Server crash via negative HTTP chunk size
Published Jul 27, 2026 · Updated Jul 27, 2026
Out-of-bounds read in facil.io 0.7.5 through 0.7.6 allows remote attackers to crash HTTP/1.1 servers with a negative chunk size. The http1_atol16 parser accepts a minus sign, and the chunk state calculation turns the negative value into a large positive length that moves the read pointer before the input buffer. A single unauthenticated chunked POST request triggers the fault, interrupting the affected server process without reported data disclosure or modification.
Summary
What happened
Out-of-bounds read in facil.io 0.7.5 through 0.7.6 allows remote attackers to crash HTTP/1.1 servers with a negative chunk size. The http1_atol16 parser accepts a minus sign, and the chunk state calculation turns the negative value into a large positive length that moves the read pointer before the input buffer. A single unauthenticated chunked POST request triggers the fault, interrupting the affected server process without reported data disclosure or modification.
The record
- CVE
- CVE-2026-66731
- Published
- Jul 27, 2026
- Updated
- Jul 27, 2026
- Vendor
- Boaz Segev
- Product
- facil.io
- Classifications
- CWE-125, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 27, 2026CVE publishedPublication date reported by the CVE source.
- Jul 27, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0.7.5 <=0.7.6
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- CVE-2026-66731.pyproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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