CVE-2009-3978 in Firefox
Summary
by MITRE
The nsGIFDecoder2::GifWrite function in decoders/gif/nsGIFDecoder2.cpp in libpr0n in Mozilla Firefox before 3.5.5 allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via an animated GIF file with a large image size, a different vulnerability than CVE-2009-3373.
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Analysis
by VulDB Data Team • 08/28/2021
The vulnerability described in CVE-2009-3978 represents a critical denial of service flaw within Mozilla Firefox's image decoding subsystem. This issue specifically affects the nsGIFDecoder2 component responsible for processing animated gif files, where the GifWrite function fails to properly validate image dimensions before attempting to write decoded data. The flaw occurs in libpr0n, which serves as Firefox's core image decoding library that handles multiple image formats including gif, png, and jpeg. When processing specially crafted animated gif files with excessively large image dimensions, the decoder attempts to dereference a null pointer during the write operation phase, leading to immediate application crash and complete denial of service for the affected browser session.
The technical implementation of this vulnerability stems from insufficient input validation within the GIF decoding pipeline. The nsGIFDecoder2::GifWrite function does not adequately check whether image dimensions exceed reasonable bounds or validate the internal state of the decoder before proceeding with data writes. When an attacker submits an animated gif file containing oversized image parameters, the decoder's memory management routines fail to handle the exceptional case properly, resulting in a null pointer dereference that terminates the application process. This behavior aligns with CWE-476 which describes null pointer dereference vulnerabilities, and demonstrates how improper error handling in image processing components can lead to complete application failure rather than merely displaying corrupted images.
The operational impact of this vulnerability extends beyond simple browser instability, as it enables remote attackers to perform denial of service attacks against Firefox users without requiring any special privileges or user interaction beyond visiting a malicious website. The attack vector is particularly concerning because it leverages the common and widely used gif format, making it difficult for users to distinguish between legitimate and malicious content. Security researchers have noted that such vulnerabilities are especially dangerous in web browsers since they can be exploited through simple web page loads, potentially allowing attackers to disrupt service for multiple users simultaneously. The vulnerability operates at the application level rather than at the system level, meaning that successful exploitation results in complete browser process termination without affecting the underlying operating system.
Mitigation strategies for this vulnerability include immediate patching of Firefox installations to version 3.5.5 or later where the issue has been resolved through proper input validation and null pointer checks. Organizations should implement security awareness training to help users recognize potentially malicious websites and avoid visiting untrusted content sources. Network-level protections such as web application firewalls can help filter out suspicious gif file content, though these measures are not foolproof against all variants of the attack. Additionally, browser hardening techniques including disabling automatic image loading and implementing strict content security policies can reduce the attack surface for such vulnerabilities. The fix implemented by Mozilla developers involved adding comprehensive boundary checks to validate image dimensions before processing and ensuring proper null pointer validation in the decoder's write operations, aligning with ATT&CK technique T1499.004 which covers network denial of service attacks through application-level vulnerabilities.