CVE-2026-26209 | cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads

Exp

cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (<100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.

Published: 2026-03-23 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-26209 is rated Exploit Available (57/100): CVSS High severity, with low exploitation likelihood (EPSS 0.42%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2026-26209

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2026-26209

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.06% 0.42% +0.36%
2 2026-03-29 0.04% 0.06% +0.01%
3 2026-03-24 — 0.04% —

Full EPSS history (3 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-26209

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.0 HIGH
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2026-26209

GitHub Security Advisory for CVE-2026-26209

GHSA-3c37-wwvx-h642 · Severity: high · Ecosystem: pip — cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads

OS Trackers for CVE-2026-26209

vendor priority summary link
alpine — CVE-2026-26209: 1 source package rows (py3-cbor2); 3 state rows across 2 repos (3.23-community, edge-community); fixed 0, open 3. https://security.alpinelinux.org/vuln/CVE-2026-26209
debian not yet assigned CVE-2026-26209 not yet assigned priority: Debian including 1 source packages (cbor2), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 3, resolved 2. https://security-tracker.debian.org/tracker/CVE-2026-26209
redhat medium — https://access.redhat.com/security/cve/CVE-2026-26209
suse high CVE-2026-26209 severity important: SUSE including 3 source package names (python311-cbor2-5.9.0-1.1, python313-cbor2-5.6.5-160000.4.1, python313-cbor2-5.9.0-1.1), 4 product×package rows across 3 product lines (SUSE Linux Enterprise Server 16.0, openSUSE Leap 16.0, openSUSE Tumbleweed): Fixed 3, First Fixed 1. https://www.suse.com/security/cve/CVE-2026-26209/
ubuntu medium CVE-2026-26209 medium priority: Ubuntu including 1 source packages (cbor2), 4 status rows across 4 suites (jammy, noble, questing, upstream): needs-triage 4. https://ubuntu.com/security/CVE-2026-26209

Affected software / configurations for CVE-2026-26209

Vendor Product Version Raw CPE
agronholm cbor2 < 5.9.0 cpe:2.3:a:agronholm:cbor2:*:*:*:*:*:python:*:*

References for CVE-2026-26209

cvelogic Threat Intelligence