6502.txt

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Rule:

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Sid:
6502

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Summary:
This event is generated when an attempt is made to exploit a heap overflow associated with Mozilla's processing of a malformed GIF file.

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Impact:
A successful attack can cause a heap overflow on the client host running Mozilla and permit execution of arbitrary code at the privilege level of the current user.

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Detailed Information:
The Mozilla browser can read images from a file formatted in Graphics Interchange Format (GIF). The GIF format has optional "extension blocks" that supply additional information to a browser that can interpret and render it.

An optional application block is one type of extension block.  There is a vulnerability with particular sub-blocks associated with the application block that permits a signed variable that should contain a positive value to be treated as a negative number.  This may cause a heap overflow.

NOTE
In order to avoid potential evasion techniques, http_inspect should be configured with "flow_depth 0" so that all HTTP server response traffic is inspected.
NOTE

WARNING
Setting flow_depth 0 will cause performance problems in some situations.
WARNING

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Affected Systems:
Mozilla Firefox and Thunderbird 1.0.1 and prior
Mozilla browser 1.7.5 and prior

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Attack Scenarios:
An attacker can entice a user to download a malicious GIF file that causes an overflow, enabling execution of arbitrary code on a vulnerable client.

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Ease of Attack:
Simple. Exploit code is available.

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False Positives:
None known.

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False Negatives:
A malicious value found in any sub-block but the first will not trigger an alert.

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Corrective Action:
Upgrade to the most current non-affected version of the product.

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Contributors:
Sourcefire Vulnerability Research Team
Brian Caswell <[e-mail]>
Judy Novak <[e-mail]>

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Additional References

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