mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata
- Exploited
- No known exploitation
- Fix
- Fixed in v1.2.1
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: v1.2.1.
- Published
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 13/100 · provisional
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: none
- Automatable: no
- Technical impact: total
Summary
Mcp-go, a Go library for building Model Context Protocol servers and clients, has a flaw in how its OAuth handler validates server metadata. A malicious MCP server can declare a different server as the resource for which tokens are issued, so a token obtained for that malicious server can be replayed against another resource; the attack is reached remotely over the network without logging in, but it requires a user to take an action such as approving the authorization.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in OAuthHandler.fetchServerMetadata that allows malicious MCP servers to obtain tokens for other resources by declaring arbitrary resources in well-known metadata. Attackers can declare a different MCP server as the RFC 8707 resource indicator, causing the authorization server to mint tokens the attacker replays against that resource.
cve.now Index
- Exploitation0/40No signal
- Exposure7/25Reachable over the network without a login · Unknown: No advisory context yet
- Reach?/20Unknown. No tracked technology or package matched
- Severity6.1/10CVSS 5.1, total technical impact per CISA
- No patch0/5No signal
What would change the score
- Would score about 68 if added to CISA KEV
- Would score about 63 if CISA's SSVC rates exploitation active
- Would score about 28 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: v1.2.1.
Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: none · Automatable: no · Technical impact: total
- Record last updated
Affected versions
- mark3labs mcp-go
From v0.48.0 · fixed in v1.2.1
Evidence 6
- Advisory
- Patch
- Patch
- Repository
- Repository
- Link
Corrections
None so far.
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Questions about CVE-2026-123272
Is CVE-2026-123272 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by CVE-2026-123272?
mcp-go: from v0.48.0 before v1.2.1.Source: cve.now feed data
How do I fix CVE-2026-123272?
Upgrade to a fixed version: v1.2.1.Source: cve.now feed data