Regular expression backtracking in Starlet chunked decoding exhausts CPU
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CISA ADP. Fix status from CNA. Impact type from CWE-1333.
Summary
Starlet versions from 0.20 before 0.33 for Perl are affected. The chunk-size parser matches a long run of hex digits with a regular expression that backtracks heavily, so processing time grows with the square of the run length (CWE-1333). A remote unauthenticated attacker can occupy a worker for about a minute of CPU time with one 128 KiB chunked request body, before the application is called.
Written with Claude Haiku 5.5 from the sources below · 2026-11-06
Description from the CNA
Starlet versions from 0.20 before 0.33 for Perl allow CPU exhaustion via regular expression backtracking when decoding chunked request bodies. A chunk-size line is matched as a run of hex digits, then .* and a CRLF. A run followed by a bare LF and then a CRLF can never match, and the engine retries the match once for each digit, so the time taken grows with the square of the run length. Nothing limits the length of the run, which can span several reads. A remote attacker can occupy a worker for about a minute of CPU time with one 128 KiB request, before the application is called.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| — | Starlet | 0.20 to before 0.33 | 0.33 | Patch available | CNA |
- Starlet Vendor— Affected 0.20 to before 0.33 Fixed in 0.33 Patch available CNA
Remediation
Upgrade Starlet to version 0.33, which fixes the issue.
Written with Claude Haiku 5.5 from the sources below · 2026-11-06
- Fix · CNA Starlet Fixed in 0.33
Vendor assessments
Timeline
- 2026-10-30 Threatist record created from CVE-2026-115866
- 2026-10-30 Alias added: GHSA-8jc4-2m4j-6vf8
- 2026-10-30 Affected versions changed · Red Hat
- 2026-10-30 CVSS score added · Red Hat
- 2026-11-06 CVSS score added · CISA ADP
- 2026-11-06 CISA SSVC decision points changed
References
Sources
- CVE-2026-115866 · CVE Program
- CVE-2026-115866 · NVD
- FIRST EPSS
- GHSA-8jc4-2m4j-6vf8 · GitHub Advisory Database · CC BY 4.0
- CVE-2026-115866 · Red Hat security data · CC BY 4.0
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