Friday, May 27, 2011

TLSSLed v1.0

When running web application security assessments it is mandatory to evaluate the security stance of the SSL/TLS (HTTPS) implementation and configuration. OWASP has a couple of references I strongly recommend to take a look at, the "OWASP-CM-001: Testing for SSL-TLS" checks, part of the OWASP Testing Guide v3, and the Transport Layer Protection Cheat Sheet.

We have had several tools to test for SSL and TLS security misconfigurations along the years, but still today, lots of people get the output from all these tools and are not very sure what they need to look at. Apart from the SSL/TLS web application best practices, it is important to also check the security of SSL/TLS at the web platform layer.

The purpose of the TLSSLed tool (named from the idea of your website being TLS/SSL-ed, that is, using "https;//") is to simplify the output of a couple of commonly used tools, and highlight the most relevant security findings of any target SSL/TLS implementation. It is based on sslscan, a thorough SSL/TLS scanner that is based on the openssl library, and on the "openssl s_client" command line tool.

TLSSLed is a Linux shell script inspired on by Aung Khant, where a few optimizations have been made to reduce the stress on the target web server (sslscan is run only once and the results are stored on a local file), and some tests have been added and tuned.

The current tests include checking if the target supports the SSLv2 protocol, the NULL cipher, weak ciphers based on their key length (40 or 56 bits), the availability of strong ciphers (like AES), if the digital certificate is MD5 signed, and the current SSL/TLS renegotiation capabilities. The tool is open to comments, suggestions, improvements and new tests from the community. Do not hesitate to contact me with ideas!

NOTE: First idea for v1.1 - Would it be useful to check for the certificate key length (<= 1024 bits)?

TLSSLed v1.0 can be downloaded from Taddong's lab.

The following output shows how to use TLSSLed and the kind of results you will get. It only requires two arguments ("yes, I know, with no verification..."): the target HTTPS server hostname (or IP address) and the target port. The example below shows a secure implementation from, because unfortunately, I'm sure you will find multiple insecure examples out there (Make your web environment look the same!):

$ ./ 443
 TLSSLed - (1.0) based on sslscan and openssl
 by Raul Siles (
 ( inspired by by Aung Khant )
+ openssl version: OpenSSL 0.9.8g 19 Oct 2007
+ sslscan version 1.8.2

[*] Analyzing SSL/TLS on ..

[*] Running sslscan on

[*] Testing for SSLv2 ...

[*] Testing for NULL cipher ...

[*] Testing for weak ciphers (based on key length) ...

[*] Testing for strong ciphers (AES) ...
    Accepted  SSLv3  256 bits  DHE-RSA-AES256-SHA
    Accepted  SSLv3  256 bits  AES256-SHA
    Accepted  SSLv3  128 bits  DHE-RSA-AES128-SHA
    Accepted  SSLv3  128 bits  AES128-SHA
    Accepted  TLSv1  256 bits  DHE-RSA-AES256-SHA
    Accepted  TLSv1  256 bits  AES256-SHA
    Accepted  TLSv1  128 bits  DHE-RSA-AES128-SHA
    Accepted  TLSv1  128 bits  AES128-SHA

[*] Testing for MD5 signed certificate ...

[*] Checking preferred server ciphers ...
  Prefered Server Cipher(s):
    SSLv3  256 bits  DHE-RSA-AES256-SHA
    TLSv1  256 bits  DHE-RSA-AES256-SHA

[*] Testing for SSLv3/TLSv1 renegotiation vuln. (CVE-2009-3555) ...
depth=3 /L=ValiCert Validation Network/O=ValiCert, Inc./OU=ValiCert Class 2 Policy ...
... Validation Authority/CN=
verify error:num=19:self signed certificate in certificate chain
verify return:0
Secure Renegotiation IS supported
12172:error:1409E0E5:SSL routines:SSL3_WRITE_BYTES:ssl handshake failure:s3_pkt.c:530:

[*] New files created:
-rw-r--r-- 1 samurai samurai 5980 2011-05-27 19:47

[*] done

Thursday, May 5, 2011

Vulnerability in Android: To add, or not to add (a Wi-Fi network), that is the question

Title: Preferred Network List (PNL) disclosure vulnerability in Android based on the method used to add Wi-Fi networks
Vulnerability ID: TAD-2011-003
Credits: This vulnerability was discovered by Raul Siles, Founder and Senior Security Analyst with Taddong (
Publication date: May 5, 2011
Vendors contacted: Android Security Team

Vulnerability Summary:
Depending on the method the user followed to add a Wi-Fi network to its Android mobile device, selecting it from the Wi-Fi networks scan list or manually through the “Add Wi-Fi Network” button, the network name could be disclosed in the air by Android and be used by an attacker to impersonate that network, forcing the victim mobile device to connect to it to capture and manipulate its traffic and launch more advanced attacks.

For all broadcast Wi-Fi networks the user has previously connected to from the “Add Wi-Fi Network” button, it is advised to delete them all and re-add them back from the scan list once the user is under the network coverage.

Android users should preferably connect to a new broadcast Wi-Fi network from the scan list and use the “Add Wi-Fi Network” button only for connecting to a non-broadcast Wi-Fi network. A non-broadcast Wi-Fi network does require a Wi-Fi client to expose the network name in its probe request packets in order to be able to successfully connect to the network, making the client vulnerable to the previously mentioned security threats.

Vulnerability Description:
Android mobile devices, as most Wi-Fi clients, keep a list of the wireless networks manually configured, plus the networks it has connected to in the past, on the Preferred Network List (PNL). Every time the Wi-Fi interface is turned on, and periodically once it has been activated (for example, to roam between access points), the device checks through 802.11 probe requests what networks in its PNL are available in the current location. Based on the responses obtained, it tries to connect to the most preferred network.

In the past this network discovery process was performed by sending a generic probe request for the broadcast (or any) network plus specific requests for every network in the PNL. This meant devices disclosed the full PNL in the air [1], exposing themselves to karma-like attacks [2], where an attacker can identify all the networks (or access points) the mobile device is trying to connect to and impersonate them, forcing the victim device to connect to the attacker's network to capture and manipulate its traffic and launch more advanced attacks.

In order to avoid this vulnerable behavior, modern operating systems and Wi-Fi supplicants changed the previous vulnerable behavior not to advertise the wireless networks in its PNL. Modern Wi-Fi clients only generate 802.11 probe requests for the broadcast network, generically asking for the networks available in the area. An exception to this behavior is presented by the existence of Wi-Fi hidden networks in the PNL: due to the fact hidden (or non-broadcast) networks do not include their SSID (Wi-Fi network name) in the beacon frames, and do not respond to generic queries asking for any network available, the Wi-Fi clients need to specifically ask for these hidden networks, disclosing its name and existence inside the device PNL. This makes devices vulnerable again to the aforementioned attacks.

Android mobile devices provide two methods to add and configure Wi-Fi networks into the device. If the network is visible, it will appear on the Wi-Fi networks scan list. By simply selecting it form the list, and after providing the network credentials, the user can add the Wi-Fi network to the device. Additionally, Android provides an “Add Wi-Fi network” button at the bottom of the scan list, to manually add Wi-Fi networks. This is the only method available to add hidden networks, as they will never appear on the scan list.

However, Android does not provide any specific configuration option through this method to specify if a network is hidden (non-broadcast) or visible (broadcast). Although the most natural way of adding a network for end users is from the scan list (fortunately, for Android, this is the secure option), unfortunately, the method of manually adding Wi-Fi networks to a device is very common too, and recommended from a security perspective, as advanced users have more control over all the Wi-Fi network settings and options.

This subtle configuration behavior has serious security implications. Depending on how the user added the Wi-Fi network to the device, selecting it from the scan list or through the "Add Wi-Fi network" button, you are vulnerable or not. As a result, all the Wi-Fi networks (hidden or visible) added to Android through the “Add Wi-Fi network” button are implicitly considered as hidden, its details will be revealed in the air, and the mobile device will be exposed to Karma-like attacks [2].

The expected non-vulnerable behavior implies the propagation of probe requests only for the broadcast (or any) network plus all the intentionally configured hidden networks in the PNL. By default, unless it is clearly specified by the user, all networks should be treated as visible, not generating any probe request frames for them.

The vulnerable behavior exists on the default Android configuration when adding a Wi-Fi network through the “Add Wi-Fi network” button; the Wi-Fi networks connected from the scan list are not exposed and hence not vulnerable.

This vulnerable behavior is similar to TAD-2010-003 [3], but in the case of Android, only those Wi-Fi networks added through the “Add Wi-Fi Network” button are disclosed, instead of the full PNL.

Security Solutions, Workarounds, and Countermeasures:
Every time a user connects to a Wi-Fi network for the first time from her Android mobile device, it must select it from the Wi-Fi networks scan list, instead of using the “Add Wi-Fi Network” button except when connecting to hidden networks (option not recommended). This method ensures the Wi-Fi network will be added to the PNL in a secure way and won’t be disclosed through probe request scans.

End users, corporate administrators, and security professionals, using or managing Android mobile devices must be aware of this behavior and ensure that all the Wi-Fi networks available on the device PNL are treated as visible.

Unfortunately, Android does not provide any indication on the user interface to be able to differentiate between the two types of networks (hidden or visible) for the already configured Wi-Fi networks. Once a Wi-Fi network has been added, the user cannot know if it was securely added or not. Thus, for all Wi-Fi networks previously added to the device the user must delete them all and re-add them again, selecting each of them from the scan list (and not using the “Add Wi-Fi Network”) once the user is under the network coverage (and it is visible).

A similar scenario occurs for those Wi-Fi networks that were configured as hidden in the past, were manually and insecurely added to Android, and are configured as visible now because the administrator learned about Karma-like attacks and improved the security of the network by making it visible. It is highly recommended not to setup or connect to Wi-Fi hidden networks, as the Wi-Fi clients will be exposed to the attacks previously mentioned.

A more granular solution is to monitor the mobile device Wi-Fi traffic, identify what Wi-Fi networks Android is generating probe requests for, and delete and re-add again only those networks.

The recommended solution would be for Android to add a new configuration setting to the user interface that allows the user to specify if the network must be considered hidden or visible every time a new Wi-Fi network is added to the mobile device, independently of the method used, or at least when it is manually added through the vulnerable “Add Wi-Fi Network” button. The default value for this new setting must reflect that the network to connect to is visible (unless the user specifies otherwise by changing the default value).

Besides that, Android users should be able to see and change this “type of network” setting at any time, that is, when the Wi-Fi network is added for the first time, or afterwards, through the "Edit network" button.

Vulnerable Platforms:
The vulnerable behavior was discovered on Android 2.2.

The Android Security Team has confirmed this vulnerable behavior also affects all currently available Android 2.x and 3.x versions (such as 2.2.1, 2.2.2, 2.3, 2.3.2, 2.3.3, or 3.0).

Vendor Information:
The Android Security Team confirmed the existence of this vulnerable behavior and is working on changing the "Add Wi-Fi Network" dialog box to read "Configure a non-broadcast network". The original intent of the "Add Wi-Fi Network" dialog box was only to add non-broadcast networks; the wording will hopefully make that clearer.

The new dialog box text will inform aware users that probe request messages will be sent from their device. They also confirmed there is no Android documentation available which describes the “scan list” versus the “Add Wi-Fi Network" behavior, hence the importance of the distribution of this security advisory in an effort to raise awareness on this issue.

In the “Vulnerability Description” section above, Taddong generally recommends from a security perspective the method of manually adding Wi-Fi networks to a device so that advanced users have more control over all the Wi-Fi network settings and options. The Android Security Team thinks that adding a network via the scan list is more secure, because more critical security information can be conveyed automatically, rather than relying on the limited options available to the user. We (at Taddong) agree this could be true for the average user, especially to avoid misconfiguration and user mistakes, IF the user connects to a secure and properly configured Wi-Fi network, but unfortunately, this is not always the case.

We at Taddong honestly believe this finding must be publicly known by end users and by the information security community in order to take appropriate countermeasures and mitigate the vulnerable behavior. Therefore, we have tried to coordinate the release of this security advisory together with the vendor, following responsible disclosure principles. This vulnerable behavior is especially relevant considering the broad market adoption of Android mobile devices (with significant increasing adoption estimations for the upcoming years), and its extensive usage to connect to Wi-Fi networks.

Vulnerability Report Timeline:
2011-04-08: Taddong contacts the Android Security Team to provide details about this vulnerable behavior. The Android Security Team requests more details and clarifies the expected behavior.
2011-04-09: Taddong provides extra details after reanalyzing the expected behavior and ratifies the vulnerable behavior only when the “Add Wi-Fi Network” button is used. Taddong asks for details to differentiate the two types of networks, available documentation, expected behavior, and future plans to mitigate the vulnerable behavior.
2011-04-12: Taddong asks for feedback, and the Android Security Team replies back clarifying the previous questions and notifying future plans to improve the Android user interface. Both parties start to coordinate the public disclosure of this issue.
2011-04-15: Taddong completes and provides an initial security advisory draft to the Android Security Team for its review and comments. The Android Security Teams confirms its reception, internal distribution, and feedback is expected for next week.
2011-04-22: The Android Security Team confirms it is still collecting feedback regarding the security advisory draft.
2011-04-28: Taddong tries to get an update of the status of the security advisory draft review process.
2011-05-04: Taddong again tries to get an update of the status of the security advisory draft review process. The Android Security Team provides its review and comments to the security advisory draft.
2011-05-05: Taddong publishes security advisory TAD-2011-003.

[1] "Trying to shut up your wireless chatty Windows". Raul Siles. 2005.
[2] "KARMA Wireless Client Security Assessment Tools". Dino A. Dai Zovi. 2005.
[3] “TAD-2010-003: Full 802.11 Preferred Network List (PNL) disclosure in windows Mobile 6.5”. Raul Siles. Taddong. 2010.

About Taddong:
Taddong ( is a company established in Spain in 2010 with the purpose of improving customer's information security, by discovering and eliminating or mitigating the real risks that threaten their networking and information technology infrastructures. To achieve this goal, Taddong's portfolio includes specialized information security services, requiring an in-depth technical knowledge and broad understanding of the information technology market, as well as training services, focused on providing customers with auto-defense skills. Taddong remains at the forefront of the security market through continuous research and education activities.

The contents of this security advisory are copyright (c) 2011 Taddong S.L., and may be distributed freely provided that no fee is charged for this distribution and proper credit is given.

Tuesday, May 3, 2011

Selective attack with a rogue GSM/GPRS base station

An attacker employing a rogue GSM/GPRS base station usually wants to compromise the communications of a particular user, while trying to generate the least possible activity for the rest of mobile users within his radio range. We call this a “selective attack”. In order to perform it, the attacker must know the victim’s IMSI (the number that identifies a SIM card) in advance.
There are two widespread misconceptions regarding this type of attack. Most people think that:

A.- It is difficult to obtain the victim’s IMSI, and
B.- It is difficult not to affect the other users in the radio range of the rogue base station

However, there are some techniques that allow the attacker to solve the aforementioned issues. In this article we explain one of them as an illustrative example.

Discovering the victim’s IMSI

To solve point A, the attacker could do the following:
  • Step 1: the attacker positions himself in the area where his victim lives (the victim’s home location) when the victim is inside and captures all the IMSIs in the range of his rogue base station. He will probably use a directional antenna to limit the geographical area where he is capturing IMSIs. During this operation, the rogue base station will reject all registration attempts from any mobile, but it will annotate the IMSI numbers of the subscribers that are trying to register.
  • Step 2: afterwards (for example the next working day), the attacker positions himself in the victim’s office location, and performs the same procedure. It is to be expected that the first IMSI that is present in both IMSI lists will be the one of the victim.
  • Step 3: after that, the attacker should authorize the registration of the victim’s IMSI (and only this one) in his rogue base station in order to intercept all its communications. The registration attempts from any other IMSI will be rejected.
Avoiding affecting the rest of users.

Once point A is solved, let’s see how the attacker can tackle point B. First, and most important, notice that the attacker didn’t leave the registration open for all mobile stations at any time, thus preventing the appearance of any symptom that could alert the mobile users in the area (such as a sudden change in the coverage indicator, any abnormal error in outgoing calls, the absence of incoming calls, etc.) and any overload problem for his rogue base station (that will typically have limited capabilities for traffic and call management).
For every aforementioned step, the attacker performs the following configuration actions to affect the least possible the rest of mobiles in his radio range:
  • Step 1: the attacker will configure its rogue base station so that each IMSI tries to register to it only once. This is convenient for him for several reasons: it will minimize any symptoms in the mobile phones in his range, and also he will reduce to the minimum redundant and useless information in his logs. One way to achieve this objective is to configure a Reject Cause Code 0x0C “Location Area Not Allowed”. This reject code is included by the base station in the “Location Update Procedure Reject” message, sent whenever the base station rejects a registration attempt. According to 3GPP 24.008 and the tests in our lab, the mobile station annotates the LAI (Location Area Identifier) in its “forbidden location areas” list and it will not try to register to any cell with this LAI (at least not until the mobile station is reset or the SIM is removed). This way, the attacker is forcing the mobile terminals in his range to try to register only once to his rogue base station, but they will continue to be able to register back to a legitimate base station.
  • Step 2: at this moment the attacker wants the victim’s mobile station to be rejected in its first registration attempt, as well as the other subscribers in his radio range. He can configure its rogue base station with a LAI different from the one used in the previous day, so that he will ensure that the mobile station will try to register again. Once identified the first IMSI matching any one stored the previous day, he shutdowns the base station.
  • Step 3: at this time, the attacker already knows the victim’s IMSI. He can then turn on again his rogue base station with a new LAI and with the victim’s IMSI authorized. When the victim’s mobile station tries to register, the registration will succeed and the attacker will be able to intercept the communications of the victim. All other mobile stations will try to register only once in the rogue base station and will register back to a legitimate cell, because the configured Reject Cause Code will still be 0x0C.
This procedure would allow an attacker to identify the victim’s IMSI with a first capture session that would last some minutes, and a second session in which he would be able to selectively intercept the victim’s communications. The only collateral effect on other mobile terminals in the attacker’s range would be that they would try to register to his rogue base station, but only once (or twice at most), and the users won’t even get an indication of this fact on their screens.

NOTE: We first published this article, in Spanish, in this post of the blog “Un inform├ítico en el lado del mal”