CVE-2025-25291: Ruby SAML allows a SAML authentication bypass due to DOCTYPE handling (parser differential)

Should you act?

High exploit likelihood

EPSS ranks it in the top 5% of all CVEs for likely exploitation in the next 30 days. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 21.0%, percentile 98: FIRST’s model estimates a 21.0% chance of exploitation activity in the next 30 days, higher than 98% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes ruby-saml (RubyGems) < 1.12.4, fixed in 1.12.4; ruby-saml (RubyGems) >= 1.13.0, < 1.18.0, fixed in 1.18.0. Check your lockfile, including transitive dependencies.

What it is

ruby-saml provides security assertion markup language (SAML) single sign-on (SSO) for Ruby. An authentication bypass vulnerability was found in ruby-saml prior to versions 1.12.4 and 1.18.0 due to a parser differential. ReXML and Nokogiri parse XML differently; the parsers can generate entirely different document structures from the same XML input. That allows an attacker to be able to execute a Signature Wrapping attack. This issue may lead to authentication bypass. Versions 1.12.4 and 1.18.0 fix the issue.

From the GitHub advisory

Summary An authentication bypass vulnerability was found in ruby-saml due to a parser differential. ReXML and Nokogiri parse XML differently, the parsers can generate entirely different document structures from the same XML input. That allows an attacker to be able to execute a Signature Wrapping attack.

Impact This issue may lead to authentication bypass.

Weakness type: CWE-347, CWE-436.

Product: ruby-saml (RubyGems) · Assigned by: GitHub_M · Published: 2025-03-12

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 20.96% probability of exploitation activity in the next 30 days; percentile 98.

Timeline

• 2025-03-12 — CVE record published by GitHub_M
• 2025-03-12 — GitHub-reviewed advisory GHSA-4vc4-m8qh-g8jm published (critical)
• 2025-11-03 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-4vc4-m8qh-g8jm github.com
2. CVE.org record CVE-2025-25291 cve.org
3. NVD entry CVE-2025-25291 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. about.gitlab.com about.gitlab.com
6. github.blog github.blog
7. github.com github.com
8. github.com github.com
9. portswigger.net portswigger.net
10. securitylab.github.com securitylab.github.com
11. security.netapp.com security.netapp.com
12. news.ycombinator.com news.ycombinator.com
13. lists.debian.org lists.debian.org
14. github.com github.com

Critical RubyGems advisories · Today’s Security Digest