CVE-2026-89321 in OpenVSX
Summary
by MITRE • 09/14/2026
Publishing limits the compressed size of a VSIX (ovsx.publishing.max-content-size, 512 MB by default) but nothing limited how large an entry becomes when opened.
On the first request to /vscode/unpkg/{namespace}/{extension}/{version}/{path}, WebResourceService opened the entry with ZipFile.getInputStream() and passed the decompressed stream to Files.copy(), which ran to the end of the stream without counting bytes written. The result was cached under java.io.tmpdir, and that cache evicted by entry count (150), not by size, so it placed no bound on disk usage.
A publisher with access only to their own namespace could therefore upload a small, highly compressible VSIX and cause the server to write far larger files to the temp filesystem — repeating with different files or versions, since a repeat request is served from the cache.
Impact observed: the temp filesystem filled; requests for files not already cached returned 500 with No space left on device; a failed extraction left a partial cache file that blocked later attempts at that path; publishing failed with Failed to read extension file. Metadata and already-cached files kept working, and the server did not stop.
Triggering the extraction needs no authentication — only the upload does.
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability described constitutes an Uncontrolled Resource Consumption flaw within the VSIX publishing infrastructure, specifically affecting the WebResourceService component responsible for handling extension artifacts. This issue arises from a critical design oversight in how compressed archive entries are processed and cached on disk. While the system enforces strict limits on the size of uploaded VSIX files through the ovsx.publishing.max-content-size parameter, which defaults to 512 MB, there is no corresponding mechanism to limit the decompressed size of individual entries within those archives. This asymmetry allows an attacker or a compromised publisher to exploit the disparity between compressed and uncompressed data sizes to exhaust server resources.
Technically, the flaw resides in the handling of HTTP requests directed at the /vscode/unpkg endpoint. When such a request is received for a specific extension version and path, the WebResourceService utilizes Java's ZipFile.getInputStream() method to extract the requested entry from the VSIX archive. The resulting decompressed input stream is then passed directly to Files.copy(), which writes the data to a temporary file in the java.io.tmpdir directory without implementing any byte-counting or size-limiting logic during the copy operation. Consequently, if an uploaded VSIX contains entries with extremely high compression ratios—such as repetitive binary patterns or large blocks of null bytes—the decompressed output can be orders of magnitude larger than the original upload limit permits.
The caching mechanism exacerbates this resource exhaustion by storing these extracted files in a cache that is evicted based on entry count rather than disk space usage. The system maintains a maximum of 150 cached entries, but since each entry's size is unbounded, filling all one hundred and fifty slots with large decompressed files can rapidly consume the available storage capacity of the temporary filesystem. This design choice fails to account for variable file sizes, treating every cache hit as equally significant regardless of its actual disk footprint.
The operational impact of this vulnerability is severe, leading to a denial-of-service condition characterized by disk space exhaustion on the server hosting the service. Once the temp filesystem reaches capacity, subsequent requests for files that are not already cached will fail with HTTP 500 errors accompanied by No space left on device messages. Furthermore, if an extraction process fails due to insufficient space or other interruptions, it may leave behind partial cache files. These orphaned files can block future attempts to access the same path, effectively creating a persistent denial-of-service for specific extension resources even after disk space is freed up through manual intervention or system restarts.
Although publishing actions require authentication and are restricted to publishers within their own namespaces, triggering the resource exhaustion does not require any form of authentication. Any unauthenticated user can send requests to the /vscode/unpkg endpoint to trigger the extraction process. This means that an external attacker could potentially contribute to disk exhaustion by repeatedly requesting large decompressed entries from existing extensions or forcing new extractions if they have access to upload malicious VSIX files under their own namespace. The server continues to operate, but its ability to serve dynamic content is degraded, while metadata and already-cached static files remain accessible until the cache itself becomes corrupted or full.
From a standards perspective, this vulnerability aligns with CWE-400: Uncontrolled Resource Consumption, as the application does not properly control the allocation and maintenance of resources based on user input. It also relates to CWE-770: Allocation of Resources Without Limits or Throttling in the context of file system storage. In terms of attack vectors, this scenario reflects aspects of ATT&CK technique T1496: Resource Hijacking, where an adversary uses computing resources for their own benefit, here resulting in a denial-of-service rather than cryptomining, but still representing unauthorized consumption of infrastructure capacity.
Mitigation strategies must address both the input validation and the caching logic. First, the system should implement strict limits on the decompressed size of individual entries within VSIX archives during extraction. This can be achieved by wrapping the InputStream in a counting filter that throws an exception if the number of bytes read exceeds a predefined threshold significantly lower than or equal to the maximum allowed uncompressed extension size. Second, the caching mechanism must be revised to evict files based on total disk usage rather than entry count. Implementing a Least Recently Used (LRU) cache with a hard cap on total byte size would prevent any single set of extensions from monopolizing storage space. Additionally, introducing temporary file cleanup routines that remove partial or corrupted extraction artifacts upon failure is essential to maintain system stability and ensure that failed requests do not permanently block access to valid resources.