AST-2008-005: HTTP Manager ID is predictable

           Asterisk Project Security Advisory - AST-2008-005

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| Product | Asterisk |
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| Summary | HTTP Manager ID is predictable |
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| Nature of Advisory | An attacker could hijack a manager session |
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| Susceptibility | All users using the HTTP manager port |
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| Severity | Minor |
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| Exploits Known | No |
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| Reported On | February 25, 2008 |
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| Reported By | Dino A. Dai Zovi < ddz AT theta44 DOT org > |
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| Posted On | March 18, 2008 |
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| Last Updated On | March 18, 2008 |
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| Advisory Contact | Tilghman Lesher < tlesher AT digium DOT com > |
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| CVE Name | CVE-2008-1390 |
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| Description | Due to the way that manager IDs are calculated, this |
| | 32-bit integer is likely to have a much larger than |
| | average number of 1s, which greatly reduces the number |
| | of guesses an attacker would have to make to |
| | successfully predict the manager ID, which is used |
| | across multiple HTTP queries to hold manager state. |
| | |
| | "The issue is the generation of session ids in the |
| | AsteriskGUI HTTP server. |
| | |
| | When using Glibc, the implementation and state of rand() |
| | and random() is |
| | |
| | shared. Asterisk uses random() to issue MD5 digest |
| | authentication |
| | |
| | challenges and rand() bitwise-ORed with a malloc'd |
| | pointer to generate |
| | |
| | AsteriskGUI session identifiers. An attacker can |
| | synchronize with |
| | |
| | random() by retrieving 32 successive challenges and |
| | predict all subsequent |
| | |
| | output of calls to random() and rand(). Because a |
| | pointer returned by |
| | |
| | malloc has at best 21 bits of entropy, the attacker will |
| | on average only |
| | |
| | need to guess 1448 session identifiers in order to steal |
| | an established |
| | |
| | session. |
| | |
| | "The crux of the problem is that under Glibc, the |
| | implementation of rand() |
| | |
| | and random() is shared. rand() is just an alias to |
| | random(). This means |
| | |
| | that they all come from the same randomizer with the |
| | same state. |
| | |
| | "A remote attacker can synchronize with all subsequent |
| | output of a remote |
| | |
| | system's random() state by just observing or retrieving |
| | 32 successive |
| | |
| | outputs. They can easily do this by generating 32 MD5 |
| | digest |
| | |
| | authentication challenges. At this point, they will be |
| | able to predict |
| | |
| | all subsequent output of random() and rand(). |
| | |
| | "The memory address returned by calloc() is also not |
| | sufficiently random. |
| | |
| | In practice, it will be in low memory, immediately |
| | following the executable. |
| | |
| | In addition, the buffer returned will be 8-byte aligned. |
| | This means that |
| | |
| | the high order 8 bits and low order 3 bits will always |
| | be zero. Finally, |
| | |
| | this value is bitwise ORed with the output of random(), |
| | so any bits that |
| | |
| | are set will be preserved. |
| | |
| | "An attacker will only have to guess 2^N session ids, |
| | where N is the number |
| | |
| | of zeros in the number return by random() between bit |
| | positions 3 and 24. |
| | |
| | On average, this will be 1448 guesses. |
| | |
| | "However, an attacker can do better than this by |
| | consuming challenges until |
| | |
| | the following number output by random() has many 1's in |
| | those significant |
| | |
| | bit positions." |
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| Resolution | To mitigate this attack, the two values are now XORed |
| | together. This will increase the entropy to approximately |
| | 2^21, which is far more difficult to predict, especially |
| | given that the random number generator is used for other |
| | purposes in Asterisk, not just manager HTTP session ID |
| | generation. |
| | |
| | Upgrade to SVN revision 104704 or greater, or upgrade to |
| | one of the releases below. That the random number |
| | generator is used for other things makes this attack |
| | extremely difficult and unlikely, so we will not produce |
| | a separate release for this security advisory. |
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| Links | |
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| Asterisk Project Security Advisories are posted at |
| http://www.asterisk.org/security |
| |
| This document may be superseded by later versions; if so, the latest |
| version will be posted at |
| http://downloads.digium.com/pub/security/AST-2008-005.pdf and |
| http://downloads.digium.com/pub/security/AST-2008-005.html |
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           Asterisk Project Security Advisory - AST-2008-005
          Copyright (c) 2008 Digium, Inc. All Rights Reserved.

Permission is hereby granted to distribute and publish this advisory in its
original, unaltered form.