Rule:

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Sid:
3535

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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.

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.

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 Research Team
Brian Caswell <bmc@sourcefire.com>
Judy Novak <judy.novak@sourcefire.com>

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Additional References
Other:

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