CVE-2026-43966
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
A vulnerability in the cowlib library (from version 2.9.0 onward) involves improper neutralization of CRLF sequences in HTTP headers, enabling HTTP response splitting attacks. The cow_http_struct_hd:escape_string/2 function in cowlib only escapes \ and ", passing all other bytes through verbatim, creating an encoder/decoder asymmetry. An attacker can inject \r\n into the serialized header, allowing new headers to be added and potentially splitting the HTTP response.
Risk Assessment
The risk includes the possibility of manipulating HTTP responses, which could lead to content injection, session hijacking, or other response splitting attacks. The vulnerability affects applications using cowlib to build HTTP headers.
Recommendation
Upgrade cowlib to a version that includes a fix for this issue. Also, validate input data thoroughly before using it in HTTP header building functions.
Other vulnerabilities in cowlib
See all- CVE-2026-43971Medium
cowlib (Erlang) versions 2.9.0 through 2.20.0 fail to escape values inserted into the Link: header in cow_link:link/1. Special characters in the URI, rel, or attribute keys can be injected into the header, allowing additional Link directives to be smuggled. As a result, victim browsers may be forced to make connections to attacker-controlled resources.
- CVE-2026-59248High
A vulnerability in the cowlib library allows an unauthenticated remote attacker to exhaust memory on a vulnerable server or client by sending specially crafted HTTP/2 or HTTP/3 frames. The issue stems from missing limits in the HPACK and QPACK decoder, which processes arbitrarily long prefixed integers, causing large bignum allocations and heavy garbage collection churn.
- CVE-2026-43970High
CVE-2026-43970 is a vulnerability related to improper handling of highly compressed data in the cowlib library, which can lead to remote denial of service attacks via memory exhaustion. It allows unauthenticated users to send specially crafted SPDY frames that can cause significant memory consumption.
- CVE-2026-43969Low
The cowlib Erlang library's cow_cookie:cookie/1 function builds a Cookie header without validating cookie names or values. This allows injection of ;, comma, CR, LF, or TAB characters, enabling cookie smuggling and HTTP request splitting. Affects versions from 2.9.0 before 2.21.0.
- CVE-2026-43968Medium
cowlib versions 2.6.0 to before 2.16.1 contain a CRLF injection vulnerability allowing SSE event splitting and injection via unvalidated field values. An attacker can inject additional SSE lines and forge a complete event with arbitrary type and data.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS