How severe is CVE-2026-43966?

No CVSS score has been assigned to CVE-2026-43966 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-43966?

CVE-2026-43966 affects the following packages: cowboy (Hex), gun (Hex). Ecosystems affected: Hex.

How do I fix CVE-2026-43966?

Update cowboy to 2.16.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-43966 is resolved across your whole dependency graph.

How do I detect CVE-2026-43966 in my Hex dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for cowboy. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-43966 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-43966?

O3 pinpoints whether CVE-2026-43966 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-43966 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-43966 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is CVE-2026-43966?

CVE-2026-43966 is classified as CWE-113 (CWE-113). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-43966 published, and has it been updated?

CVE-2026-43966 was published on June 8, 2026 and was last updated on July 29, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💧 Hex

CVE-2026-43966

CVE-2026-43966 is a CWE-113 vulnerability in cowboy. O3 Security confirms whether CVE-2026-43966 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2

Also known asEEF-CVE-2026-43966GHSA-w4f7-4cxr-rv3c

Published

Jun 8, 2026

Updated

Jul 29, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Blast Radius

2 pkgs affected

💧cowboy💧gun

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Hex packages — download data is not available via public APIs for these ecosystems.

Description

cow_http_struct_hd:escape_string/2 in cowlib only escapes \ and ", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding " and ), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-level wrapper such as cow_http_hd:wt_protocol/1) from attacker-controlled input can have \r\n injected into the serialized header value. Once on the wire, the injected CRLF terminates the current header and any following bytes are interpreted as a new header, enabling HTTP response splitting.

This issue affects cowlib from 2.9.0.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
💧Hex	`cowboy`	all versions	2.16.0
💧Hex	`gun`	all versions	2.16.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for cowboy. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update cowboy to 2.16.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-43966 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-43966 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-43966. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values. cow_http_struct_hd:escape_string/2 in cowlib only escapes \ and ", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding " and \), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-leve

O3 Security · Impact-Aware SCA

Is CVE-2026-43966 in your dependencies?

O3 detects CVE-2026-43966 across Hex dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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