CVE
AWS API MCP Server Security Policy Bypass via Incorrect Operation Name Normalization
Only affects awslabs.aws-api-mcp-server from 0.2.13 up to but not including 1.5.1, when run locally with a custom security policy that has denyList or elicitList entries. Fixed in 1.5.1.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 1.5.1
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. Custom security policy with denyList or elicitList entries for the affected operations Affected versions of awslabs.aws-api-mcp-server from 0.2.13 up to but not including 1.5.1 Operation names containing consecutive capital letters
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- 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
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- 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
Summary
The AWS API MCP Server, an awslabs tool, has a flaw in how it normalizes operation names when enforcing custom security policies. A local attacker with no login who needs a user to take an action can use operation names with consecutive capital letters to run AWS API operations that the operator's policy was set to deny or require confirmation for, across AWS services.
Read the original description
Incorrect normalization of operation names in the custom security policy enforcement in awslabs aws-api-mcp-server before 1.5.1 might allow an actor to execute AWS API operations across AWS services that the operator's security policy was configured to deny or require elicitation for, via calls to operations whose names contain consecutive capital letters.
To remediate this issue, users should upgrade to version 1.5.1.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure5/25Specific setups only
- Reach?/20Unknown. No tracked technology or package matched
- Severity7.8/10CVSS 7.8
- No patch0/5No signal
What would change the score
- Would score about 63 if added to CISA KEV
- Would score about 58 if CISA's SSVC rates exploitation active
- Would score about 23 if CISA's SSVC rates it automatable
What to do
Check whether your setup matches what the bug needs.
Custom security policy with denyList or elicitList entries for the affected operations Affected versions of awslabs.aws-api-mcp-server from 0.2.13 up to but not including 1.5.1 Operation names containing consecutive capital lettersUpgrade to a fixed version: 1.5.1.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-125271 (AWS API MCP Server Security Policy Bypass via Incorrect Operation Name Normalization), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-125271 (entry last updated 2026-11-10):
Affected:
- AWS aws-api-mcp-server: from 0.2.13, fixed in 1.5.1
Fixed versions: 1.5.1.
Conditions (all must be true for the bug to be hit):
- Custom security policy with denyList or elicitList entries for the affected operations (source: https://github.com/awslabs/mcp/security/advisories/GHSA-4wq9-grf3-qg2p)
- Affected versions of awslabs.aws-api-mcp-server from 0.2.13 up to but not including 1.5.1 (source: https://aws.amazon.com/security/security-bulletins/2026-176-aws/)
- Operation names containing consecutive capital letters (source: https://aws.amazon.com/security/security-bulletins/2026-176-aws/)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: local access, no login, a user action.
CISA KEV: CVE-2026-125271 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-125271 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://aws.amazon.com/security/security-bulletins/2026-176-aws/
- https://pypi.org/project/awslabs.aws-api-mcp-server/1.5.1/
- https://github.com/awslabs/mcp/security/advisories/GHSA-4wq9-grf3-qg2p
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-125271
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-125271"}.Timeline
- CVE published
- Record last updated
- First seen by cve.now
Affected versions
- AWS aws-api-mcp-server
From 0.2.13 · fixed in 1.5.1
Evidence 5
Vendor advisory · 2
Establishes: Affected and fixed versions.
- Advisory
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 1
- Patch
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-125271
Is CVE-2026-125271 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
What does this mean?
Operation names containing consecutive capital letters never matched a correctly written denyList or elicitList entry, so the security policy could fail to deny or gate those AWS API operations. An actor might then execute AWS API operations across services the operator's policy was configured to deny or require elicitation for.Source: github.com, aws.amazon.com
Which versions are affected by CVE-2026-125271?
aws-api-mcp-server: from 0.2.13 before 1.5.1.Source: cve.now feed data
Am I affected, and how do I check?
Affected versions are awslabs.aws-api-mcp-server 0.2.13 up to but not including 1.5.1, and the issue requires a locally run server with a custom security policy that lists affected operations. To check, look at the installed awslabs.aws-api-mcp-server package version and whether a custom security policy with denyList or elicitList entries is in use.Source: github.com, aws.amazon.com
How do I fix CVE-2026-125271?
Upgrade awslabs.aws-api-mcp-server to version 1.5.1, which addresses the issue.Source: github.com, aws.amazon.com