VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-28498: Authlib · Fail-Open Cryptographic Verification in OIDC Hash Binding

Affects weaviate-cliSource GCVE Database

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Not known to be exploited · CVSS 8.2 High : upgrade to 3.3.0_3 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

8.2High

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

Patch: Available – fixed in 3.3.0_3

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

Attack Vector: Network
Attack Complexity: High
Attack Requirements: Present
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: High: The successful attack depends on the evasion or circumvention of security-enhancing techniques in place that would otherwise hinder the attack. These include: Evasion of exploit mitigation techniques, for example, circumvention of address space randomization (ASLR) or data execution prevention (DEP) must be performed for the attack to be successful; Obtaining target-specific secrets. The attacker must gather some target-specific secret before the attack can be successful. A secret is any piece of information that cannot be obtained through any amount of reconnaissance. To obtain the secret the attacker must perform additional attacks or break otherwise secure measures (e.g. knowledge of a secret key may be needed to break a crypto channel). This operation must be performed for each attacked target.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
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 weaviate-cli to 3.3.0_3.

Affected software

Product	Affected	Fixed in
Homebrewweaviate-cli`pkg:brew/weaviate-cli`Other threats	Affected >=3.0.2 <3.3.0_3	Fixed in 3.3.0_3

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

Description

## 1. Executive Summary A critical library-level vulnerability was identified in the **Authlib** Python library concerning the validation of OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (`_verify_hash`) responsible for validating the `at_hash` (Access Token Hash) and `c_hash` (Authorization Code Hash) claims exhibits a **fail-open** behavior when encountering an unsupported or unknown cryptographic algorithm. This flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a deliberately unrecognized `alg` header parameter. The library intercepts the unsupported state and silently returns `True` (validation passed), inherently violating fundamental cryptographic design principles and direct OIDC specifications. --- ## 2. Technical Details & Root Cause The vulnerability resides within the `_verify_hash(signature, s, alg)` function in `authlib/oidc/core/claims.py`: ```python def _verify_hash(signature, s, alg): hash_value = create_half_hash(s, alg) if not hash_value: # ← VULNERABILITY: create_half_hash returns None for unknown algorithms return True # ← BYPASS: The verification silently passes return hmac.compare_digest(hash_value, to_bytes(signature)) ``` When an unsupported algorithm string (e.g., `"XX999"`) is processed by the helper function `create_half_hash` in `authlib/oidc/core/util.py`, the internal `getattr(hashlib, hash_type, None)` call fails, and the function correctly returns `None`. However, instead of triggering a `Fail-Closed` cryptographic state (raising an exception or returning `False`), the `_verify_hash` function misinterprets the `None` return value and explicitly returns `True`. Because developers rely on the standard `.validate()` method provided by Authlib's `IDToken` class—which internally calls this flawed function—there is **no mechanism for the implementing developer to prevent this bypass**. It is a strict library-level liability. --- ## 3. Attack Scenario This vulnerability exposes applications utilizing Hybrid or Implicit OIDC flows to **Token Substitution Attacks**. 1. An attacker initiates an OIDC flow and receives a legitimately signed ID Token, but wishes to substitute the bound Access Token (`access_token`) or Authorization Code (`code`) with a malicious or mismatched one. 2. The attacker re-crafts the JWT header of the ID Token, setting the `alg` parameter to an arbitrary, unsupported value (e.g., `{"alg": "CUSTOM_ALG"}`). 3. The server uses Authlib to validate the incoming token. The JWT signature validation might pass (or be previously cached/bypassed depending on state), progressing to the claims validation phase. 4. Authlib attempts to validate the `at_hash` or `c_hash` claims. 5. Because `"CUSTOM_ALG"` is unsupported by `hashlib`, `create_half_hash` returns `None`. 6. Authlib's `_verify_hash` receives `None` and silently returns `True`. 7. **Result:** The application accepts the substituted/malicious Access Token or Authorization Code without any cryptographic verification of the binding hash. --- ## 4. Specification & Standards Violations This explicit fail-open behavior violates multiple foundational RFCs and Core Specifications. A secure cryptographic library **MUST** fail and reject material when encountering unsupported cryptographic parameters. **OpenID Connect Core 1.0** * **§ 3.2.2.9 (Access Token Validation):** "If the ID Token contains an `at_hash` Claim, the Client MUST verify that the hash value of the Access Token matches the value of the `at_hash` Claim." Silencing the validation check natively contradicts this absolute requirement. * **§ 3.3.2.11 (Authorization Code Validation):** Identically mandates the verification of the `c_hash` Claim. **IETF JSON Web Token (JWT) Best Current Practices (BCP)** * **RFC 8725 § 3.1.1:** "Libraries MUST NOT trust the signature without verifying it according to the algorithm... if validation fails, the token MUST be rejected." Authlib's implementation effectively "trusts" the hash when it cannot verify the algorithm. **IETF JSON Web Signature (JWS)** * **RFC 7515 § 5.2 (JWS Validation):** Cryptographic validations must reject the payload if the specified parameters are unsupported. By returning `True` for an `UnsupportedAlgorithm` state, Authlib violates robust application security logic. --- ## 5. Remediation Recommendation The `_verify_hash` function must be patched to enforce a `Fail-Closed` posture. If an algorithm is unsupported and cannot produce a hash for comparison, the validation **must** fail immediately. **Suggested Patch (`authlib/oidc/core/claims.py`):** ```python def _verify_hash(signature, s, alg): hash_value = create_half_hash(s, alg) if hash_value is None: # FAIL-CLOSED: The algorithm is unsupported, reject the token. return False return hmac.compare_digest(hash_value, to_bytes(signature)) ``` --- ## 6. Proof of Concept (PoC)

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