PatchTheCVECVE-2026-77640: tor before 0.4.9.9 was prone to an infinite loop when decompressing a truncated zlib/gzip stream with done=1
CVE-2026-77640 is a low-severity flaw (CVSS 3.7) affecting tor. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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.
Why this priority
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 3.7 (Low) |
| Attack vector | Network (remote) |
| Attack complexity | High |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 0.4% (top 68%) |
| CISA exploited list | No |
| Weakness | CWE-1284 |
| Published | 2026-08-20 |
| Last modified | 2026-10-18 |
Last updated 2026-10-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-77640?
tor: upgrade to 0.4.9.9. Exact commands for each OS are on this page.
Is CVE-2026-77640 being exploited?
CVE-2026-77640 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-77640 affect?
CVE-2026-77640 affects tor. Severity is low (CVSS 3.7). 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_compre
How urgent is CVE-2026-77640?
Priority 26/100. Remotely exploitable, no login, no user action
Software pages
Related CVEs
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