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A lenient DER parser can turn into a signature bypass when a verifier accepts an encoding or structure that the signature scheme does not permit. The gap is between what the scheme requires and what the implementation accepts—not a general property of ASN.1 parsers, and not a way to make an arbitrary altered signature valid. Whether it is exploitable depends on the verifier’s checks, the signature scheme, key parameters, and deployment.
Why DER’s one-encoding rule matters to signatures
ASN.1 describes data structures; BER defines ways to encode them. DER is a restricted profile of BER that selects one canonical encoding for a given value. That distinction matters when cryptographic rules depend on the encoded representation: accepting multiple encodings for what a parser considers the same value can widen the verifier’s acceptance rule beyond the format the signature scheme specifies.
RFC 7468, Appendix B, “DER Expectations,” puts the point directly: “A digital signature is (supposed to be) computed over the DER encoding of the semantic content, so providing anything other than the DER encoding is senseless.” The appendix is informative and directs readers to the relevant standards for normative requirements. Its key distinction is that every DER encoding is a BER encoding, but a value has only one DER encoding. Definite-length forms are also part of DER’s predictable representation.
How a permissive interpretation can bypass verification
A verifier should accept only the structure and semantics prescribed for the signature, then validate the cryptographic operation against them. If it instead accepts extra fields, a non-canonical encoding, or other unexpected structure and extracts a digest or value from it, a crafted input may satisfy the implementation even though it is not a valid instance of the expected signed structure. That mismatch is an interpretation gap between the format’s rules and the verifier’s behavior.
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This does not mean that changing a byte in a correctly verified signature makes it valid. A successful attack requires a specific weakness in the verifier and any other conditions the relevant scheme imposes. The practical question is not only “Did the parser decode an object?” but “Did the verifier enforce the exact object and constraints that this signature profile requires?”
Why consuming all input is not enough
Forge’s advisory illustrates the difference. It reports that _parseAllDigestBytes ensures all bytes are consumed, but does not ensure the parsed structure is the canonical, minimal DigestInfo shape expected by the RFC 8017 verification semantics. The advisory also identifies missing enforcement of the specified minimum eight-byte PKCS#1 v1.5 padding string. A parser can therefore reach the end of its input and still have accepted a structurally invalid signature block. These findings describe the Forge versions and defaults covered by that advisory, not a universal property of RSA libraries.
What real parser-related vulnerabilities show
Two classes of failure are often conflated: a malformed input that crashes or disrupts a service, and an input that makes a verifier authenticate a signature it should reject. Both can cross a security boundary, but a denial of service is not itself a signature forgery.
Libreswan: denial of service and forgery are separate impacts
Red Hat’s Libreswan advisory describes inadequate validation of the DER-encoded ASN.1 digest in an IKEv2 RSASSA-PKCS1-v1_5 authentication path. A malicious hash shorter than expected can trigger an assertion failure, causing the daemon to restart. The advisory separately says a Bleichenbacher-style signature forgery and authentication bypass requires weak public RSA exponents such as e=3. Red Hat notes that its modern enterprise policy blocks those weak exponents, which changes the practical severity in that environment; it should not be assumed to describe every operating system or deployment.
For this documented issue, Red Hat recommends upgrading or restricting authentication to modern algorithms. Its guidance discusses configuring ECDSA and RSASSA-PSS authentication, with a compatibility trade-off: native Windows VPN clients that do not support RSASSA-PSS may no longer connect under that configuration.
Other ASN.1 mistakes can affect trust decisions differently
Historical non-canonical serialization acceptance has been associated with Bleichenbacher-style RSA signature forgery, but that does not make every permissive parser exploitable. The Microsoft Research paper on X.509 parsing highlights a separate semantic challenge: an extension’s payload may be carried inside an OCTET STRING and must be interpreted according to its object identifier. A certificate validator must reject an unrecognized critical extension. Correctly decoding low-level TLV structures alone does not ensure correct certificate validation.
The SSTIC 2019 paper documents other parser failures that reached security boundaries: a crafted keyUsage extension enabled a secure-boot bypass on listed NXP processors, while a Nintendo 3DS RSA PKCS#1 v1.5 issue involved unchecked bounds for an embedded signed hash, changing what the BootROM checked. These are examples of parser mistakes with security consequences, but they are distinct mechanisms from accepting non-canonical DER in signature verification.
What to check in a verifier or security review
Review the parser and the code that decides whether a signature is valid as one verification path. A memory-safe parser can still be semantically too permissive; conversely, a strict DER decoder does not by itself guarantee sound certificate policy checks.
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- Canonical encoding: Where the protocol or signature profile requires DER, reject malformed and non-canonical encodings rather than permissively normalizing them before verification.
- Exact scheme structure: Check the expected algorithm identifier, digest,
DigestInfoshape, and signature padding constraints. Do not treat complete input consumption as proof that these requirements were met. - Bounds and nested content: Validate lengths, integer and bit-string bounds, embedded payload types, and resource limits. Include X.509 extension semantics—not just low-level decoding—in the review.
- Key constraints: Enforce appropriate key-parameter policy. The Libreswan case demonstrates why exponent assumptions can affect whether a forgery path is practical.
- Negative tests: Test alternate BER encodings, trailing or embedded ASN.1 fields, malformed digest lengths, and padding at the minimum boundary. Confirm that each invalid structure is rejected rather than merely parsed.
- Failure behavior: Test malformed inputs for crashes, assertions, resource exhaustion, and service restart separately from tests of incorrect signature acceptance.
When comparing implementations, assess these dimensions separately: canonical-encoding enforcement, exact scheme-level validation, treatment of extra data, key-parameter constraints, and failure behavior. Passing one check—for example, consuming every byte—does not establish that the others are correct.
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