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Fencer Vulns
medium 5.3

CVE-2020-28042 json

Signature validation bypass in ServiceStack

published 2020-11-01 6 y ago modified 2026-06-17 4 mo ago also GHSA-v5rv-hpxg-8x49 , EUVD-2021-0516

cvss
5.3
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
2.3%
EPSS 2.3% chance of exploitation in 30 days, percentile 83%
public exploits
0
none known

Summary

ServiceStack before 5.9.2 mishandles JWT signature verification unless an application has a custom ValidateToken function that establishes a valid minimum length for a signature.

Patch / fix

  • nuget ServiceStack 5.9.2
  • servicestack servicestack 5.9.2

Exploit prediction

2.34% chance of exploitation in the next 30 days, higher than 83% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • shielder.it https://www.shielder.it/blog/2020/11/re-discovering-a-jwt-authentication-bypass-in-servicestack/ referenced PoC, tagged "exploit" by a source, unverified
  • shielder.it https://www.shielder.it/advisories/servicestack-jwt-signature-verification-bypass/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:P/A:N

Affected packages

ecosystem package affected fixed in
nuget ServiceStack
<5.9.2
5.9.2

Classification

Weakness
  • CWE-347 Improper Verification of Cryptographic Signature

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.