CVE-2026-97370 in Junrar
Summary
by MITRE • 10/15/2026
Junrar is an open source Java RAR archive library. Prior to 7.6.1, RarVM.ExecuteCode in src/main/java/com/github/junrar/unpack/vm/RarVM.java can repeatedly execute a VM_JMP instruction whose target is greater than or equal to the prepared program's codeSize. The setIP() function returns success without changing the instruction pointer or decrementing maxOpCount, and the jump handler continues before loop-end accounting, leaving both values unchanged. A crafted RAR4 archive can therefore keep an extraction thread in an infinite CPU loop and exhaust archive-processing worker capacity. The RAR virtual machine does not expose native process execution, so the supported impact is availability loss rather than code execution. This issue is fixed in version 7.6.1.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in Junrar, specifically within versions prior to 7.6.1, centers on a critical logic error in the RAR Virtual Machine implementation located in the src/main/java/com/github/junrar/unpack/vm/RarVM.java file. This open-source Java library is widely used for handling RAR archive formats, and its robustness is essential for applications that process compressed data from untrusted sources. The core flaw resides within the ExecuteCode method, which manages the execution flow of virtual machine instructions during archive extraction. Specifically, the vulnerability affects how jump instructions are handled when their target addresses exceed or equal the prepared program's codeSize boundary. In a correctly implemented state machine, such out-of-bounds jumps should trigger an error condition or terminate execution to prevent undefined behavior. However, in this flawed implementation, the setIP function incorrectly returns success without actually updating the instruction pointer register nor decrementing the maximum operation count counter. This deviation from expected control flow logic creates a scenario where the virtual machine enters a state of infinite recursion or looping.
From a technical perspective, the issue stems from the jump handler continuing its execution before performing loop-end accounting operations. Because both the instruction pointer and the maxOpCount remain unchanged after processing an invalid JMP instruction with a target address greater than or equal to codeSize, the virtual machine effectively gets stuck in a tight loop. It repeatedly attempts to execute the same out-of-bounds instruction without advancing state or respecting termination limits designed to prevent infinite execution. This behavior allows a crafted RAR4 archive containing maliciously constructed internal structures to exploit this logic gap. When an application processes such a file, the extraction thread responsible for decompressing the data becomes trapped in an endless CPU-intensive cycle. The lack of proper bounds checking and state management means that the library fails to detect or halt execution when encountering these malformed instructions, leading directly to resource exhaustion on the host system.
The operational impact of this vulnerability is primarily focused on availability rather than confidentiality or integrity. Since the RAR virtual machine implemented in Junrar does not expose native process execution capabilities, an attacker cannot leverage this flaw to achieve arbitrary code execution or remote command injection. The primary consequence is a denial-of-service condition where one or more archive-processing worker threads are consumed indefinitely by the infinite loop. In server environments or applications that handle multiple concurrent requests involving RAR archives, this can lead to thread pool exhaustion. As available worker threads are tied up processing malicious files, legitimate users experience service degradation or complete unavailability of the application's features related to file extraction and compression. This represents a significant risk for web services, email gateways, or any software component that accepts user-uploaded RAR archives without strict validation prior to decompression.
This vulnerability aligns with CWE-675, which describes operations on multiple resources without sufficient monitoring of resource consumption limits, as well as CWE-835 regarding loop errors where a condition fails to terminate properly due to incorrect state updates. In the context of the MITRE ATT&CK framework, this behavior is consistent with techniques used in denial-of-service attacks aimed at exhausting computational resources, specifically falling under tactics related to impact and resource hijacking. The lack of proper input validation for internal virtual machine instructions allows an external actor to manipulate system resources effectively without needing elevated privileges or complex exploitation chains beyond the delivery of a crafted file.
To mitigate this risk, organizations relying on Junrar must upgrade immediately to version 7.6.1 or later, where the logic error in the jump handler and instruction pointer management has been corrected. The fix ensures that out-of-bounds jumps are properly detected and handled, preventing the infinite loop condition. Additionally, defensive measures should include implementing strict limits on archive extraction depth and complexity at the application layer to provide a secondary defense-in-depth strategy against malformed archives. Input validation mechanisms should be employed to reject RAR files with suspicious internal structures before they reach the decompression engine. Regular security audits of dependencies are also recommended to ensure that similar logic flaws in other compression libraries do not pose comparable threats to system availability and stability.