] then
return 0
else
return 1
@ -68,6 +81,9 @@ function body()
html("<form method='post' action='")
html_attr(cgit["login"])
html("'>")
html("<input type='hidden' name='redirect' value='")
html_attr(secure_value(cgit["url"], 0))
html("' />")
html("<table>")
html("<tr><td><label for='username'>Username:</label></td><td><input id='username' name='username' autofocus /></td></tr>")
html("<tr><td><label for='password'>Password:</label></td><td><input id='password' name='password' type='password' /></td></tr>")
@ -78,81 +94,10 @@ function body()
end
--
--
-- Cookie construction and validation helpers.
--
--
local crypto = require("crypto")
-- Returns username of cookie if cookie is valid. Otherwise returns nil.
function validate_cookie(cookie)
local i = 0
local username = ""
local expiration = 0
local salt = ""
local hmac = ""
if cookie:len() < 3 or cookie:sub(1, 1) == "|" then
return nil
end
for component in string.gmatch(cookie, "[^|]+") do
if i == 0 then
username = component
elseif i == 1 then
expiration = tonumber(component)
if expiration == nil then
expiration = 0
end
elseif i == 2 then
salt = component
elseif i == 3 then
hmac = component
else
break
end
i = i + 1
end
if hmac == nil or hmac:len() == 0 then
return nil
end
-- TODO: implement time invariant comparison to prevent against timing attack.
if hmac ~= crypto.hmac.digest("sha1", username .. "|" .. tostring(expiration) .. "|" .. salt, secret) then
return nil
end
if expiration <= os.time() then
return nil
end
return username:lower()
end
function construct_cookie(username, cookie)
local authstr = ""
if username:len() > 0 then
-- One week expiration time
local expiration = os.time() + 604800
local salt = crypto.hex(crypto.rand.bytes(16))
authstr = username .. "|" .. tostring(expiration) .. "|" .. salt
authstr = authstr .. "|" .. crypto.hmac.digest("sha1", authstr, secret)
end
html("Set-Cookie: " .. cookie .. "=" .. authstr .. "; HttpOnly")
if http["https"] == "yes" or http["https"] == "on" or http["https"] == "1" then
html("; secure")
end
html("\n")
end
--
--
-- Wrapper around filter API follows below, exposing the http table, the cgit table, and the post table to the above functions.
-- Wrapper around filter API, exposing the http table, the cgit table, and the post table to the above functions.
--
--
@ -197,7 +142,7 @@ end
--
--
-- Utility functions follow below, based on keplerproject/wsapi.
-- Utility functions based on keplerproject/wsapi.
--
--
@ -211,6 +156,16 @@ function url_decode(str)
return str
end
function url_encode(str)
if not str then
return ""
end
str = string.gsub(str, "\n", "\r\n")
str = string.gsub(str, "([^%w ])", function (c) return string.format("%%%02X", string.byte(c)) end)
str = string.gsub(str, " ", "+")
return str
end
function parse_qs(qs)
local tab = {}
for key, val in string.gmatch(qs, "([^&=]+)=([^&=]*)&?") do
@ -223,3 +178,90 @@ function get_cookie(cookies, name)
cookies = string.gsub(";" .. cookies .. ";", "%s*;%s*", ";")
return url_decode(string.match(cookies, ";" .. name .. "=(.-);"))
end
--
--
-- Cookie construction and validation helpers.
--
--
local crypto = require("crypto")
-- Returns value of cookie if cookie is valid. Otherwise returns nil.
function validate_value(cookie)
local i = 0
local value = ""
local expiration = 0
local salt = ""
local hmac = ""
if cookie == nil or cookie:len() < 3 or cookie:sub(1, 1) == "|" then
return nil
end
for component in string.gmatch(cookie, "[^|]+") do
if i == 0 then
value = component
elseif i == 1 then
expiration = tonumber(component)
if expiration == nil then
expiration = 0
end
elseif i == 2 then
salt = component
elseif i == 3 then
hmac = component
else
break
end
i = i + 1
end
if hmac == nil or hmac:len() == 0 then
return nil
end
-- TODO: implement time invariant comparison to prevent against timing attack.
if hmac ~= crypto.hmac.digest("sha1", value .. "|" .. tostring(expiration) .. "|" .. salt, secret) then
return nil
end
if expiration ~= 0 and expiration <= os.time() then
return nil
end
return url_decode(value)
end
function secure_value(value, expiration)
if value == nil or value:len() <= 0 then
return ""
end
local authstr = ""
local salt = crypto.hex(crypto.rand.bytes(16))
value = url_encode(value)
authstr = value .. "|" .. tostring(expiration) .. "|" .. salt
authstr = authstr .. "|" .. crypto.hmac.digest("sha1", authstr, secret)
return authstr
end
function set_cookie(cookie, value)
html("Set-Cookie: " .. cookie .. "=" .. value .. "; HttpOnly")
if http["https"] == "yes" or http["https"] == "on" or http["https"] == "1" then
html("; secure")
end
html("\n")
end
function redirect_to(url)
html("Status: 302 Redirect\n")
html("Cache-Control: no-cache, no-store\n")
html("Location: " .. url .. "\n")
end
function not_found()
html("Status: 404 Not Found\n")
html("Cache-Control: no-cache, no-store\n\n")
end