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Vulnerabilities

Vulnerability · HSEC-2026-0010

Improper parsing of fractional NumericDate from JWT exp claims

Published Sep 29, 2026 · catalog updated Sep 29, 2026 · Improper parsing of fractional

Open the advisory

Exploitation likelihood is not on this record for HSEC-2026-0010. A missing EPSS score is not a low risk. Compare Improper parsing of fractional with the linked advisory before you change production.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Improper parsing of fractional).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
  • EPSS is missing. A missing score is not a low risk.
Severity
Unknown—
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
Awaiting synchronization with FIRST
Remediation
Not established
Affected products
Hackage jwt

# Improper parsing of fractional from JWT claims Since its [initial commit][], package jwt uses [ ][scientific] to extract the fractional mantissa in a way that improperly *discards the exponent*. This can lead to incorrect parsing of the claim in JWT payload body. PoC: The encoded claim is: 1. Explicitly allowed by RFC 7519 [section 2][rfc-2] to contain fractional part (*"... non-integer values can be represented."*); 2. Decodes correctly as (including in JS implementations on jwt.io & token.dev); 3. Decodes incorrectly as (== posix epoch ) by the library. This happens because jwt's only extracts the parsed while ignoring [ ][scientific], which in this case becomes negative non-zero. Since the standard claim carries credential expiration time: > ### [4.1.4][rfc-4.1.4]. "exp" (Expiration Time) Claim > > The "exp" (expiration time) claim identifies the expiration time on > or after which the JWT MUST NOT be accepted for processing. The > processing of the "exp" claim requires that the current date/time > MUST be before the expiration date/time listed in the "exp" claim. — this parsing mistake of dropping the exponent can have security-relevant consequences, should an attacker be able to forge JWTs with fractional expiry dates and get them fed into software that uses the library for consuming JWTs. [initial commit]: https://github.com/puffnfresh/haskell-jwt/commit/76567342c71f62f3f7554da8987579d90e532209 [rfc-2]: https://www.rfc-editor.org/info/rfc7519/#section-2 [rfc-4.1.4]: https://www.rfc-editor.org/info/rfc7519/#section-4.1.4 [scientific]: https://hackage-content.haskell.org/package/scientific-0.3.8.1/docs/Data-Scientific.html#g:2

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Hackage jwt0.1.0 → 0.12.0

How to fix HSEC-2026-0010

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

Exploitation likelihood is not on this record for HSEC-2026-0010. A missing EPSS score is not a low risk. Compare Improper parsing of fractional with the linked advisory before you change production.

Which versions are affected?

Compare what you have installed with 0.1.0 → 0.12.0. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

No weakness class is stored. EPSS is not on this record yet. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Sep 29, 2026 · OpenSSF OSV / OSV Scanner

    Detected

    # Improper parsing of fractional from JWT claims Since its [initial commit][], package jwt uses [ ][scientific] to extract the fractional mantissa in a way that improperly *discards the exponent*. This can lead to incorrect parsing of the claim in JWT payload body. PoC: The encoded claim is: 1. Explicitly allowed by RFC 7519 [section 2][rfc-2] to contain fractional part (*"... non-integer values c

  2. Sep 29, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    jwt fixed in 0.12.0

  3. Sep 29, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    jwt fixed in 0.12.0

  4. Sep 29, 2026 · OpenSSF OSV / OSV Scanner

    Updated

    # Improper parsing of fractional from JWT claims Since its [initial commit][], package jwt uses [ ][scientific] to extract the fractional mantissa in a way that improperly *discards the exponent*. This can lead to incorrect parsing of the claim in JWT payload body. PoC: The encoded claim is: 1. Explicitly allowed by RFC 7519 [section 2][rfc-2] to contain fractional part (*"... non-integer values c

Sources

References

Freshness

Last verified in this catalog Sep 29, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.