CVE-2026-84267 in Red Hat
Summary
by MITRE • 09/01/2026
A flaw was found in the SFTP backend in gvfs. When mounting a share, a malicious SFTP server can cause read_string() to allocate a buffer with a certain length but the function does not verify that the buffer is completely filled, leaving the remainder of the buffer containing uninitialized heap contents. If the server sends a short FXP_HANDLE reply, these uninitialized bytes are taken as the file handle. The client will then echo these uninitialized bytes back to the server on all subsequent requests using that handle. With a length of 128 bytes, this issue allows the malicious server to deterministically read uninitialized heap memory from the gvfsd-sftp process, leaking its heap base and the load address of the libgio library, resulting in a deterministic defeat of Address Space Layout Randomization (ASLR).
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Analysis
by VulDB Data Team • 09/01/2026
The vulnerability identified within the SFTP backend of GVFS represents a critical information disclosure flaw rooted in improper memory handling during protocol negotiation. Specifically, the issue resides in the read_string function which is responsible for parsing incoming data from an SFTP server. When mounting a share, the client allocates a buffer to receive specific fields from server replies. In this instance, while the allocation size is determined by a length field provided by the malicious server, the implementation fails to verify that the received data actually fills the entire allocated buffer. This discrepancy creates a scenario where the remainder of the heap-allocated memory retains its previous state, containing uninitialized or stale data rather than being zeroed out as security best practices would dictate.
The operational impact becomes severe when a malicious SFTP server sends an FXP_HANDLE reply that is shorter than expected. Because the buffer was not fully overwritten by the incoming packet, the uninitialized heap contents remain in the upper portion of the allocated memory block. The gvfsd-sftp process incorrectly interprets these leftover bytes as part of the file handle data structure. Consequently, when the client subsequently issues requests using this corrupted handle, it echoes these uninitialized bytes back to the server. This behavior transforms a simple parsing error into a powerful remote information leak mechanism, allowing an attacker positioned on the network or controlling the SFTP endpoint to read arbitrary portions of the process heap memory deterministically.
This vulnerability has profound implications for system security, particularly regarding Address Space Layout Randomization (ASLR). ASLR is a fundamental memory protection technique used in modern operating systems to randomize the memory addresses where key program components are loaded, thereby making it difficult for attackers to predict locations of executable code or data structures needed for exploitation. By leaking uninitialized heap contents that include pointers and offsets related to the gvfsd-sftp process, an attacker can deterministically calculate the base address of the heap and the load address of critical libraries such as libgio. This effectively defeats ASLR, providing the precise memory layout information required to craft reliable exploits against other vulnerabilities in the same or related processes.
From a classification perspective, this flaw aligns with CWE-120 Buffer Copy without Checking Size of Input which describes cases where data is copied into a buffer without verifying that it fits within the allocated space, although here the specific failure is more accurately categorized under CWE-908 Use of Uninitialized Resource or CWE-457 Use of Uninitialized Variable. The exploitation technique leverages memory disclosure to bypass security mitigations, which maps closely to ATT&CK techniques related to Memory Disclosure and potentially Defense Evasion through ASLR bypass. Such vulnerabilities are particularly dangerous because they do not require code execution on the target but instead rely on network interaction to extract sensitive internal state information that should remain confidential.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary fix involves ensuring that all buffers allocated for receiving protocol data are explicitly zeroed out before use or after parsing, guaranteeing that any portion of the buffer not overwritten by valid input contains null bytes rather than stale heap data. Developers should also implement strict length validation checks to ensure that received packets match expected sizes and handle short reads gracefully without propagating uninitialized memory. Additionally, enabling compiler-based protections such as stack canaries and full ASLR enforcement on supported platforms provides a layer of defense-in-depth, although these measures are secondary to fixing the root cause in the application logic. Users should apply vendor-provided patches for gvfs promptly and restrict access to SFTP services to trusted networks or authenticated users where possible to reduce the attack surface available to malicious actors attempting this memory disclosure exploit.