EEF-CVE-2026-43971 CVE-2026-43971
Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1
- Severity
- 0.0 / 10
- Unknown · CVSS 4.0
- Exploit chance (EPSS)
- 0.52%
- Chance of exploitation in the next 30 days. Higher than 44.44% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-116 Improper Encoding or Escaping of Output
- Fix
2.20.0- Upgrade cowlib to this version or later
Description
Summary
Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.
cow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A > byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A " or \ in rel escapes the quoted string and opens new parameters. Any byte — including whitespace, =, and " — in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel="preconnect", rel="preload", and rel="prerender", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins.
This issue affects cowlib: from 2.9.0 before 2.20.0.
Workarounds
Sanitize values before passing them to cow_link:link/1: reject or strip any target value containing >, any rel value containing " or \, and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input.
Configurations
The application must pass values that originate from, or can be influenced by, untrusted input into cow_link:link/1 — specifically the target URI, rel value, or attribute keys. Applications that construct Link entries exclusively from trusted, application-controlled values are not affected.
Affected versions
Hex
| Package | Vulnerable | Fixed in |
|---|---|---|
| cowlib | >= 2.9.0, < 2.20.0 |
2.20.0 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
cowlib Hex → 2.20.0
- mix
{:cowlib, "~> 2.20.0"} # then: mix deps.update cowlib
Severity breakdown
CVSS 4.0 0.0
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
- Attack vector
- Network
- Attack complexity
- Low
- Attack requirements
- Present
- Privileges required
- None
- User interaction
- None
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- Low
- Availability (vulnerable system)
- None
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- Low
- Availability (subsequent systems)
- None
Weakness
CWE-116: Improper Encoding or Escaping of Output
The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
How to prevent it
- Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using the ESAPI Encoding control [REF-45] or a similar tool, library, or framework. These will help the programmer encode outputs in a manner less prone to error. Alternately, use built-in functions, but consider using wrappers in case those functions are discovered to have a vulnerability.
- Architecture and Design: If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated. For example, stored procedures can enforce database query structure and reduce the likelihood of SQL injection.
- Architecture and Design, Implementation: Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies.
- Architecture and Design: In some cases, input validation may be an important strategy when output encoding is not a complete solution. For example, you may be providing the same output that will be processed by multiple consumers that use different encodings or representations. In other cases, you may be required to allow user-supplied input to contain control information, such as limited HTML tags that support formatting in a wiki or bulletin board. When this type of requirement must be met, use an extremely strict allowlist to limit which control sequences can be used. Verify that the resulting syntactic structure is what you expect. Use your normal encoding methods for the remainder of the input.
- Architecture and Design: Use input validation as a defense-in-depth measure to reduce the likelihood of output encoding errors (see CWE-20).
- Requirements: Fully specify which encodings are required by components that will be communicating with each other.
- Implementation: When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.
References
Fixes and patches
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| EEF-CVE-2026-59248 Unbounded HPACK/QPACK prefixed-integer decoding in Cowlib causes memory-exhaustion DoS | Not rated yet | Also affects cowlib |
| EEF-CVE-2026-43966 HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2 | Medium | Also affects cowlib |
| EEF-CVE-2026-43970 Decompression Bomb in cow_spdy:inflate/2 Allows Memory Exhaustion via Crafted SPDY Frame | High | Also affects cowlib |
| EEF-CVE-2026-43968 CR Injection in SSE Encoder Enables Event Splitting via cow_sse:event/1 | Medium | Also affects cowlib |
| EEF-CVE-2026-7790 Unbounded chunk-size hex digits in cowlib cause quadratic CPU and memory DoS | High | Also affects cowlib |
| EEF-CVE-2026-43969 Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1 | Low | Also affects cowlib |
| GHSA-qxg6-89mg-rqv8 Vitest: Code injection via unauthenticated manual mock registration | Medium | Same weakness, CWE-116 |
| GHSA-333x-w4r6-6crp fast-uri vulnerable to scheme injection via an unescaped first-segment colon in suffix references | Low | Same weakness, CWE-116 |
Credits: Jonatan Männchen / EEF, Loïc Hoguin, Peter Ullrich
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