CVE-2026-77640 in Tor
Summary
by MITRE • 08/21/2026
tor before 0.4.9.9 was prone to an infinite loop when decompressing a truncated zlib/gzip stream with done=1. A truncated stream never reaches Z_STREAM_END, causing zlib to return Z_BUF_ERROR with no input remaining, which buf_add_compress() mistook for a full output buffer and retried forever. Fixed by returning TOR_COMPRESS_ERROR in that case so the caller can abort cleanly. This is TROVE-2026-021.
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Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified as CVE ID associated with TROVE-2026-021 affects versions of Tor prior to 0.4.9.9 and represents a significant reliability issue rooted in the handling of compressed data streams within the application's networking layer. The core technical flaw lies in the decompression logic, specifically when processing zlib or gzip formatted data that has been truncated or is otherwise incomplete. Under normal operation, a valid compression stream will eventually reach an end state signaled by Z_STREAM_END from the underlying library. However, when presented with a truncated stream, the zlib library returns Z_BUF_ERROR to indicate that more input data is required but none remains available in the current buffer. This specific return code signifies an incomplete read rather than a fatal error or a full output condition.
The operational failure occurs because the Tor function buf_add_compress() incorrectly interprets this Z_BUF_ERROR response as an indication that the output buffer has reached capacity, prompting it to retry the decompression operation with zero new input data. Since the underlying zlib state remains unchanged and still reports insufficient input, the application enters a tight infinite loop. This behavior effectively consumes all available CPU resources on the affected process without making progress or producing any useful network traffic. The lack of an exit condition for this specific error case transforms what should be a graceful handling of malformed data into a denial-of-service vector that can crash or hang the Tor relay or client instance depending on how the connection is managed by higher-level protocols.
From a classification perspective, this vulnerability aligns with CWE-835, which describes loops that never terminate due to logic errors in condition checking. It also relates to CWE-20 regarding improper input validation, as the application fails to correctly distinguish between different types of buffer-related error codes returned by external libraries. In terms of adversary behavior and defensive mapping, this flaw can be leveraged for Denial-of-Service attacks under MITRE ATT&CK technique T1499, specifically Endpoint Denial of Service via resource exhaustion. An attacker could craft a malicious circuit or connection that sends truncated compressed payloads to trigger the infinite loop on vulnerable relays, thereby degrading network performance and availability for other users relying on those nodes.
The remediation strategy involves updating Tor to version 0.4.9.9 or later, where the decompression logic has been corrected to recognize Z_BUF_ERROR in the context of truncated streams as a fatal compression error rather than a retry condition. The fix ensures that TOR_COMPRESS_ERROR is returned immediately when no input remains and the stream has not ended cleanly, allowing the calling function to abort the operation gracefully. To mitigate this risk in environments where immediate patching may be delayed, operators should monitor for abnormal CPU usage spikes on Tor processes associated with specific connections or circuits. Additionally, implementing network-level rate limiting or payload inspection that rejects malformed compression headers before they reach the decompression engine can provide a layer of defense against exploitation attempts targeting this logic flaw.