VulnerabilityPublished 23 Mar 2026

CVE-2026-26209: agronholm cbor2 · Uncontrolled Recursion

Affects agronholm cbor2Source CVE Database V5

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Not known to be exploited · CVSS 7.5 High : upgrade to 5.9.0 soon.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.0 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 33% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 5.9.0

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 23 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade agronholm cbor2 to 5.9.0.

Affected software

Product	Affected	Fixed in
agronholmcbor2	Affected <5.9.0	Fixed in 5.9.0
PyPIcbor2`pkg:pypi/cbor2`Other threats	Affected <5.9.0	Fixed in 5.9.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade cbor2 to version 5.9.0 or later immediately to apply the official patch that enforces recursion depth limits.
Implement input validation and rate limiting on CBOR payloads to detect and block unusually deeply nested or large CBOR structures before decoding.
Use application-level circuit breakers or worker process supervisors that can gracefully handle RecursionError exceptions without crashing the entire worker.
Consider sandboxing or isolating CBOR decoding in separate processes or containers to limit the blast radius of crashes.
Monitor application logs for RecursionError or worker crashes indicative of exploitation attempts.
If upgrading is not immediately possible, patch or monkey-patch the cbor2 library to add explicit depth limits on recursion during decoding.
Educate developers and DevOps teams about this vulnerability and incorporate CBOR payload fuzz testing into CI/CD pipelines to detect similar issues early.

Description

CVE-2026-26209 is a high-severity vulnerability in the cbor2 Python library versions prior to 5.9.0. It arises from uncontrolled recursion when decoding deeply nested CBOR data structures, allowing an attacker to cause a Denial of Service (DoS) by crashing the worker process. The vulnerability affects both the pure Python and C extension implementations, which rely on Python's recursion limits rather than enforcing a strict depth limit. An attacker can craft a CBOR payload with approximately 100,000 nested arrays to exhaust the stack, triggering a RecursionError and terminating the process. This can be exploited repeatedly with small payloads to disrupt applications using cbor2, such as web servers and task queues. Version 5.9.0 patches this issue.

Impact

This vulnerability can cause complete Denial of Service for applications relying on cbor2 for CBOR data parsing. Since many Python web servers and task queues use cbor2, attackers can repeatedly crash worker processes by sending crafted CBOR payloads, leading to service outages and degraded availability. The attack does not compromise confidentiality or integrity but can severely disrupt business operations, especially for services that rely on CBOR for communication or data serialization. The low size of malicious payloads makes the attack feasible even in bandwidth-constrained environments. Organizations using vulnerable versions of cbor2 in critical infrastructure, microservices, or cloud-native applications risk significant downtime and potential cascading failures if worker processes are not properly isolated or restarted. The vulnerability also increases operational costs due to increased resource consumption and incident response efforts.

CWE-674Uncontrolled Recursion (opens in a new tab)
Class weakness
The product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack.
Implementation
Ensure that an end condition will be reached under all logic conditions. The end condition may include checking against the depth of recursion and exiting with an error if the recursion goes too deep. The complexity of the end condition contributes to the effectiveness of this action.
Implementation
Increase the stack size.
Attack patterns
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 30 Mar 2026, 20:28 UTC

WeaknessesCWE-674

Technical details

References

Community reviews

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