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Vulnerability details: VCID-bu8w-z79j-c3dh
Vulnerability ID VCID-bu8w-z79j-c3dh
Aliases CVE-2021-39155
GHSA-7774-7vr3-cc8j
Summary Authorization Policy Bypass Due to Case Insensitive Host Comparison ### Impact 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 Envoy 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 * Istio 1.11.1 and above * Istio 1.10.4 and above * Istio 1.9.8 and above ### Workarounds 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. ### References More details can be found in the [Istio Security Bulletin](https://istio.io/latest/news/security/istio-security-2021-008). ### For more information If you have any questions or comments about this advisory, please email us at istio-security-vulnerability-reports@googlegroups.com
Status Published
Exploitability 0.5
Weighted Severity 8.0
Risk 4.0
Affected and Fixed Packages Package Details
Weaknesses (2)
CWE-178 Improper Handling of Case Sensitivity
CWE-863 Incorrect Authorization
System Score Found at
cvssv3 8.3 https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2021-39155.json
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
epss 0.00211 https://api.first.org/data/v1/epss?cve=CVE-2021-39155
cvssv3.1 8.3 https://datatracker.ietf.org/doc/html/rfc4343
generic_textual HIGH https://datatracker.ietf.org/doc/html/rfc4343
cvssv3.1 8.3 https://github.com/istio/istio
generic_textual HIGH https://github.com/istio/istio
cvssv3.1 8.3 https://github.com/istio/istio/commit/084b417a486dbe9b9024d4812877016a484572b1
generic_textual HIGH https://github.com/istio/istio/commit/084b417a486dbe9b9024d4812877016a484572b1
cvssv3.1 8.3 https://github.com/istio/istio/commit/76ed51413ddd2a7fa253a368ab20a9cec5fb1cbe
generic_textual HIGH https://github.com/istio/istio/commit/76ed51413ddd2a7fa253a368ab20a9cec5fb1cbe
cvssv3.1 8.3 https://github.com/istio/istio/commit/90b00bdf891e6c770cb3235c14a9b1fda96cc7c5
generic_textual HIGH https://github.com/istio/istio/commit/90b00bdf891e6c770cb3235c14a9b1fda96cc7c5
cvssv3.1 8.3 https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j
generic_textual HIGH https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j
cvssv2 5.0 https://nvd.nist.gov/vuln/detail/CVE-2021-39155
cvssv3.1 7.5 https://nvd.nist.gov/vuln/detail/CVE-2021-39155
cvssv3.1 8.3 https://nvd.nist.gov/vuln/detail/CVE-2021-39155
generic_textual HIGH https://nvd.nist.gov/vuln/detail/CVE-2021-39155
archlinux High https://security.archlinux.org/AVG-2321
Reference id Reference type URL
https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2021-39155.json
https://api.first.org/data/v1/epss?cve=CVE-2021-39155
https://datatracker.ietf.org/doc/html/rfc4343
https://github.com/istio/istio
https://github.com/istio/istio/commit/084b417a486dbe9b9024d4812877016a484572b1
https://github.com/istio/istio/commit/76ed51413ddd2a7fa253a368ab20a9cec5fb1cbe
https://github.com/istio/istio/commit/90b00bdf891e6c770cb3235c14a9b1fda96cc7c5
https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j
https://nvd.nist.gov/vuln/detail/CVE-2021-39155
1996929 https://bugzilla.redhat.com/show_bug.cgi?id=1996929
AVG-2321 https://security.archlinux.org/AVG-2321
cpe:2.3:a:istio:istio:*:*:*:*:*:*:*:* https://nvd.nist.gov/vuln/search/results?adv_search=true&isCpeNameSearch=true&query=cpe:2.3:a:istio:istio:*:*:*:*:*:*:*:*
RHSA-2021:3272 https://access.redhat.com/errata/RHSA-2021:3272
RHSA-2021:3273 https://access.redhat.com/errata/RHSA-2021:3273
No exploits are available.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L Found at https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2021-39155.json
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://datatracker.ietf.org/doc/html/rfc4343
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://github.com/istio/istio
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://github.com/istio/istio/commit/084b417a486dbe9b9024d4812877016a484572b1
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://github.com/istio/istio/commit/76ed51413ddd2a7fa253a368ab20a9cec5fb1cbe
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://github.com/istio/istio/commit/90b00bdf891e6c770cb3235c14a9b1fda96cc7c5
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: AV:N/AC:L/Au:N/C:P/I:N/A:N Found at https://nvd.nist.gov/vuln/detail/CVE-2021-39155
Exploitability (E) Access Vector (AV) Access Complexity (AC) Authentication (Au) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

high

functional

unproven

proof_of_concept

not_defined

local

adjacent_network

network

high

medium

low

multiple

single

none

none

partial

complete

none

partial

complete

none

partial

complete
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N Found at https://nvd.nist.gov/vuln/detail/CVE-2021-39155
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L Found at https://nvd.nist.gov/vuln/detail/CVE-2021-39155
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Exploit Prediction Scoring System (EPSS)
Percentile 0.43824
EPSS Score 0.00211
Published At July 30, 2025, 12:55 p.m.
Date Actor Action Source VulnerableCode Version
2025-07-31T08:51:14.926063+00:00 GithubOSV Importer Import https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2021/08/GHSA-7774-7vr3-cc8j/GHSA-7774-7vr3-cc8j.json 37.0.0