CVE-2021-21239Public exploitNetwork attack vector

Authentication bypass via attacker-supplied signature key

Published Jan 21, 2021 · Updated Aug 3, 2024

Improper signature verification in IdentityPython PySAML2 6.4.1 and earlier allows remote attackers to forge signed SAML documents. CryptoBackendXmlSec1 invokes xmlsec1 without restricting accepted key data, so verification can trust an attacker-supplied HMAC key or embedded certificate instead of the configured X.509 certificate. The target must process the crafted document with the default backend, which allows attackers to substitute trusted SAML assertion contents and impersonate a federated identity.

CVSS severity6.5
Medium
EPSS probability1.27%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper signature verification in IdentityPython PySAML2 6.4.1 and earlier allows remote attackers to forge signed SAML documents. CryptoBackendXmlSec1 invokes xmlsec1 without restricting accepted key data, so verification can trust an attacker-supplied HMAC key or embedded certificate instead of the configured X.509 certificate. The target must process the crafted document with the default backend, which allows attackers to substitute trusted SAML assertion contents and impersonate a federated identity.

The record

CVE
CVE-2021-21239
Published
Jan 21, 2021
Updated
Aug 3, 2024
Vendor
IdentityPython
Product
PySAML2
Classifications
T1606.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 21, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 6.5.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

IdentityPython · PySAML2version=< 6.5.0

Published CVSS scores

6.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

T1606.002Forge Web Credentials: SAML Tokens

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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