CVE-2026-43969 (GCVE-0-2026-43969)
Vulnerability from cvelistv5 – Published: 2026-05-11 18:06 – Updated: 2026-10-09 06:57
VLAI
EPSS
VEX
Title
Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1
Summary
Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.
cow_cookie:cookie/1 in cowlib builds a client-side Cookie: request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject ;, ,, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting ; admin=1 to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (parse_cookie_name/1, parse_cookie_value/1) and setcookie/3 already validate and reject these characters; the encoder alone is missing the check.
This issue affects cowlib: from 2.9.0 before 2.21.0.
Severity
SSVC
Exploitation: none
Automatable: no
Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-05-11 18:55 UTC
CWE
- CWE-93 - Improper Neutralization of CRLF Sequences ('CRLF Injection')
Assigner
References
5 references
| URL | Tags |
|---|---|
| https://github.com/erlef/cowlib/commit/177953dd51… | patch |
| https://cna.erlef.org/cves/CVE-2026-43969.html | relatedthird-party-advisory |
| https://osv.dev/vulnerability/EEF-CVE-2026-43969 | related |
| https://github.com/ninenines/cowlib/commit/f017f8… | related |
| https://github.com/ninenines/cowlib/commit/c17fd7… | patch |
Impacted products
2 products
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"affected": [
{
"collectionURL": "https://repo.hex.pm",
"cpes": [
"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"
],
"defaultStatus": "unaffected",
"modules": [
"cow_cookie"
],
"packageName": "cowlib",
"packageURL": "pkg:hex/cowlib",
"product": "cowlib",
"programFiles": [
"src/cow_cookie.erl"
],
"programRoutines": [
{
"name": "cow_cookie:cookie/1"
}
],
"repo": "https://github.com/ninenines/cowlib",
"vendor": "ninenines",
"versions": [
{
"lessThan": "2.21.0",
"status": "affected",
"version": "2.9.0",
"versionType": "semver"
}
]
},
{
"collectionURL": "https://github.com",
"cpes": [
"cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*"
],
"defaultStatus": "unaffected",
"modules": [
"cow_cookie"
],
"packageName": "ninenines/cowlib",
"packageURL": "pkg:github/ninenines/cowlib",
"product": "cowlib",
"programFiles": [
"src/cow_cookie.erl"
],
"programRoutines": [
{
"name": "cow_cookie:cookie/1"
}
],
"repo": "https://github.com/ninenines/cowlib",
"vendor": "ninenines",
"versions": [
{
"lessThan": "c17fd73945ef536cccecc492d286fb6b6d1c3307",
"status": "affected",
"version": "f017f8a0ecbffd5033d9ab49bf180186f7a523a7",
"versionType": "git"
}
]
}
],
"configurations": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003eThe application must pass attacker-controlled bytes as cookie names or values to \u003ccode\u003ecow_cookie:cookie/1\u003c/code\u003e. Applications that construct cookie lists exclusively from trusted, application-controlled values are not affected.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "The application must pass attacker-controlled bytes as cookie names or values to `cow_cookie:cookie/1`. Applications that construct cookie lists exclusively from trusted, application-controlled values are not affected."
}
],
"value": "The application must pass attacker-controlled bytes as cookie names or values to cow_cookie:cookie/1. Applications that construct cookie lists exclusively from trusted, application-controlled values are not affected."
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*",
"versionEndExcluding": "2.21.0",
"versionStartIncluding": "2.9.0",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
],
"operator": "AND"
}
],
"credits": [
{
"lang": "en",
"type": "finder",
"value": "Peter Ullrich"
},
{
"lang": "en",
"type": "remediation developer",
"value": "Lo\u00efc Hoguin"
},
{
"lang": "en",
"type": "coordinator",
"value": "Jonatan M\u00e4nnchen / EEF"
}
],
"descriptions": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003eImproper Neutralization of CRLF Sequences (\u0027CRLF Injection\u0027) vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.\u003c/p\u003e\n\u003cp\u003e\u003ccode\u003ecow_cookie:cookie/1\u003c/code\u003e in cowlib builds a client-side \u003ccode\u003eCookie:\u003c/code\u003e request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject \u003ccode\u003e;\u003c/code\u003e, \u003ccode\u003e,\u003c/code\u003e, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting \u003ccode\u003e; admin=1\u003c/code\u003e to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (\u003ccode\u003eparse_cookie_name/1\u003c/code\u003e, \u003ccode\u003eparse_cookie_value/1\u003c/code\u003e) and \u003ccode\u003esetcookie/3\u003c/code\u003e already validate and reject these characters; the encoder alone is missing the check.\u003c/p\u003e\n\u003cp\u003eThis issue affects cowlib: from 2.9.0 before 2.21.0.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "Improper Neutralization of CRLF Sequences (\u0027CRLF Injection\u0027) vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.\n\n`cow_cookie:cookie/1` in cowlib builds a client-side `Cookie:` request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject `;`, `,`, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting `; admin=1` to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (`parse_cookie_name/1`, `parse_cookie_value/1`) and `setcookie/3` already validate and reject these characters; the encoder alone is missing the check.\n\nThis issue affects cowlib: from 2.9.0 before 2.21.0."
}
],
"value": "Improper Neutralization of CRLF Sequences (\u0027CRLF Injection\u0027) vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.\n\ncow_cookie:cookie/1 in cowlib builds a client-side Cookie: request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject ;, ,, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting ; admin=1 to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (parse_cookie_name/1, parse_cookie_value/1) and setcookie/3 already validate and reject these characters; the encoder alone is missing the check.\n\nThis issue affects cowlib: from 2.9.0 before 2.21.0."
}
],
"impacts": [
{
"capecId": "CAPEC-105",
"descriptions": [
{
"lang": "en",
"value": "CAPEC-105 HTTP Request Splitting"
}
]
}
],
"metrics": [
{
"cvssV4_0": {
"Automatable": "NOT_DEFINED",
"Recovery": "NOT_DEFINED",
"Safety": "NOT_DEFINED",
"attackComplexity": "LOW",
"attackRequirements": "PRESENT",
"attackVector": "LOCAL",
"baseScore": 2.1,
"baseSeverity": "LOW",
"privilegesRequired": "NONE",
"providerUrgency": "NOT_DEFINED",
"subAvailabilityImpact": "NONE",
"subConfidentialityImpact": "NONE",
"subIntegrityImpact": "LOW",
"userInteraction": "NONE",
"valueDensity": "NOT_DEFINED",
"vectorString": "CVSS:4.0/AV:L/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"
},
"format": "CVSS",
"scenarios": [
{
"lang": "en",
"value": "GENERAL"
}
]
}
],
"problemTypes": [
{
"descriptions": [
{
"cweId": "CWE-93",
"description": "CWE-93 Improper Neutralization of CRLF Sequences (\u0027CRLF Injection\u0027)",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-10-09T06:57:15.234Z",
"orgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"shortName": "EEF"
},
"references": [
{
"name": "GitHub Advisory",
"tags": [
"patch"
],
"url": "https://github.com/erlef/cowlib/commit/177953dd51540da11090666c1f007214127a1144"
},
{
"name": "EEF CNA record for CVE-2026-43969",
"tags": [
"related",
"third-party-advisory"
],
"url": "https://cna.erlef.org/cves/CVE-2026-43969.html"
},
{
"name": "OSV record EEF-CVE-2026-43969",
"tags": [
"related"
],
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-43969"
},
{
"name": "Introducing commit f017f8a in ninenines/cowlib",
"tags": [
"related"
],
"url": "https://github.com/ninenines/cowlib/commit/f017f8a0ecbffd5033d9ab49bf180186f7a523a7"
},
{
"name": "Fix commit c17fd73 in ninenines/cowlib",
"tags": [
"patch"
],
"url": "https://github.com/ninenines/cowlib/commit/c17fd73945ef536cccecc492d286fb6b6d1c3307"
}
],
"source": {
"discovery": "EXTERNAL"
},
"title": "Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1",
"workarounds": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003eValidate inputs into \u003ccode\u003ecow_cookie:cookie/1\u003c/code\u003e to only include valid cookie name and value characters as defined in RFC 6265 Section 4.1.1 before passing them to the function.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "Validate inputs into `cow_cookie:cookie/1` to only include valid cookie name and value characters as defined in RFC 6265 Section 4.1.1 before passing them to the function."
}
],
"value": "Validate inputs into cow_cookie:cookie/1 to only include valid cookie name and value characters as defined in RFC 6265 Section 4.1.1 before passing them to the function."
}
]
}
},
"cveMetadata": {
"assignerOrgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"assignerShortName": "EEF",
"cveId": "CVE-2026-43969",
"datePublished": "2026-05-11T18:06:40.667Z",
"dateReserved": "2026-05-04T18:23:25.573Z",
"dateUpdated": "2026-10-09T06:57:15.234Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Drag nodes to arrange them. Double-click a related vulnerability to open it, or a source to visit its link. The Physics button on the left tunes the layout and the simulation; the legend at the bottom left hides or shows nodes by kind and by sighting type.
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.
Vulnerability timeline
Sightings and recorded dates, grouped by day · UTC
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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.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.