CVE-2026-43969
Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1
Published May 11, 2026 · corrected/updated Oct 9, 2026 · ninenines cowlib · evidence confidence high
## 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. in cowlib builds a client-side 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 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 ( , ) and 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. ## Workarounds Validate inputs into to only include valid cookie name and value characters as defined in RFC 6265 Section 4.1.1 before passing them to the function. ## Configurations The application must pass attacker-controlled bytes as cookie names or values to . Applications that construct cookie lists exclusively from trusted, application-controlled values are not affected.
Grounded briefings
Catalog interpretations for three audiences. Each brief cites ingested metadata and the original advisories below. They are not independent research and they do not invent exploit steps.
Executive
CVE-2026-43969 is a low public vulnerability record for ninenines cowlib. It is not currently listed in the CISA Known Exploited catalog. Check the linked vendor or CERT advisories for patch status. This summary is a catalog interpretation of the ingested metadata and cites the original advisories below.
System administrator
Identify whether ninenines cowlib is in your estate, then compare installed versions with the affected ranges. Affected packages or ranges on record: cowlib (2.9.0 → 2.21.0); ninenines/cowlib. Check the linked vendor or CERT advisories for patch status. Prefer the vendor or distribution advisory linked in Sources before changing production systems. This is a catalog interpretation, not a runbook.
Security analyst
Signals on file: CVSS 2.1. It is not currently listed in the CISA Known Exploited catalog. Weakness classes: CWE-93. Sources contributing to this record: cve_org, github, nvd, osv. Public exploit references, when present, are outbound links only and are not proof of active exploitation. This is a catalog interpretation of stored metadata.
Exploitation and remediation
Exploitation evidence
Not listed as known exploited in CISA KEV. Not in CISA KEV or VulnCheck KEV.
Patch and remediation
No required-action text is stored yet. Use the vendor or CERT links in Sources for patch availability.
Safe detection guidance
Compare installed versions with the affected ranges below and verify the advisory from the original publisher. This site stores metadata and outbound links only; it does not ship exploit code or proof-of-concept payloads.
Code
The project this record applies to. Exploit listings stay in References.
Timeline
Detection, updates, and resolution for this record.
May 11, 2026 · GitHub Advisory Database
Detected
## 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. in cowlib builds a client-side 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
Oct 9, 2026 · GitHub Advisory Database
Fix available
cowlib fixed in 2.21.0
Oct 9, 2026 · GitHub Advisory Database
Fix available
cowlib fixed in 2.21.0
Oct 9, 2026 · GitHub Advisory Database
Updated
## 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. in cowlib builds a client-side 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
Oct 9, 2026 · NIST NVD
Updated
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 fu
Affected
- Hex cowlib2.9.0 → 2.21.0
- cpe ninenines/cowlibversion unspecified
Sources
- CVE.orgVulnerability recordCVE-2026-43969
Official CVE identifier
cve.org - GitHub Advisory DatabasePackage advisoryGHSA-g2wm-735q-3f56
GitHub Advisory Database record mirrored by OSV.
github.com - NIST NVDVulnerability recordCVE-2026-43969
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 o
nvd.nist.gov - OpenSSF OSV / OSV ScannerPackage advisoryEEF-CVE-2026-43969
Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1
osv.dev