Fix redirect URL validation bypass
It turns out that browsers silently convert backslash characters into forward slashes, while apr_uri_parse() does not. This mismatch allows an attacker to bypass the redirect URL validation by using an URL like: https://sp.example.org/mellon/logout?ReturnTo=https:%5c%5cmalicious.example.org/ mod_auth_mellon will assume that it is a relative URL and allow the request to pass through, while the browsers will use it as an absolute url and redirect to https://malicious.example.org/ . This patch fixes this issue by rejecting all redirect URLs with backslashes.
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@ -927,6 +927,13 @@ int am_check_url(request_rec *r, const char *url)
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"Control character detected in URL.");
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return HTTP_BAD_REQUEST;
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}
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if (*i == '\\') {
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/* Reject backslash character, as it can be used to bypass
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* redirect URL validation. */
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AM_LOG_RERROR(APLOG_MARK, APLOG_ERR, HTTP_BAD_REQUEST, r,
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"Backslash character detected in URL.");
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return HTTP_BAD_REQUEST;
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}
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}
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return OK;
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