GitHub Security Advisory: GHSA-ppp5-5v6c-4jwp
Package: node-forge (NPM)
Severity: HIGH (CVSS 7.5)
CWE: CWE-20: Improper Input Validation, CWE-347: Improper Verification of Cryptographic Signature
Vulnerable versions: < 1.4.0
Patched in: 1.4.0

Summary: Forge has signature forgery in RSA-PKCS due to ASN.1 extra field  

Details:
## Summary
RSASSA PKCS#1 v1.5 signature verification accepts forged signatures for low public exponent keys (e=3). Attackers can forge signatures by stuffing “garbage” bytes within the ASN structure in order to construct a signature that passes verification, enabling [Bleichenbacher style forgery](https://mailarchive.ietf.org/arch/msg/openpgp/5rnE9ZRN1AokBVj3VqblGlP63QE/). This issue is similar to [CVE-2022-24771](https://github.com/digitalbazaar/forge/security/advisories/GHSA-cfm4-qjh2-4765), but adds bytes in an addition field within the ASN structure, rather than outside of it. 

Additionally, forge does not validate that signatures include a minimum of 8 bytes of padding as [defined by the specification](https://datatracker.ietf.org/doc/html/rfc2313#section-8), providing attackers additional space to construct Bleichenbacher forgeries. 

## Impacted Deployments
**Tested commit:** `8e1d527fe8ec[PII_REDACTED]db783172d4fb9012e5`
**Affected versions:** tested on v1.3.3 (latest release) and recent prior versions.

**Configuration assumptions:**
- Invoke key.verify with defaults (default `scheme` uses RSASSA-PKCS1-v1_5).
- `_parseAllDigestBytes: true` (default setting).

## Root Cause

In `lib/rsa.js`, `key.verify(...)`, forge decrypts the signature block, decodes PKCS#1 v1.5 padding (`_decodePkcs1_v1_5`), parses ASN.1, and compares `capture.digest` to the provided digest.

Two issues are present with this logic:

1. Strict DER byte-consumption (`_parseAllDigestBytes`) only guarantees all bytes are parsed, not that the parsed structure is the canonical minimal DigestInfo shape expected by RFC 8017 verification semantics. A forged EM with attacker-controlled additional ASN.1 content inside the parsed container can still pass forge verification while OpenSSL rejects it.
2. `_decodePkcs1_v1_5` comments mention that PS < 8 bytes should be rejected, but does not implement this logic.

## Reproduction Steps
1. Use Node.js (tested with `v24.9.0`) and clone `digitalbazaar/forge` at commit `8e1d527fe8ec[PII_REDACTED]db783172d4fb9012e5`.
4. Place and run the PoC script (`repro_min.js`) with `node repro_min.js` in the same level as the `forge` folder.
5. The script generates a fresh RSA keypair (`4096` bits, `e=3`), creates a normal control signature, then computes a forged candidate using cube-root interval construction.
6. The script verifies both signatures with:
  - forge verify (`_parseAllDigestBytes: true`), and
  - Node/OpenSSL verify (`crypto.verify` with `RSA_PKCS1_PADDING`).
7. Confirm output includes:
  - `control-forge-strict: true`
  - `control-node: true`
  - `forgery (forge library, strict): true`
  - `forgery (node/OpenSSL): false`

## Proof of Concept

**Overview:**
- Demonstrates a valid control signature and a forged signature in one run.
- Uses strict forge parsing mode explicitly (`_parseAllDigestBytes: true`, also forge default).
- Uses Node/OpenSSL as an differential verification baseline.
- Observed output on tested commit:

```text
control-fo