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CVE-2026-43966

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

6.3 MediumFix available

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-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 onward.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
None

Exploit likelihood

0.26%Up 0.2 points since Jun 9, 2026
90%50%10%1%

What each source reports

CVE List Score used 6.3

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

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-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 onward.

Record
CVE-2026-43966
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Weakness
CWE-113
Updated
2026-11-02
EUVDEUVD-2026-35131 6.3

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-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 onward.

Record
EUVD-2026-35131
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-w4f7-4cxr-rv3c 6.3

cowboy and gun affected by an HTTP Request/Response Splitting vulnerability

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-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.

Record
GHSA-w4f7-4cxr-rv3c
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Weakness
CWE-113
Updated
2026-11-02
NVD 6.3

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-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 onward.

Record
CVE-2026-43966
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/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
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Weakness
CWE-113
Updated
2026-11-02
OSV 6.3

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

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-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 onward.

Record
CVE-2026-43966
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Weakness
CWE-113
Updated
2026-11-02
OSVDEBIAN-CVE-2026-43966 5.3

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-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 onward.

Record
DEBIAN-CVE-2026-43966
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Updated
2026-11-02
Show 4 moreShow less
OSVEEF-CVE-2026-43966 6.3

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

Summary

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-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 onward.

Workarounds

Validate all values passed into structured-fields header builders (directly via cow_http_struct_hd:item/1 or indirectly via higher-level wrappers) before calling the encoder. Reject any value that is not from a trusted, application-controlled source or that contains CR (\r) or LF (\n) bytes.

Applications using cowboy 2.16.0 or later are protected on the server side by the invalid_response_headers option (defaults to error_terminate), which rejects any outgoing response header value containing CR or LF before it reaches the wire. Applications using gun 2.4.0 or later are protected on the client side by the invalid_request_headers request option (defaults to raise), which raises an exception when an outgoing request header value contains CR or LF.

Configurations

The application must pass attacker-controlled data as a string value into cow_http_struct_hd:item/1 (or a wrapper that delegates to it). Applications that construct structured-fields header values exclusively from trusted, application-controlled values are not affected.

Record
EEF-CVE-2026-43966
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Weakness
CWE-113
Updated
2026-11-02
OSVGHSA-w4f7-4cxr-rv3c 6.3

cowboy and gun affected by an HTTP Request/Response Splitting vulnerability

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-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.

Record
GHSA-w4f7-4cxr-rv3c
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Weakness
CWE-113
Updated
2026-11-02
OSVUBUNTU-CVE-2026-43966 6.3

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-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.

Record
UBUNTU-CVE-2026-43966
CVSS 4.0
6.3 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
Updated
2026-11-02
OSVAZL-90162 No score

CVE-2026-43966 affecting package rabbitmq-server for versions less than 3.13.7-8

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-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 onward.

Record
AZL-90162
Updated
2026-11-05

Affected products

Affected packages

Affected packages
PackageAffectedFixed
cowboy
Hex
 before 2.16.0
2.16.0
gun
Hex
 before 2.4.0
2.4.0
rabbitmq-server
Azure Linux:3
 before 3.13.7-8
3.13.7-8
erlang-cowboy
Debian:14
 before 2.16.1+dfsg-1, plus listed versions
0.10.0+dfsg1-10.6.1-10.6.1-20.6.1-30.6.1-4
Show 7 moreShow less
0.8.6+dfsg-10.8.6+dfsg1-11.0.0+dfsg1-12.0.0~pre.1+dfsg1-12.0.0~pre.1+dfsg1-22.0.0~pre.1+dfsg1-32.0.0~pre.1+dfsg1-4
2.16.1+dfsg-1
rabbitmq-server
Debian:12
 All versions, plus listed versions
3.10.8-1.13.10.8-1.1+deb12u13.10.8-23.10.8-33.10.8-4
Show 24 moreShow less
3.12.1-13.12.1-1.14.0.5-14.0.5-104.0.5-114.0.5-124.0.5-134.0.5-144.0.5-24.0.5-34.0.5-44.0.5-54.0.5-64.0.5-84.0.5-94.3.0-14.3.0-24.3.0-34.3.2-14.3.2-24.3.2-34.3.2-44.3.2-54.3.6-1
 None known
rabbitmq-server
Debian:13
 All versions, plus listed versions
4.0.5-104.0.5-114.0.5-124.0.5-134.0.5-14
Show 14 moreShow less
4.0.5-64.0.5-6+deb13u14.0.5-6+deb13u24.0.5-84.0.5-94.3.0-14.3.0-24.3.0-34.3.2-14.3.2-24.3.2-34.3.2-44.3.2-54.3.6-1
 None known
rabbitmq-server
Debian:14
 All versions, plus listed versions
4.0.5-104.0.5-114.0.5-124.0.5-134.0.5-14
Show 12 moreShow less
4.0.5-64.0.5-84.0.5-94.3.0-14.3.0-24.3.0-34.3.2-14.3.2-24.3.2-34.3.2-44.3.2-54.3.6-1
 None known
cowlib
Hex
 2.9.0 and later, plus listed versions
2.10.02.10.12.11.02.12.02.12.1
Show 12 moreShow less
2.13.02.14.02.15.02.16.02.16.12.17.02.17.12.18.02.19.02.20.02.21.02.9.1
 None known
cowboy
Hex
 before 2.16.0, plus listed versions
1.0.01.0.11.0.21.0.31.0.4
Show 26 moreShow less
1.1.01.1.11.1.22.0.02.1.02.10.02.11.02.12.02.13.02.14.02.14.12.14.22.15.02.2.02.2.12.2.22.3.02.4.02.5.02.6.02.6.12.6.22.6.32.7.02.8.02.9.0
2.16.0
gun
Hex
 before 2.16.0, plus listed versions
1.0.01.0.0-pre.11.0.0-pre.21.0.0-pre.41.0.0-pre.4b
Show 18 moreShow less
1.0.0-pre.51.1.01.2.01.3.01.3.11.3.21.3.32.0.02.0.0-rc.12.0.0-rc.22.0.12.1.02.2.02.3.02.4.02.4.12.5.02.6.0
2.16.0

History

History
RecordedChangedSources
factsOSV
affected
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-43966 is a medium vulnerability (CVSS 6.3) in Ninenines Cowlib, published June 8, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.26%, the 17th percentile. Fixed in cowboy 2.16.0.

References

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