CVE-2021-47353 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
udf: Fix NULL pointer dereference in udf_symlink function
In function udf_symlink, epos.bh is assigned with the value returned by udf_tgetblk. The function udf_tgetblk is defined in udf/misc.c and returns the value of sb_getblk function that could be NULL. Then, epos.bh is used without any check, causing a possible NULL pointer dereference when sb_getblk fails.
This fix adds a check to validate the value of epos.bh.
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Analysis
by VulDB Data Team • 06/06/2025
The vulnerability identified as CVE-2021-47353 represents a critical NULL pointer dereference flaw within the Linux kernel's Universal Disk Format UDF implementation. This issue specifically affects the udf_symlink function where improper handling of block allocation results in potential system crashes and denial of service conditions. The vulnerability stems from the function's failure to validate the return value from udf_tgetblk, which itself delegates to sb_getblk and can legitimately return NULL under certain error conditions. When epos.bh receives this NULL value and subsequent code attempts to dereference it without prior validation, the kernel encounters a critical failure that can terminate system operations.
The technical exploitation of this vulnerability occurs within the UDF filesystem driver's symlink handling mechanism where the kernel attempts to process symbolic links in filesystems formatted with UDF standards. The flaw manifests when the udf_tgetblk function fails to allocate a block structure, returning NULL to the udf_symlink function which then proceeds to use this invalid pointer. This pattern of error handling failure creates a direct path to system instability, as the kernel's memory management subsystem cannot safely process a NULL reference. The vulnerability directly maps to CWE-476 which describes NULL pointer dereference conditions in software implementations, and represents a classic example of inadequate error checking in kernel space code where resource allocation failures are not properly accounted for in subsequent operations.
From an operational perspective, this vulnerability presents significant risks to Linux systems that utilize UDF filesystems, particularly those employing optical media such as DVDs and Blu-ray discs, or systems that mount UDF formatted storage devices. Attackers could potentially trigger this condition through maliciously crafted filesystem structures or by manipulating the environment to force allocation failures, leading to system crashes, kernel panics, and complete service disruption. The impact extends beyond simple denial of service as the kernel's stability is compromised, potentially allowing for more sophisticated exploitation techniques that could leverage the system's instability to gain unauthorized access or escalate privileges. The vulnerability affects all Linux kernel versions that implement the UDF filesystem driver and are susceptible to the specific error handling pattern described.
Mitigation strategies for CVE-2021-47353 focus primarily on applying the kernel patch that implements the necessary NULL pointer validation check before using the epos.bh value. System administrators should prioritize updating their kernel versions to include the fix that adds proper validation logic to the udf_symlink function. The recommended approach involves verifying that the returned value from udf_tgetblk is checked for NULL before proceeding with any operations that might dereference the pointer. Additionally, monitoring systems should be configured to detect unusual kernel crashes or memory allocation failures that might indicate exploitation attempts. Organizations utilizing UDF filesystems should conduct vulnerability assessments to ensure all systems are patched and maintain continuous monitoring for any signs of attempted exploitation. The fix aligns with ATT&CK technique T1059.003 which involves system service manipulation and T1499.004 which covers network disruption, as the vulnerability enables denial of service conditions that can compromise system availability and potentially provide a foothold for further attacks.