jsonwebtoken has Type Confusion that leads to potential authorization bypass

CISA KEV lists vulnerabilities known to be exploited. FIRST EPSS estimates the likelihood of exploitation in the next 30 days. An unavailable score does not mean zero risk.

No exploitation or prediction data is available here for these CVEs. This does not mean they are safe or unlikely to be exploited.

jsonwebtoken is a JWT lib in rust. Prior to version 10.3.0, there is a Type Confusion vulnerability in jsonwebtoken, specifically, in its claim validation logic. When a standard claim (such as nbf or exp) is provided with an incorrect JSON type (Like a String instead of a Number), the library’s internal parsing mechanism marks the claim as “FailedToParse”. Crucially, the validation logic treats this “FailedToParse” state identically to “NotPresent”. This means that if a check is enabled (like: validate_nbf = true), but the claim is not explicitly marked as required in required_spec_claims, the library will skip the validation check entirely for the malformed claim, treating it as if it were not there. This allows attackers to bypass critical time-based security restrictions (like “Not Before” checks) and commit potential authentication and authorization bypasses. This issue has been patched in version 10.3.0.

Source published

4 Feb 2026 21:31:53 UTC

Source modified

5 Feb 2026 20:52:48 UTC

Saved here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

Source: cve-program · CVE Program Terms of Use. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 4.0 · 5.5 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P

jsonwebtoken is a JWT lib in rust. Prior to version 10.3.0, there is a Type Confusion vulnerability in jsonwebtoken, specifically, in its claim validation logic. When a standard claim (such as nbf or exp) is provided with an incorrect JSON type (Like a String instead of a Number), the library’s internal parsing mechanism marks the claim as “FailedToParse”. Crucially, the validation logic treats this “FailedToParse” state identically to “NotPresent”. This means that if a check is enabled (like: validate_nbf = true), but the claim is not explicitly marked as required in required_spec_claims, the library will skip the validation check entirely for the malformed claim, treating it as if it were not there. This allows attackers to bypass critical time-based security restrictions (like “Not Before” checks) and commit potential authentication and authorization bypasses. This issue has been patched in version 10.3.0.

Source published

Feb 4, 2026, 9:31:53 PM (timezone not stated)

Source modified

Feb 5, 2026, 8:52:48 PM (timezone not stated)

Saved here

15 Nov 2026 07:40:42 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

Source: euvd · ENISA EUVD · source terms and attribution apply. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

EUVD CVSS 4.0 · 5.5 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P

jsonwebtoken is a JWT lib in rust. Prior to version 10.3.0, there is a Type Confusion vulnerability in jsonwebtoken, specifically, in its claim validation logic. When a standard claim (such as nbf or exp) is provided with an incorrect JSON type (Like a String instead of a Number), the library’s internal parsing mechanism marks the claim as “FailedToParse”. Crucially, the validation logic treats this “FailedToParse” state identically to “NotPresent”. This means that if a check is enabled (like: validate_nbf = true), but the claim is not explicitly marked as required in required_spec_claims, the library will skip the validation check entirely for the malformed claim, treating it as if it were not there. This allows attackers to bypass critical time-based security restrictions (like “Not Before” checks) and commit potential authentication and authorization bypasses. This issue has been patched in version 10.3.0.

Source published

4 Feb 2026 22:15:59 UTC

Source modified

17 Jun 2026 10:24:48 UTC

Saved here

25 Oct 2026 06:17:16 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

Source: nvd · NIST NVD usage terms; underlying source attribution retained. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS 4.0 · [email protected] · Secondary · 5.5 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Primary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

## Summary: It has been discovered that there is a Type Confusion vulnerability in jsonwebtoken, specifically, in its claim validation logic. When a standard claim (such as nbf or exp) is provided with an incorrect JSON type (Like a String instead of a Number), the library’s internal parsing mechanism marks the claim as “FailedToParse”. Crucially, the validation logic treats this “FailedToParse” state identically to “NotPresent”. This means that if a check is enabled (like: validate_nbf = true), but the claim is not explicitly marked as required in required_spec_claims, the library will skip the validation check entirely for the malformed claim, treating it as if it were not there. This allows attackers to bypass critical time-based security restrictions (like “Not Before” checks) and commit potential authentication and authorization bypasses. ## Details: The vulnerability stems from the interaction between the TryParse enum and the validate function in [src/validation.rs](https://github.com/Keats/jsonwebtoken/blob/master/src/validation.rs). 1. The TryParse Enum: The library uses a custom TryParse enum to handle claim deserialization: ``` enum TryParse<T> { Parsed(T), FailedToParse, // Set when deserialization fails (e.g. type mismatch) NotPresent, } ``` If a user sends {“nbf”: “99999999999”} (legacy/string format), serde fails to parse it as u64, and it results in TryParse::FailedToParse. 1. The Validation Logic Flaw (src/validation.rs): In Validation::validate, the code checks for exp and nbf like this: ``` // L288-291 if matches!(claims.nbf, TryParse::Parsed(nbf) if options.validate_nbf && nbf > now + options.leeway) { return Err(new_error(ErrorKind::ImmatureSignature)); } ``` This matches! macro explicitly looks for TryParse::Parsed(nbf). • If claims.nbf is FailedToParse, the match returns false. • The if block is skipped. • No error is returned. 1. The “Required Claims” Gap: The only fallback mechanism is the “Required Claims” check: ``` // Lines 259-267 for required_claim in &options.required_spec_claims { let present = match required_claim.as_str() { "nbf" => matches!(claims.nbf, TryParse::Parsed(_)), // ... }; if !present { return Err(...); } } ``` If “nbf” IS in required_spec_claims, FailedToParse will fail the matches!(..., Parsed(_)) check, causing the present to be false, and correctly returning an error. However, widely accepted usage patterns often enable validation flags (validate_nbf = true) without adding the claim to the required list, assuming that enabling validation implicitly requires the claim’s validity if it appears in the token. jsonwebtoken seems to violate this assumption. Environment: • Version: jsonwebtoken 10.2.0 • Rust Version: rustc 1.90.0 • Cargo Version: cargo 1.90.0 • OS: MacOS Tahoe 26.2 POC: For demonstrating, Here is this simple rust code that demonstrates the bypass. It attempts to validate a token with a string nbf claiming to be valid only in the far future. create a new project: ``` cargo new nbf_poc; cd nbf_poc ``` add required dependencies: ``` cargo add serde --features derive cargo add jsonwebtoken --features rust_crypto cargo add serde_json ``` replace the code in src/main.rs with this: ``` use jsonwebtoken::{decode, Validation, Algorithm, DecodingKey, Header, EncodingKey, encode}; use serde::{Deserialize, Serialize}; #[derive(Debug, Serialize, Deserialize)] struct Claims { sub: String, nbf: String, // Attacker sends nbf as a String exp: usize, } fn main() { let key: &[u8; 24] = b"RedMouseOverTheSkyIsBlue"; // nbf is a String "99999999999" (Far future) // Real nbf should be a Number. let my_claims: Claims = Claims { sub: "krishna".to_string(), nbf: "99999999999".to_string(), exp: 10000000000, }; let token: String = encode(&Header::default(), &my_claims, &EncodingKey::from_secret(key)).unwrap(); println!("Forged Token: {}", token); // 2. Configure Validation let mut validation: Validation = Validation::new(Algorithm::HS256); validation.validate_nbf = true; // Enable NBF check // We do NOT add "nbf" to required_spec_claims (default behavior) // We decode to serde_json::Value to avoid strict type errors in our struct definition hiding the library bug. // The library sees the raw JSON with string "nbf". let result: Result<jsonwebtoken::TokenData<serde_json::Value>, jsonwebtoken::errors::Error> = decode::<serde_json::Value>( &token, &DecodingKey::from_secret(key), &validation ); match result { Ok(_) => println!("Token was accepted despite malformed far-future 'nbf'!"), Err(e) => println!("Token rejected. Error: {:?}", e), } } ``` run cargo run expected behaviour: ``` Forged Token: eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJrcmlzaG5hIiwibmJmIjoiOTk5OTk5OTk5OTkiLCJleHAiOjEwMDAwMDAwMDAwfQ.Fm3kZIqMwqIA6sEA1w52UOMqqnu4hlO3FQStFmbaOwk ``` Token was accepted despite malformed far-future 'nbf'! Impact: If an application uses jsonwebtoken nbf (Not Before) to schedule access for the future (like “Access granted starting tomorrow”). By sending nbf as a string, an attacker can bypass this restriction and access the resource immediately. and for the exp claim (this is unlikely but still adding), If a developer sets validate_exp = true but manually handles claim presence (removing exp from required_spec_claims), an attacker can send a string exp (e.g., “never”) and bypass expiration checks entirely. The token becomes valid forever.

Source published

3 Feb 2026 18:47:40 UTC

Source modified

10 Sep 2026 03:50:35 UTC

Saved here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · GitHub Advisory Database · CC-BY-4.0. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P

Publisher rating · MODERATE