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.
- Published
- Jun 8, 2026
- Updated
- Aug 17, 2026
- EPSS
- 0.4% · 35th percentile
- CWE
- CWE-113
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Report tools
Action score
25
Monitor
Low priority for now.
- CVSS
- 25 / 40 · 6.3 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 0.4%
CISA SSVC decision
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- —
- CNA · EEF
- 6.3
- NVD has not scored this yet; the score shown is the CNA’s.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Noroxi analysis
No Noroxi analysis for this record yet
We don't hand-write analysis for the hundreds of thousands of vulnerabilities in the database; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.
We use this product, ask for helpAffected systems
| Vendor | Product | CPE |
|---|---|---|
| ninenines | cowlib | cpe:2.3:a:ninenines:cowlib |
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- ninenines cowlib2.9.0 and later · before 2.16.0
Versions reported by the vendor
Affected version ranges reported by the assigning authority (EEF). Independent of NVD's CPE analysis and usually ahead of it.
ninenines cowlib
- 2.9.0, a8b793db3d6ffe91d62f81baf41b1dab4cd78fb6affected
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| Debian:12 | rabbitmq-server | all versions | — |
| Debian:14 | erlang-cowboy | before 2.16.1+dfsg-1 | 2.16.1+dfsg-1 |
| Hex | cowboy | before 2.16.0 | 2.16.0 |
| Hex | cowlib | from 2.9.0 | — |
| Hex | gun | before 2.16.0 | 2.16.0 |
Same product
ninenines: all recordsOther highest-scoring records for the same primary product.
- CVE-2026-59248Unbounded HPACK/QPACK prefixed-integer decoding in Cowlib causes memory-exhaustion DoS34Monitor
- CVE-2026-7790Unbounded chunk-size hex digits in cowlib cause quadratic CPU and memory DoS34Monitor
- CVE-2026-43970Decompression Bomb in cow_spdy:inflate/2 Allows Memory Exhaustion via Crafted SPDY Frame32Monitor
- CVE-2026-43971Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/125Monitor
- CVE-2026-43968CR Injection in SSE Encoder Enables Event Splitting via cow_sse:event/125Monitor
- CVE-2026-43969Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/18Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| debian:erlang-cowboy | 2.16.1+dfsg-1 · Debian:14 | Package registry (OSV) |
| Hex:cowboy | 2.16.0 | Package registry (OSV) |
| Hex:gun | 2.16.0 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
- Peter Ullrichfinder
- Loïc Hoguinremediation developer
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
No nightly-computed relations.
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- No user action is required.
- No special conditions are required; it is repeatable.
If successful
- Confidentiality
- Integrity
- Availability
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- X
Weakness class (CWE)
CWE-113 · Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')CVSS vector
CVSS: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
nvd-secondary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
Attack patterns (CAPEC)
ATT&CK techniques
Change log
- Fix✗ → ✓
For records you follow, these changes also arrive as notifications. →
References
- cna.erlef.org/cves/CVE-2026-43966.html
- github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef
- github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42
- osv.dev/vulnerability/EEF-CVE-2026-43966
Vendor advisories and official records. Exploit/PoC links are deliberately left out.