VulnerabilityPublished 6 Oct 2026Updated 29 Oct 2026

CVE-2026-104874: Reference leak in CIMultiDict/MultiDict items-view union and subtraction

Affects lexiconSource GCVE Database

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Authoritative record:CVE-2026-104874· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 3.25.2_4 when convenient.

Data as of 30 Oct 2026, 07:44 UTC

CVSS 3.1 base score

5.3Medium

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

Patch: Available – fixed in 3.25.2_4

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

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

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: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 2 Oct 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 lexicon to 3.25.2_4.

Affected software

Product	Affected	Fixed in
Homebrewlexicon`pkg:brew/lexicon`Other threats	Affected >=3.25.2_3 <3.25.2_4	Fixed in 3.25.2_4

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

AI-suggested

A patch is available in version 3.25.2_4 that fixes the reference leak. Users should upgrade to this fixed version or later to remediate the vulnerability. Since the issue is a memory leak triggered by specific set operations, upgrading the library is the recommended and effective mitigation. No other vendor advisories indicate alternative mitigations or that the issue is already mitigated.

Description

CVE-2026-104874 is a reference leak vulnerability in the items-view union and subtraction operators of the aio-libs/multidict Python library versions 6.7.0 through 6.9.0. The flaw causes unbounded memory growth by leaking key-identity and value objects during certain set operations on dictionary views. This leak occurs because new references are created but never released in specific code paths. The vulnerability can be triggered remotely by supplying large HTTP headers or query strings to servers using this library, causing memory exhaustion until the process is terminated.

Impact

The vulnerability allows a remote attacker to cause unbounded, unreclaimable memory growth in affected servers using aio-libs/multidict by exploiting the reference leak in items-view union and subtraction operations. This can lead to denial of service due to process termination from resource exhaustion. There is no direct impact on confidentiality or integrity, only availability is affected.

Summary

The vulnerability is a reference leak in the C extension of aio-libs/multidict (versions 6.7.0 to 6.9.0) affecting the items-view union (operand | d.items()) and subtraction (d.items() - operand) operators. The root cause is that the first parse loop in these operators obtains new strong references for identity and value objects but fails to release them, leading to permanent memory leaks. This behavior was introduced due to a change in key-identity ownership semantics between versions 6.6.4 and 6.7.0. The leak is exploitable remotely because these views are built over attacker-controlled HTTP headers and query strings in servers using aio-libs. Garbage collection does not reclaim the leaked objects, causing resident memory to grow monotonically until process termination. Proof-of-concept code demonstrates the leak and its impact on memory usage.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 07:44 UTC

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