Service interruption via recursive nested-container decoding
Published Mar 23, 2026 · Updated Mar 23, 2026
Uncontrolled recursion in cbor2 before 5.9.0 allows remote attackers to interrupt services by submitting deeply nested CBOR data. The CBORDecoder decode_array and decode_map paths recursively invoke decode for nested values without tracking a data-specific depth limit, causing RecursionError at Python's recursion ceiling. Exploitation requires an application to pass attacker-controlled CBOR to cbor2.loads without containing the exception; repeated payloads can terminate and exhaust worker processes.
Summary
What happened
Uncontrolled recursion in cbor2 before 5.9.0 allows remote attackers to interrupt services by submitting deeply nested CBOR data. The CBORDecoder decode_array and decode_map paths recursively invoke decode for nested values without tracking a data-specific depth limit, causing RecursionError at Python's recursion ceiling. Exploitation requires an application to pass attacker-controlled CBOR to cbor2.loads without containing the exception; repeated payloads can terminate and exhaust worker processes.
The record
- CVE
- CVE-2026-26209
- Published
- Mar 23, 2026
- Updated
- Mar 23, 2026
- Vendor
- Alex Grönholm
- Product
- cbor2
- Classifications
- CWE-674, CAPEC-230, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 23, 2026CVE publishedPublication date reported by the CVE source.
- Mar 23, 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=< 5.9.0
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
- Upstream nested-array RecursionError proof of conceptproof of concept · unverified
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Technologies
Your stack
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Your stack
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