VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-49852: joserfc · HS256/HS384/HS512 verify accepts empty/nil HMAC key (cross-language sibling of CVE-2026-45363)

Affects mcpmSource GCVE Database

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Mentioned vulnerabilities

CVE-2026-45363

Not known to be exploited · CVSS 8.7 High : upgrade to 2.15.0_1 soon.

Data as of 30 Oct 2026, 07:00 UTC

CVSS 4.0 base score

8.7High

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

EPSS: 0.1% chance of exploitation in 30 days – higher than 6% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.15.0_1

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: High
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 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 version of joserfc is available in 2.15.0_1, which rejects empty HMAC keys during signing and verification. Users should upgrade to this version or later. Until then, ensure that the JWT secret key is never empty or None by validating configuration sources such as environment variables, database entries, or key finder functions. Avoid defaulting to empty strings for missing keys. No other mitigations are indicated by the vendor advisory.

Description

The joserfc library versions up to 1.6.7 accept attacker-forged HMAC-signed JWT tokens when the verification key is empty or None. This occurs because the library does not reject zero-length HMAC keys, allowing an attacker to forge tokens without secret knowledge if the configured secret is empty or unset. This vulnerability is a cross-language sibling of CVE-2026-45363 affecting ruby-jwt. No patched release existed at the time of this report, but a fix is available in version 2.15.0_1. The vulnerability requires no authentication and can be triggered if the JWT secret is sourced from an unset environment variable, missing database entry, or similar misconfiguration.

Impact

An attacker can forge valid JWT tokens without knowledge of the secret key if the configured HMAC key is empty or None. This allows bypassing authentication or authorization mechanisms relying on JWT verification, potentially leading to unauthorized access. The vulnerability requires no privileges and can be triggered remotely if the application calls joserfc.jwt.decode with a misconfigured or missing secret key. The CVSS 4.0 score is 8.7 (high severity), reflecting the network attack vector, no required privileges, and high impact on integrity and availability.

Summary

The joserfc library's HMACAlgorithm class uses the Python hmac module to verify JWT tokens signed with HS256, HS384, or HS512 algorithms. However, the import_key function only warns when the key length is less than 14 bytes and does not reject empty keys. As a result, if the verification key is empty or None, the library accepts attacker-forged tokens because hmac.new with an empty key produces a predictable digest. This issue is similar to CVE-2026-45363 in ruby-jwt, which added explicit checks to reject empty keys. The vulnerability affects joserfc versions up to 1.6.7, with a fix introduced in 2.15.0_1. The flaw can be exploited without authentication if the application uses an empty or unset JWT secret.

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Affected software

Description

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

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