VulnerabilityPublished 18 Nov 2025Updated 19 Nov 2025

CVE-2025-64076: cbor2 · Integer Underflow

Affects cbor2Source CVE Database V5

Not known to be exploited · CVSS 7.5 High : upgrade to 5.7.1 soon.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 41% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 5.7.1

CVSS:3.1/AC:L/AV:N/A:H/C:N/I:N/PR:N/S:U/UI:N

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

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

Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
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).
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).
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted 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.
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.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 18 Nov 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 cbor2 to 5.7.1.

Affected software

Product	Affected	Fixed in
PyPIcbor2`pkg:pypi/cbor2`Other threats	Affected <5.7.1	Fixed in 5.7.1

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

AI-suggested

European organizations should immediately upgrade all instances of the cbor2 library to version 5.7.1 or later, which contains the fixes for these vulnerabilities. For environments where immediate upgrade is not feasible, implement strict input validation and filtering to block or sanitize CBOR payloads containing large definite-length text strings with multi-byte UTF-8 characters, especially those near 65536-byte chunk boundaries. Deploy runtime application monitoring to detect abnormal memory usage patterns or frequent CBORDecodeEOF exceptions indicative of exploitation attempts. Network-level protections such as Web Application Firewalls (WAFs) or Intrusion Prevention Systems (IPS) should be configured to detect and block suspicious CBOR traffic patterns. Additionally, isolate systems processing untrusted CBOR data within segmented network zones to limit potential impact. Regularly audit dependencies and maintain an up-to-date software bill of materials (SBOM) to quickly identify and remediate vulnerable library versions. Finally, educate development and security teams about the risks associated with CBOR data processing and the importance of timely patching.

Description

Multiple vulnerabilities exist in cbor2 through version 5.7.0 in the decode_definite_long_string() function of the C extension decoder (source/decoder.c): (1) Integer Underflow Leading to Out-of-Bounds Read (CWE-191, CWE-125): An incorrect variable reference and missing state reset in the chunk processing loop causes buffer_length to not be reset to zero after UTF-8 character consumption. This results in subsequent chunk_length calculations producing negative values (e.g., chunk_length = 65536 - buffer_length), which are passed as signed integers to the read() method, potentially triggering unlimited read operations and resource exhaustion. (2) Memory Leak via Missing Reference Count Release (CWE-401): The main processing loop fails to release Python object references (Py_DECREF) for chunk objects allocated in each iteration. For CBOR strings longer than 65536 bytes, this causes cumulative memory leaks proportional to the payload size, enabling memory exhaustion attacks through repeated processing of large CBOR payloads. Both vulnerabilities can be exploited remotely without authentication by sending specially-crafted CBOR data containing definite-length text strings with multi-byte UTF-8 characters positioned at 65536-byte chunk boundaries. Successful exploitation results in denial of service through process crashes (CBORDecodeEOF exceptions) or memory exhaustion. The vulnerabilities affect all applications using cbor2's C extension to process untrusted CBOR data, including web APIs, IoT data collectors, and message queue processors. Fixed in commit 851473490281f82d82560b2368284ef33cf6e8f9 pushed with released version 5.7.1.

Impact

For European organizations, the impact of CVE-2025-64076 is significant, especially for those relying on cbor2 for processing CBOR-encoded data in critical infrastructure, IoT deployments, or cloud-based APIs. Exploitation can lead to denial of service conditions, causing application crashes or memory exhaustion, which disrupts service availability and may impact operational continuity. In sectors such as healthcare, manufacturing, and smart city infrastructure, where IoT data collectors and message queues are prevalent, these disruptions could have cascading effects on safety and productivity. Additionally, denial of service attacks could be leveraged as part of multi-vector campaigns targeting European enterprises, potentially amplifying the impact on critical services. Since exploitation requires no authentication or user interaction, attackers can remotely trigger these conditions by sending malicious CBOR payloads, increasing the risk of widespread exploitation if vulnerable systems are exposed to untrusted data sources.

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 25 Nov 2025, 19:12 UTC

WeaknessesCWE-191

Technical details

References

Community reviews

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