Unrated

HTTP Response Header Injection in undici Cookie Parser

IdentifiersCVE-2026-9679CWE-113

CVE-2026-9679 is an HTTP response header injection issue in undici introduced in version 7.0.0 via PR #3789. The vulnerability stems from undici's cookie parsing logic, specifically parseSetCookie and related helpers such as parseCookie and getSetCookies, which percent-decode cookie values using qsUnescape. As a result, encoded sequences such as %0D%0A, %00, %3B, and %3D are converted into literal CR, LF, NUL, semicolon, and equals characters. This behavior is inconsistent with RFC 6265 §5.4 and browser behavior, which do not decode Set-Cookie values in this way. In applications that parse an upstream Set-Cookie header with undici and then reuse the parsed cookie value in a downstream HTTP response header, an attacker controlling the upstream response can inject arbitrary response headers.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows downstream HTTP response header injection. An attacker-controlled upstream can cause the vulnerable application to emit attacker-chosen headers such as Set-Cookie, Location, or Cache-Control in its response. Depending on application behavior, this can enable session fixation, open redirect, cache poisoning, and other header-manipulation attacks. The impact is limited to applications that both use the affected undici cookie parsing functions and propagate the parsed values into response headers.

Mitigation

If you can’t patch tonight, do this now.

If immediate upgrade is not possible, do not forward values returned by parseSetCookie, parseCookie, or getSetCookies directly into response headers. Sanitize or reject parsed values before reuse in headers, specifically stripping or blocking CR, LF, NUL, semicolon, and equals characters. More generally, avoid reflecting upstream-controlled cookie values into downstream response headers without strict validation.

Remediation

Patch, then assume compromise.

Upgrade undici to a fixed release. The provided advisory indicates fixed versions are v6.27.0, v7.28.0, and v8.5.0, depending on the deployed major branch. Validate that no affected parsing path remains in use after upgrade.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

NodejsUndiciapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

6 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

6 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

What this page doesn’t show

The version that knows your environment.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity6

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2026-9679

NVD

nvd.nist.gov/vuln/detail/CVE-2026-9679

MITRE CWE · CWE-113

cwe.mitre.org