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NVD / CVE Program· CNA GitHub_M

CVE-2021-39155: Authorization Policy Bypass Due to Case Insensitive Host Comparison

Noroxi assessment

Priority: Monitor (action score 30/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 1.2% (FIRST EPSS); higher than 69.3% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: istio.io/istio (Go) 1.10.4, 1.11.1, 1.9.8; servicemesh (Red Hat:service_mesh:1.1::el8) 0:1.1.17-3.el8; servicemesh-citadel (Red Hat:service_mesh:1.1::el8) 0:1.1.17-3.el8.

Low priority for now. Upgrade to istio.io/istio (Go) 1.10.4, 1.11.1, 1.9.8; servicemesh (Red Hat:service_mesh:1.1::el8) 0:1.1.17-3.el8; servicemesh-citadel (Red Hat:service_mesh:1.1::el8) 0:1.1.17-3.el8 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

Istio is an open source platform for providing a uniform way to integrate microservices, manage traffic flow across microservices, enforce policies and aggregate telemetry data. According to [RFC 4343](https://datatracker.ietf.org/doc/html/rfc4343), Istio authorization policy should compare the hostname in the HTTP Host header in a case insensitive way, but currently the comparison is case sensitive. The proxy will route the request hostname in a case-insensitive way which means the authorization policy could be bypassed. As an example, the user may have an authorization policy that rejects request with hostname "httpbin.foo" for some source IPs, but the attacker can bypass this by sending the request with hostname "Httpbin.Foo". Patches are available in Istio 1.11.1, Istio 1.10.4 and Istio 1.9.8. As a work around a Lua filter may be written to normalize Host header before the authorization check. This is similar to the Path normalization presented in the [Security Best Practices](https://istio.io/latest/docs/ops/best-practices/security/#case-normalization) guide.

HighCVSS 7.5 · v3.1—No exploit Fix available
Published
Aug 24, 2021
Updated
Jun 17, 2026
EPSS
1.2% · 69th percentile

Report tools

JSON

Action score

30

Monitor

Low priority for now.

CVSS
30 / 40 · 7.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 1.2%

CNA vs NVD score

NVD
7.5
CNA · GitHub_M
8.3
0.8 point gap

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-39155

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
istioistio

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • istio istiobefore 1.9.8
  • istio istio1.10.0 and later · before 1.10.4
  • istio istio1.11.0 and later · before 1.11.1

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • istio istio

    • <= 1.9.8, >= 1.10.0, < 1.10.4, >= 1.11.0, < 1.11.1affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Goistio.io/istiofrom 1.10.0 · before 1.10.41.10.4
Goistio.io/istiofrom 1.11.0 · before 1.11.11.11.1
Goistio.io/istiobefore 1.9.81.9.8
Red Hat:service_mesh:1.1::el8servicemeshbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-citadelbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-galleybefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-istioctlbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-mixcbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-mixsbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-pilot-agentbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-pilot-discoverybefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:1.1::el8servicemesh-proxybefore 0:1.1.17-2.el80:1.1.17-2.el8
Red Hat:service_mesh:1.1::el8servicemesh-sidecar-injectorbefore 0:1.1.17-3.el80:1.1.17-3.el8
Red Hat:service_mesh:2.0::el8servicemeshbefore 0:2.0.7-3.el80:2.0.7-3.el8
Red Hat:service_mesh:2.0::el8servicemesh-istioctlbefore 0:2.0.7-3.el80:2.0.7-3.el8
Red Hat:service_mesh:2.0::el8servicemesh-mixcbefore 0:2.0.7-3.el80:2.0.7-3.el8
Red Hat:service_mesh:2.0::el8servicemesh-mixsbefore 0:2.0.7-3.el80:2.0.7-3.el8
Red Hat:service_mesh:2.0::el8servicemesh-pilot-agentbefore 0:2.0.7-3.el80:2.0.7-3.el8
Red Hat:service_mesh:2.0::el8servicemesh-pilot-discoverybefore 0:2.0.7-3.el80:2.0.7-3.el8
Red Hat:service_mesh:2.0::el8servicemesh-proxybefore 0:2.0.7-3.el80:2.0.7-3.el8

Same product

istio: all records

Other highest-scoring records for the same primary product.

  • CVE-2022-31045Ill-formed headers may lead to unexpected behavior in Istio
    39Monitor
  • CVE-2022-21679Authorization Policy bypass in Istio
    39Monitor
  • CVE-2021-31921Istio before 1.8.6 and 1.9.x before 1.9.5 contains a remotely exploitable vulnerability where an external client can access unexpected servi
    39Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2021-31920May 27, 2021 · Istio before 1.8.6 and 1.9.x before 1.9.5 has a remotely exploitable vulnerability where an HTTP request path with multiple slashes or escap
    26Monitor
  • ← EarlierCVE-2021-34824Jun 29, 2021 · Istio (1.8.x, 1.9.0-1.9.5 and 1.10.0-1.10.1) contains a remotely exploitable vulnerability where credentials specified in the Gateway and De
    36Monitor
  • Later →CVE-2021-39156Aug 24, 2021 · Fragments in Path May Lead to Authorization Policy Bypass
    30Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Go:istio.io/istio1.10.4Package registry (OSV)
Go:istio.io/istio1.11.1Package registry (OSV)
Go:istio.io/istio1.9.8Package registry (OSV)
red hat:servicemesh0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-citadel0:1.1.17-3.el8 · Red Hat:service_mesh:1.1::el8Package registry (OSV)
red hat:servicemesh-galley0:1.1.17-3.el8 · Red Hat:service_mesh:1.1::el8Package registry (OSV)
red hat:servicemesh-istioctl0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-mixc0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-mixs0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-pilot-agent0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-pilot-discovery0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-proxy0:2.0.7-3.el8 · Red Hat:service_mesh:2.0::el8Package registry (OSV)
red hat:servicemesh-sidecar-injector0:1.1.17-3.el8 · Red Hat:service_mesh:1.1::el8Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
none
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
None
Availability impact
None

Weakness class (CWE)

CWE-178 · Improper Handling of Case Sensitivity

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (1 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records