Tor Project prior 0.4.9.9 Decompression buf_add_compress infinite loop

Summaryinfo

A vulnerability classified as problematic has been found in Tor Project Tor. This impacts the function buf_add_compress of the component Decompression. The manipulation leads to infinite loop. This vulnerability is referenced as CVE-2026-77640. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability was found in Tor Project Tor. It has been rated as problematic. This issue affects the function buf_add_compress of the component Decompression. The manipulation with an unknown input leads to a infinite loop vulnerability. Using CWE to declare the problem leads to CWE-835. The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. Impacted is availability. The summary by CVE is:

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.

It is possible to read the advisory at gitlab.torproject.org. The identification of this vulnerability is CVE-2026-77640 since 08/20/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/21/2026). The attack technique deployed by this issue is T1499 according to MITRE ATT&CK.

The vulnerability scanner Nessus provides a plugin with the ID 338657 (Linux Distros Unpatched Vulnerability : CVE-2026-77640), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 0.4.9.9 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (338657). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Project Management Software

Vendor

• Tor Project

Name

• Tor

License

• open-source

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CPE 2.2info

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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