VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-48990: joserfc · b64=false RFC7797 JWS payloads bypass JWSRegistry payload-size limits during deserialization

Affects mcpmSource GCVE Database

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Authoritative record:CVE-2026-48990· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 2.15.0_1 when convenient.

Data as of 30 Oct 2026, 07:00 UTC

CVSS 3.1 base score

5.3Medium

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

EPSS: 0.1% chance of exploitation in 30 days – higher than 5% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.15.0_1

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 17 Jun 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 mcpm to 2.15.0_1.

Affected software

Product	Affected	Fixed in
Homebrewmcpm`pkg:brew/mcpm`Other threats	Affected >=2.14.0_4 <2.15.0_1	Fixed in 2.15.0_1

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

AI-suggested

A fixed release (2.15.0_1) is available that enforces payload size limits correctly for RFC7797 b64=false JWS payloads. Users should upgrade to this version or later to remediate the issue. Until upgraded, applications should consider additional application-layer payload size checks for unencoded JWS payloads to mitigate resource exhaustion risks.

Description

The joserfc library version 1.6.5 and earlier allow oversized RFC7797 b64=false JWS payloads to bypass the configured payload size limit during deserialization. This occurs because the payload size validation is not applied to unencoded payloads as specified by RFC7797, unlike normal base64url-encoded JWS payloads. This flaw can lead to moderate availability or resource exhaustion risks in applications that rely on joserfc to enforce payload size limits for security.

Impact

The vulnerability allows attackers to submit JWS tokens with unencoded payloads larger than the configured maximum size, bypassing payload size restrictions. This can lead to resource exhaustion or denial of service conditions in applications that process such tokens, impacting availability. There is no impact on confidentiality or integrity reported. Exploitation requires sending oversized RFC7797 b64=false JWS tokens to the vulnerable application.

Summary

In joserfc versions up to 1.6.5, the JWSRegistry.max_payload_length setting enforces a maximum payload size for JWS tokens. However, when deserializing RFC7797-compliant JWS tokens with the b64=false header parameter (indicating an unencoded payload), the library does not invoke the payload size validation function. The normal compact and flattened JSON JWS deserialization paths correctly reject payloads exceeding the size limit with an ExceededSizeError, but the RFC7797 unencoded payload paths accept payloads larger than the configured limit. This discrepancy allows oversized payloads to be deserialized successfully, potentially causing resource exhaustion or availability issues in applications that accept lower-trust JWS tokens and depend on joserfc for payload size enforcement. The issue affects multiple tested releases including 1.3.4 through 1.6.5. A fix is available in version 2.15.0_1, which addresses this validation bypass.

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Exploitation evidence

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 07:00 UTC

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