Summary
Forge: AES-GCM authentication bypass via unvalidated tagLength option
The AES-GCM mode in forge.cipher does not validate the tagLength option passed to start(). With a tagLength of 0 or a value that converts to 0 (such as null, false, '' or '0'), the authentication tag is empty, and decipher.finish() returns true for any ciphertext and additional data when the supplied tag is empty or missing. Fixed in node-forge 1.4.1.
Details
modes.gcm.prototype.start() in lib/cipherModes.js stored the tagLength option without validation, and lib/cipher.js documented it as accepting "0-128". After processing the data, afterFinish() truncates the 16-byte tag with truncate(16 % (tagLength / 8)). For a tagLength that converts to 0, this is 16 % 0 = NaN, and truncating by NaN empties the tag. On decryption, the start-time check that the supplied tag has tagLength / 8 bytes passes for an empty tag, and the final tag comparison compares two empty strings, so decipher.finish() returns true for any ciphertext and additional data.
NIST SP 800-38D section 5.2.1.2 allows only tag lengths of 128, 120, 112, 104 and 96 bits, plus 64 and 32 bits for certain applications.
The fix validates tagLength in start(), for both encryption and decryption. A missing, null or undefined tagLength uses the default of 128 bits. Any other value is converted with Number() and must be one of 128, 120, 112, 104, 96, 64 or 32; otherwise start() throws "Invalid tag length.". Numeric strings such as '128' are still accepted. With a validated tagLength, an empty tag no longer matches the expected tag length and is rejected.
Workarounds
Do not derive tagLength from received data. Pass a fixed tagLength (or omit it to use the default of 128 bits), and check that the received tag has the expected length before calling start(), for example if(tag.length !== 16) { throw new Error('Invalid tag.'); } before decipher.start({iv: iv, tag: tag});.
Proof of concept
Start an AES-GCM decipher with tagLength: 0 and an empty tag, as an application does when it derives the tag length from a received empty tag, and feed it arbitrary bytes that were never encrypted under the key:
var decipher = forge.cipher.createDecipher('AES-GCM', key);
decipher.start({iv: iv, tag: '', tagLength: 0});
decipher.update(forge.util.createBuffer('any attacker-chosen bytes'));
decipher.finish();
// <= 1.4.0: true
// 1.4.1: start() throws "Invalid tag length."
tagLength values null, false and '0' (with an empty tag), and tagLength: 0 with no tag option, give the same result on <= 1.4.0. The fix adds regression tests in tests/unit/aes.js.
Impact
Applications that decrypt AES-GCM data with forge.cipher.createDecipher() ('AES-GCM') and pass a zero-like tagLength are affected. Typical cases are a tag length derived from the received tag, or an unset configuration value:
var decipher = forge.cipher.createDecipher('AES-GCM', key);
decipher.start({iv: iv, tag: receivedTag, tagLength: receivedTag.length * 8});
decipher.update(forge.util.createBuffer(receivedCiphertext));
if(decipher.finish()) { /* attacker-supplied data accepted */ }
An attacker who can send a ciphertext with an empty tag to such an application can make it accept attacker-chosen ciphertext as authentic. Because GCM encryption is a stream cipher, an attacker who knows (part of) a captured message's plaintext can also change that plaintext to chosen values. The default (no tagLength option, 128 bits) and any fixed valid tagLength are not affected.
CVE-2026-119186 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Fixed in node-forge 1.4.1. cipher.start() now throws an error unless tagLength is one of the tag lengths allowed by NIST SP 800-38D (128, 120, 112, 104, 96, 64 or 32 bits); a null or undefined tagLength means the default of 128 bits.
Frequently Asked Questions
- What is CVE-2026-119186? CVE-2026-119186 is a medium-severity security vulnerability in node-forge (npm), affecting versions <= 1.4.0. It is fixed in 1.4.1.
- How severe is CVE-2026-119186? CVE-2026-119186 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of node-forge are affected by CVE-2026-119186? node-forge (npm) versions <= 1.4.0 is affected.
- Is there a fix for CVE-2026-119186? Yes. CVE-2026-119186 is fixed in 1.4.1. Upgrade to this version or later.
- Is CVE-2026-119186 exploitable, and should I be worried? Whether CVE-2026-119186 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-119186 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-119186? Upgrade
node-forgeto 1.4.1 or later.