CVE-2026-43971 (GCVE-0-2026-43971)
Vulnerability from cvelistv5 – Published: 2026-08-18 09:01 – Updated: 2026-08-18 09:01
VLAI
EPSS
VEX
Title
Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1
Summary
Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.
cow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A > byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A " or \ in rel escapes the quoted string and opens new parameters. Any byte — including whitespace, =, and " — in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel="preconnect", rel="preload", and rel="prerender", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins.
This issue affects cowlib: from 2.9.0 onward.
Severity
CWE
- CWE-116 - Improper Encoding or Escaping of Output
Assigner
References
2 references
| URL | Tags |
|---|---|
| https://cna.erlef.org/cves/CVE-2026-43971.html | relatedthird-party-advisory |
| https://osv.dev/vulnerability/EEF-CVE-2026-43971 | related |
Impacted products
{
"containers": {
"cna": {
"affected": [
{
"collectionURL": "https://repo.hex.pm",
"cpes": [
"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"
],
"defaultStatus": "unaffected",
"modules": [
"cow_link"
],
"packageName": "cowlib",
"packageURL": "pkg:hex/cowlib",
"product": "cowlib",
"programFiles": [
"src/cow_link.erl"
],
"programRoutines": [
{
"name": "cow_link:link/1"
},
{
"name": "cow_link:do_link/1"
},
{
"name": "cow_link:escape/2"
}
],
"repo": "https://github.com/ninenines/cowlib",
"vendor": "ninenines",
"versions": [
{
"lessThan": "*",
"status": "affected",
"version": "2.9.0",
"versionType": "semver"
}
]
},
{
"collectionURL": "https://github.com",
"cpes": [
"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"
],
"defaultStatus": "unaffected",
"modules": [
"cow_link"
],
"packageName": "ninenines/cowlib",
"packageURL": "pkg:github/ninenines/cowlib",
"product": "cowlib",
"programFiles": [
"src/cow_link.erl"
],
"programRoutines": [
{
"name": "cow_link:link/1"
},
{
"name": "cow_link:do_link/1"
},
{
"name": "cow_link:escape/2"
}
],
"repo": "https://github.com/ninenines/cowlib",
"vendor": "ninenines",
"versions": [
{
"lessThan": "*",
"status": "affected",
"version": "485d58dfa91b91d98135dc95e5615f421715dae5",
"versionType": "git"
}
]
}
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"configurations": [
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"lang": "en",
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"value": "\u003cp\u003eThe application must pass values that originate from, or can be influenced by, untrusted input into \u003ccode\u003ecow_link:link/1\u003c/code\u003e \u2014 specifically the target URI, rel value, or attribute keys. Applications that construct Link entries exclusively from trusted, application-controlled values are not affected.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "The application must pass values that originate from, or can be influenced by, untrusted input into `cow_link:link/1` \u2014 specifically the target URI, rel value, or attribute keys. Applications that construct Link entries exclusively from trusted, application-controlled values are not affected."
}
],
"value": "The application must pass values that originate from, or can be influenced by, untrusted input into cow_link:link/1 \u2014 specifically the target URI, rel value, or attribute keys. Applications that construct Link entries exclusively from trusted, application-controlled values are not affected."
}
],
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{
"nodes": [
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"cpeMatch": [
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"criteria": "cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*",
"versionStartIncluding": "2.9.0",
"vulnerable": true
}
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"negate": false,
"operator": "OR"
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"operator": "AND"
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"credits": [
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"lang": "en",
"type": "finder",
"value": "Peter Ullrich"
}
],
"descriptions": [
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"lang": "en",
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"value": "\u003cp\u003eImproper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in \u003ccode\u003ecow_link:link/1\u003c/code\u003e.\u003c/p\u003e\n\u003cp\u003e\u003ccode\u003ecow_link:do_link/1\u003c/code\u003e in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized \u003ccode\u003eLink:\u003c/code\u003e header value without escaping or token-grammar validation. A \u003ccode\u003e\u0026gt;\u003c/code\u003e byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A \u003ccode\u003e\"\u003c/code\u003e or \u003ccode\u003e\\\u003c/code\u003e in rel escapes the quoted string and opens new parameters. Any byte \u2014 including whitespace, \u003ccode\u003e=\u003c/code\u003e, and \u003ccode\u003e\"\u003c/code\u003e \u2014 in an attribute key is emitted verbatim. Because browsers act on \u003ccode\u003eLink:\u003c/code\u003e directives such as \u003ccode\u003erel=\"preconnect\"\u003c/code\u003e, \u003ccode\u003erel=\"preload\"\u003c/code\u003e, and \u003ccode\u003erel=\"prerender\"\u003c/code\u003e, an attacker who can influence these fields in an application that round-trips parsed Link headers through \u003ccode\u003ecow_link:link/1\u003c/code\u003e can force victim browsers to make out-of-band connections to attacker-controlled origins.\u003c/p\u003e\n\u003cp\u003eThis issue affects cowlib: from 2.9.0 onward.\u003c/p\u003e"
},
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"base64": false,
"type": "text/markdown",
"value": "Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in `cow_link:link/1`.\n\n`cow_link:do_link/1` in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized `Link:` header value without escaping or token-grammar validation. A `\u003e` byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A `\"` or `\\` in rel escapes the quoted string and opens new parameters. Any byte \u2014 including whitespace, `=`, and `\"` \u2014 in an attribute key is emitted verbatim. Because browsers act on `Link:` directives such as `rel=\"preconnect\"`, `rel=\"preload\"`, and `rel=\"prerender\"`, an attacker who can influence these fields in an application that round-trips parsed Link headers through `cow_link:link/1` can force victim browsers to make out-of-band connections to attacker-controlled origins.\n\nThis issue affects cowlib: from 2.9.0 onward."
}
],
"value": "Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.\n\ncow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A \u003e byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A \" or \\ in rel escapes the quoted string and opens new parameters. Any byte \u2014 including whitespace, =, and \" \u2014 in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel=\"preconnect\", rel=\"preload\", and rel=\"prerender\", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins.\n\nThis issue affects cowlib: from 2.9.0 onward."
}
],
"impacts": [
{
"capecId": "CAPEC-137",
"descriptions": [
{
"lang": "en",
"value": "CAPEC-137 Parameter Injection"
}
]
}
],
"metrics": [
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"Automatable": "NOT_DEFINED",
"Recovery": "NOT_DEFINED",
"Safety": "NOT_DEFINED",
"attackComplexity": "LOW",
"attackRequirements": "PRESENT",
"attackVector": "NETWORK",
"baseScore": 6.3,
"baseSeverity": "MEDIUM",
"privilegesRequired": "NONE",
"providerUrgency": "NOT_DEFINED",
"subAvailabilityImpact": "NONE",
"subConfidentialityImpact": "NONE",
"subIntegrityImpact": "LOW",
"userInteraction": "NONE",
"valueDensity": "NOT_DEFINED",
"vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N",
"version": "4.0",
"vulnAvailabilityImpact": "NONE",
"vulnConfidentialityImpact": "NONE",
"vulnIntegrityImpact": "LOW",
"vulnerabilityResponseEffort": "NOT_DEFINED"
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"format": "CVSS",
"scenarios": [
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"lang": "en",
"value": "GENERAL"
}
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"problemTypes": [
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"descriptions": [
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"cweId": "CWE-116",
"description": "CWE-116 Improper Encoding or Escaping of Output",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-18T09:01:53.199Z",
"orgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"shortName": "EEF"
},
"references": [
{
"tags": [
"related",
"third-party-advisory"
],
"url": "https://cna.erlef.org/cves/CVE-2026-43971.html"
},
{
"tags": [
"related"
],
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-43971"
}
],
"source": {
"discovery": "EXTERNAL"
},
"title": "Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1",
"workarounds": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003eSanitize values before passing them to \u003ccode\u003ecow_link:link/1\u003c/code\u003e: reject or strip any target value containing \u003ccode\u003e\u0026gt;\u003c/code\u003e, any rel value containing \u003ccode\u003e\"\u003c/code\u003e or \u003ccode\u003e\\\u003c/code\u003e, and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "Sanitize values before passing them to `cow_link:link/1`: reject or strip any target value containing `\u003e`, any rel value containing `\"` or `\\`, and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input."
}
],
"value": "Sanitize values before passing them to cow_link:link/1: reject or strip any target value containing \u003e, any rel value containing \" or \\, and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input."
}
]
}
},
"cveMetadata": {
"assignerOrgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"assignerShortName": "EEF",
"cveId": "CVE-2026-43971",
"datePublished": "2026-08-18T09:01:53.199Z",
"dateReserved": "2026-05-04T18:23:25.574Z",
"dateUpdated": "2026-08-18T09:01:53.199Z",
"state": "PUBLISHED"
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"vulnerability-lookup:meta": {
"epss": {
"cve": "CVE-2026-43971",
"date": "2026-08-18",
"epss": "0.00324",
"percentile": "0.25183"
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"nvd": "{\"cve\":{\"id\":\"CVE-2026-43971\",\"sourceIdentifier\":\"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\",\"published\":\"2026-08-18T09:17:14.340\",\"lastModified\":\"2026-08-18T09:17:14.340\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.\\n\\ncow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A \u003e byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A \\\" or \\\\ in rel escapes the quoted string and opens new parameters. Any byte \u2014 including whitespace, =, and \\\" \u2014 in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel=\\\"preconnect\\\", rel=\\\"preload\\\", and rel=\\\"prerender\\\", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins.\\n\\nThis issue affects cowlib: from 2.9.0 onward.\"}],\"affected\":[{\"source\":\"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\",\"affectedData\":[{\"vendor\":\"ninenines\",\"product\":\"cowlib\",\"defaultStatus\":\"unaffected\",\"collectionURL\":\"https://repo.hex.pm\",\"packageName\":\"cowlib\",\"cpes\":[\"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*\"],\"modules\":[\"cow_link\"],\"programFiles\":[\"src/cow_link.erl\"],\"programRoutines\":[{\"name\":\"cow_link:link/1\"},{\"name\":\"cow_link:do_link/1\"},{\"name\":\"cow_link:escape/2\"}],\"repo\":\"https://github.com/ninenines/cowlib\",\"packageURL\":\"pkg:hex/cowlib\",\"versions\":[{\"version\":\"2.9.0\",\"lessThan\":\"*\",\"versionType\":\"semver\",\"status\":\"affected\"}]},{\"vendor\":\"ninenines\",\"product\":\"cowlib\",\"defaultStatus\":\"unaffected\",\"collectionURL\":\"https://github.com\",\"packageName\":\"ninenines/cowlib\",\"cpes\":[\"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*\"],\"modules\":[\"cow_link\"],\"programFiles\":[\"src/cow_link.erl\"],\"programRoutines\":[{\"name\":\"cow_link:link/1\"},{\"name\":\"cow_link:do_link/1\"},{\"name\":\"cow_link:escape/2\"}],\"repo\":\"https://github.com/ninenines/cowlib\",\"packageURL\":\"pkg:github/ninenines/cowlib\",\"versions\":[{\"version\":\"485d58dfa91b91d98135dc95e5615f421715dae5\",\"lessThan\":\"*\",\"versionType\":\"git\",\"status\":\"affected\"}]}]}],\"metrics\":{\"cvssMetricV40\":[{\"source\":\"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"4.0\",\"vectorString\":\"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X\",\"baseScore\":6.3,\"baseSeverity\":\"MEDIUM\",\"attackVector\":\"NETWORK\",\"attackComplexity\":\"LOW\",\"attackRequirements\":\"PRESENT\",\"privilegesRequired\":\"NONE\",\"userInteraction\":\"NONE\",\"vulnConfidentialityImpact\":\"NONE\",\"vulnIntegrityImpact\":\"LOW\",\"vulnAvailabilityImpact\":\"NONE\",\"subConfidentialityImpact\":\"NONE\",\"subIntegrityImpact\":\"LOW\",\"subAvailabilityImpact\":\"NONE\",\"exploitMaturity\":\"NOT_DEFINED\",\"confidentialityRequirement\":\"NOT_DEFINED\",\"integrityRequirement\":\"NOT_DEFINED\",\"availabilityRequirement\":\"NOT_DEFINED\",\"modifiedAttackVector\":\"NOT_DEFINED\",\"modifiedAttackComplexity\":\"NOT_DEFINED\",\"modifiedAttackRequirements\":\"NOT_DEFINED\",\"modifiedPrivilegesRequired\":\"NOT_DEFINED\",\"modifiedUserInteraction\":\"NOT_DEFINED\",\"modifiedVulnConfidentialityImpact\":\"NOT_DEFINED\",\"modifiedVulnIntegrityImpact\":\"NOT_DEFINED\",\"modifiedVulnAvailabilityImpact\":\"NOT_DEFINED\",\"modifiedSubConfidentialityImpact\":\"NOT_DEFINED\",\"modifiedSubIntegrityImpact\":\"NOT_DEFINED\",\"modifiedSubAvailabilityImpact\":\"NOT_DEFINED\",\"Safety\":\"NOT_DEFINED\",\"Automatable\":\"NOT_DEFINED\",\"Recovery\":\"NOT_DEFINED\",\"valueDensity\":\"NOT_DEFINED\",\"vulnerabilityResponseEffort\":\"NOT_DEFINED\",\"providerUrgency\":\"NOT_DEFINED\"}}]},\"weaknesses\":[{\"source\":\"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\",\"type\":\"Secondary\",\"description\":[{\"lang\":\"en\",\"value\":\"CWE-116\"}]}],\"references\":[{\"url\":\"https://cna.erlef.org/cves/CVE-2026-43971.html\",\"source\":\"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\"},{\"url\":\"https://osv.dev/vulnerability/EEF-CVE-2026-43971\",\"source\":\"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\"}]}}"
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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