CVE-2021-3782 in Wayland
Summary
by MITRE • 09/23/2022
An internal reference count is held on the buffer pool, incremented every time a new buffer is created from the pool. The reference count is maintained as an int; on LP64 systems this can cause the reference count to overflow if the client creates a large number of wl_shm buffer objects, or if it can coerce the server to create a large number of external references to the buffer storage. With the reference count overflowing, a use-after-free can be constructed on the wl_shm_pool tracking structure, where values may be incremented or decremented; it may also be possible to construct a limited oracle to leak 4 bytes of server-side memory to the attacking client at a time.
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Analysis
by VulDB Data Team • 01/04/2026
The vulnerability described in CVE-2021-3782 represents a critical memory safety issue within the Wayland protocol's shared memory implementation, specifically affecting the wl_shm_pool tracking structure. This flaw exists in the reference counting mechanism that manages buffer pool resources, where an internal counter tracks buffer usage through an integer data type. The vulnerability becomes exploitable when the reference count exceeds the maximum value that can be represented by a 32-bit signed integer, creating a scenario where the counter wraps around to negative values or zero, fundamentally altering the memory management behavior of the system.
The technical implementation of this vulnerability stems from the way Wayland servers handle shared memory buffer management, particularly in the context of LP64 systems where pointer sizes exceed 32 bits. When a client creates numerous wl_shm buffer objects or manipulates external references to buffer storage, the reference count integer can overflow due to its limited size. This overflow condition creates a use-after-free scenario where the wl_shm_pool tracking structure may be freed while still referenced, or where the structure's memory is accessed after deallocation. The integer overflow specifically occurs because the reference count is maintained as a standard int type rather than a larger data type that could accommodate the potential number of buffer operations that might occur during normal operation.
The operational impact of this vulnerability extends beyond simple memory corruption, as it provides attackers with potential for information disclosure and remote code execution. The overflow creates conditions where attackers can construct a limited memory leak oracle, allowing them to read 4 bytes of server-side memory at a time, which can reveal sensitive information such as stack canaries, heap addresses, or other critical data structures. This memory leakage capability, combined with the use-after-free condition, can be leveraged to bypass security mitigations like address space layout randomization and stack canaries, ultimately enabling more sophisticated exploitation techniques.
Mitigation strategies for CVE-2021-3782 must address both the immediate memory safety issue and the underlying architectural flaw in the reference counting implementation. The most effective approach involves increasing the size of the reference count variable to prevent overflow conditions, typically by using a 64-bit integer type or similar larger data structure that can accommodate the maximum expected number of buffer operations. Additionally, implementing proper bounds checking and validation of buffer creation operations can prevent attackers from triggering the overflow condition through crafted requests. Security controls should also include enhanced memory management practices such as proper deallocation sequences and validation of buffer pool structures before and after operations. This vulnerability aligns with CWE-190, Integer Overflow or Wraparound, and can be mapped to ATT&CK techniques involving privilege escalation and information gathering through memory corruption exploits. The fix requires careful consideration of the protocol's memory management model and should be implemented with thorough testing to ensure that legitimate buffer operations continue to function correctly while preventing the overflow conditions that enable exploitation.