CVE-2026-43969 (GCVE-0-2026-43969)
Vulnerability from cvelistv5 – Published: 2026-05-11 18:06 – Updated: 2026-08-18 11:26
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 onward.
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
3 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 |
Impacted products
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"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:*:*:*:*:*:*:*:*",
"versionStartIncluding": "2.9.0",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
],
"operator": "AND"
}
],
"credits": [
{
"lang": "en",
"type": "finder",
"value": "Peter Ullrich"
}
],
"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 onward.\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 onward."
}
],
"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 onward."
}
],
"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": "2026-08-18T11:26:01.022Z",
"orgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"shortName": "EEF"
},
"references": [
{
"tags": [
"patch"
],
"url": "https://github.com/erlef/cowlib/commit/177953dd51540da11090666c1f007214127a1144"
},
{
"tags": [
"related",
"third-party-advisory"
],
"url": "https://cna.erlef.org/cves/CVE-2026-43969.html"
},
{
"tags": [
"related"
],
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-43969"
}
],
"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": "2026-08-18T11:26:01.022Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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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.
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.
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