CVE-2026-82454 in Omnivore
Summary
by MITRE • 08/29/2026
The Omnivore API (packages/api) before the fix in commit abf53d6 contains an authentication bypass in Apple sign-in token verification. The decodeAppleToken function extracted the 'alg' field from the attacker-supplied JWT header and passed it as the sole allowed algorithm to jwt.verify(). Using jsonwebtoken v8 (which does not validate key/algorithm compatibility), an attacker can set alg=HS256 and sign a forged token using Apple's publicly available RSA public key as the HMAC secret, bypassing signature verification and impersonating any Apple-linked account.
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Analysis
by VulDB Data Team • 08/29/2026
The vulnerability identified in the Omnivore API prior to commit abf53d6 represents a critical authentication bypass rooted in improper validation of JSON Web Tokens during the Apple Sign-In integration process. The core technical flaw resides within the decodeAppleToken function, which is responsible for verifying the integrity and authenticity of tokens issued by Apple's identity services. When processing these tokens, the function extracts the algorithm identifier from the JWT header field labeled alg and passes this value directly to the jwt.verify method as the sole permitted signing algorithm. This design choice creates a severe security gap because it relies entirely on the external input for determining how signature verification should be performed without adequately ensuring that the specified algorithm is cryptographically compatible with the provided public key material.
The severity of this flaw is significantly amplified by the specific version of the jsonwebtoken library in use, specifically version 8.x. In these versions, the jwt.verify function does not strictly enforce compatibility between the declared signing algorithm and the type of cryptographic key supplied for verification. Consequently, an attacker can manipulate the JWT header to specify HMAC-SHA256 (HS256) as the algorithm instead of the expected RSA-based algorithms such as RS256 or ES256. Since Apple publishes its public keys publicly for developers to verify signatures, this information is readily available to potential adversaries. By setting alg=HS256 and using one of Apple's published RSA public keys as if it were an HMAC secret key, the attacker can forge valid-looking tokens that pass signature verification checks despite being cryptographically invalid under standard security assumptions.
This mechanism allows for a complete authentication bypass where an adversary can impersonate any user account linked to an Apple ID without possessing the corresponding private signing key from Apple. The attack vector involves crafting a malicious JWT with the altered algorithm header, signing it using the public RSA key as the HMAC secret, and submitting this forged token to the Omnivore API endpoints that rely on decodeAppleToken for authentication. Because the verification logic accepts the HS256 signature generated by the attacker's use of the public key, the system erroneously treats the malicious token as authentic. This results in unauthorized access to user data, potential account takeover, and a complete compromise of the application's trust model regarding third-party identity providers.
From a classification perspective, this vulnerability aligns with CWE-287 Improper Authentication, specifically involving the failure to validate cryptographic parameters correctly during authentication processes. It also maps to MITRE ATT&CK technique T1078 Valid Accounts, as it enables an attacker to assume legitimate user identities through token forgery rather than credential theft or brute force attacks. The root cause is a classic example of algorithm confusion or algorithm substitution attack, where the system's reliance on client-supplied metadata for security decisions leads to exploitation when that metadata can be manipulated by an adversary.
To mitigate this vulnerability, it is imperative to upgrade the jsonwebtoken library to version 9.x or later, which enforces stricter validation rules regarding key-algorithm compatibility and prevents the use of asymmetric public keys as symmetric HMAC secrets. Additionally, developers should implement explicit allowlists for acceptable algorithms rather than dynamically accepting any algorithm specified in the JWT header. For Apple Sign-In specifically, the application must strictly enforce RS256 or ES256 as the only valid signing algorithms regardless of what is declared in the token header. Implementing these controls ensures that even if an attacker attempts to manipulate the alg field, the verification process will reject tokens with mismatched cryptographic parameters, thereby preserving the integrity of the authentication flow and preventing unauthorized access.