CVE-2026-43971

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.

CVSS

No CVSS.

References

Link	Resource
https://cna.erlef.org/cves/CVE-2026-43971.html
https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea
https://osv.dev/vulnerability/EEF-CVE-2026-43971

Configurations

No configuration.

History

No history.

Information

Published : 2026-08-18 09:17

Updated : 2026-08-20 13:02

NVD link : CVE-2026-43971

Mitre link : CVE-2026-43971

CVE.ORG link : CVE-2026-43971

JSON object : View

Products Affected

No product.

CWE

CWE-116

Improper Encoding or Escaping of Output

{"id": "CVE-2026-43971", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-43971", "role": "CISA Coordinator", "options": [{"exploitation": "none"}, {"automatable": "no"}, {"technicalImpact": "partial"}], "version": "2.0.3", "timestamp": "2026-08-18T14:11:55.645955Z"}}], "cvssMetricV40": [{"type": "Secondary", "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db", "cvssData": {"Safety": "NOT_DEFINED", "version": "4.0", "Recovery": "NOT_DEFINED", "baseScore": 6.3, "Automatable": "NOT_DEFINED", "attackVector": "NETWORK", "baseSeverity": "MEDIUM", "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/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", "exploitMaturity": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED", "userInteraction": "NONE", "attackComplexity": "LOW", "attackRequirements": "PRESENT", "privilegesRequired": "NONE", "subIntegrityImpact": "LOW", "vulnIntegrityImpact": "LOW", "integrityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "subAvailabilityImpact": "NONE", "vulnAvailabilityImpact": "NONE", "availabilityRequirement": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "subConfidentialityImpact": "NONE", "vulnConfidentialityImpact": "NONE", "confidentialityRequirement": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED"}}]}, "affected": [{"source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db", "affectedData": [{"cpes": ["cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"], "repo": "https://github.com/ninenines/cowlib", "vendor": "ninenines", "modules": ["cow_link"], "product": "cowlib", "versions": [{"status": "affected", "version": "2.9.0", "lessThan": "*", "versionType": "semver"}], "packageURL": "pkg:hex/cowlib", "packageName": "cowlib", "programFiles": ["src/cow_link.erl"], "collectionURL": "https://repo.hex.pm", "defaultStatus": "unaffected", "programRoutines": [{"name": "cow_link:link/1"}, {"name": "cow_link:do_link/1"}, {"name": "cow_link:escape/2"}]}, {"cpes": ["cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"], "repo": "https://github.com/ninenines/cowlib", "vendor": "ninenines", "modules": ["cow_link"], "product": "cowlib", "versions": [{"status": "affected", "version": "485d58dfa91b91d98135dc95e5615f421715dae5", "lessThan": "89da27ee4c241f5d649ba7d9b7f2188918af6cea", "versionType": "git"}], "packageURL": "pkg:github/ninenines/cowlib", "packageName": "ninenines/cowlib", "programFiles": ["src/cow_link.erl"], "collectionURL": "https://github.com", "defaultStatus": "unaffected", "programRoutines": [{"name": "cow_link:link/1"}, {"name": "cow_link:do_link/1"}, {"name": "cow_link:escape/2"}]}]}], "published": "2026-08-18T09:17:14.340", "references": [{"url": "https://cna.erlef.org/cves/CVE-2026-43971.html", "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db"}, {"url": "https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea", "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db"}, {"url": "https://osv.dev/vulnerability/EEF-CVE-2026-43971", "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db"}], "vulnStatus": "Awaiting Analysis", "weaknesses": [{"type": "Secondary", "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db", "description": [{"lang": "en", "value": "CWE-116"}]}], "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 > 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."}], "lastModified": "2026-08-20T13:02:12.153", "sourceIdentifier": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db"}